(Study Material) Geology Study Material

Glossary of Selected Important Terms

Absorption Spectrum : Colors of light least absorbed combining to produce the color of the stone. The stone, when viewed by spectroscope, will show as dark bands in characteristic positions the colors most strongly absorbed.

Acicular : Needlelike; refers to the growth of a mineral in long and slender crystals.

Adamantine :  Very high luster.

Aggregate : Intergrowth of several crystals, these may be globular, fibrous, reniform, or radiating fibrous.

Adularescence : The sheen of color seen in moonstone and other feldspars of the adularia variety.

Allochromatic Minerals : Minerals that are colorless when pure, the color coming from coloring agents, most of which are, cobalt, copper, chromium, titanium, vanadium, manganese, and iron. Examples of this are beryl,corundum, quartz, and spinel.

Alluvium : Continental sediments due to transport and deposition of gravel, sand, and clay by running water, rivers, and streams. (See alluvial)

Alpha rays : Helium atoms with double positive charge.

Alpine cavities (vugs) : Hollows in silicate rock, they may be partially filled with mineral formations.
Alteration pseudomorph : One mineral has been replaced by another that is unrelated while preserving the original crystal form.

Amorphous : Has no characteristic external form or shape. The arrangement of the atoms and molecules are irregular

Amphiboles : A group of closely related, dark colored rock forming silicate minerals, as in, actinolite, hornblende.

Anisotropic :  when applied to crystals it is the display of unequal physical properties in different directions. An example would be a mineral which has a different hardness when tested in different directions.

Amygdaloidal : (amygdule) gas filled volcanic rock.

Amygdule : A rounded mass of mineral formed in a gas cavity.

Angstrom Unit : Unit and length of measurement of wavelengths of visible light and most xrays. (1 A.= .0000001 millimeter)

Anisotropic : Crystals in which the optical properties vary with direction. All crystals except those in the cubic system are in this category, and exhibit double refraction.

Aphanitic rock :  That rock in which the crystals are too small to be seen by the unaided eye.

Arid :  dry or desert like.

Arsenates : Minerals in which AsO4 radical is an important constituent.

Asterism : Stones containing suitably oriented rod like inclusions or channels, that are cut as cabochon in the correct direction show this star effect. Star effect.

Atom : The smallest part of a chemical element which remains unchanged during all chemical reactions. Atomic Weight : Weight of an atom compared with an atom of oxygen (16.00).

Batholith :  A huge body of plutonic rock that has been intruded deep into the earth's crust and latter exposed by erosion.

Bean (pisolitic) iron ore :  Globular aggregates of limonite that occur in karst cavities as weathering formations.

Beta rays : Electron rays

Bezel :  A rim of metal surrounding a gemstone securing it.

Biaxial : Two optic axes or double refraction. Usually crystals in the rhombic, monoclinic, and triclinic system.

Bipyramid (dipyramid) : Crystals that form symmetrically about a plane dividing it into two pyramids.

Birefringence : Same as double refraction. Splits rays of light passing through a transparent object as glass or crystal.

Botryoidal :  Resembling a bunch of grapes in rounded masses of a mineral.

Boule : The form and shape of a synthetic stone when created by the inverted blowpipe of a Verneuil furnace, somewhat carrot shaped.

Breccia : An aggregate of angular fragments of stone or mineral cemented together as in calcite and chalcedony.

Brilliant : The cut of a gemstone that is round and has 32 facets plus the table above the girdle, (crown), and 24 facets plus any culet below the girdle, (pavilion).

Cabochon : The cut of a gemstone that has a convex surface. A cab.

Cameo :  A carved shell, sometimes cut from onyx or other mineral containing bands of different colors, To cut in relief, the opposite of intaglio.

Carat : Unit of weight used to weigh gemstones, equal to 200 milligrams, or .200 grams. 1 gr. = 5 ct. 100 points = 1 ct. metric system.

Cataclastic rock :  A metamorphic rock produced by the crushing and grinding of preexisting rocks, which are still visible as crushed fragments.

Chatoyancy : Cat's eye effect produced by some gemstones when cut properly in cabochon. See asterism.

Chelsea Filter : A dichromatic color filter transmitting light of only two wavelengths, one deep red the other yellow green. Used to discriminate between emerald and synth. spinel and green glass colored with cobalt.

Chemical Element : Matter composed of atoms of only one chemical type which cannot be decomposed into simpler substances by chemical methods.

Clastic rock :  Sedimentary rock made up of fragments of preexisting rocks and transported into the place of deposition.
Cleavage : The tendency of stones to split along one or more definite directions, always parallel to a possible crystal face.

Conchoidal..(fracture), A breakage which leaves a conchoidal shell shaped surface.

Conglomerate ( as in geology) .. Conglomerates, as well as sedimentary breccias, are coarse-grained SEDIMENTARY ROCKS formed by the consolidation and hardening of, respectively, rounded and angular gravel deposited in oceans. More than 30 percent of the large particles of these rocks exceed 2 mm (0.08 in) in diameter. The particles may be pebbles, cobbles, or boulders, or mixtures of these sizes. Both conglomerates and sedimentary breccias may be named and classified by the proportion of gravel - sized particles; the type of matrix,and the types of gravel-sized particles. The proportion of gravel is a function of the highest current speed at the time of deposition and the availability of particles of such coarse size. A sample that is more than 80 percent pebbles, cobbles, or boulders is called a conglomerate proper, whereas one that is 30 to 80 percent is an arenaceous (sandy) conglomerate or an argillaceous (shaley) conglomerate. The matrix between the layers of coarse particles may also be calcareous (that is, containing calcium carbonate) or sideritic (containing ferrous carbonate). On the basis of the variety of pebbles, cobbles, and boulders in conglomerates, they can be classified as oligomictic, consisting of a single kind of rock (such as one of various varieties of chert and quartzite or other rock), or polymictic, containing many kinds of rock.

Concretion : Knobby or rounded mineral concentrations in sedimentary rocks that are completely surrounded by rock.

Contact metamorphism : The change of rock due to the effect of high temperatures during contact with a lava flow, magma sloping, or igneous intrusion.

Critical Angle : The angle at which a ray of light passes from one medium to another, as a gemstone and air. Cryptocrystalline :  : The structure of a substance as chalcedony, that consist of very small crystals but show no external sign of crystal structure.

Crystal..A homogeneous body in the form of a geometric solid bonded by polyhedral faces, the nature of which is expression of the orderly and periodic arrangement of its constituent atoms.

Crystal Axes : "Lines" passing through a crystal in important symmetric directions, intersecting at the center of the crystal.

Crystal Systems : The six main groups into which crystals can be classified: triclinic, monoclinic, orthorhombic, cubic, tetragonal and hexagonal.

Decrepitation :  The explosive shattering of mineral grains on heating.

Dendrites :  Skeletal crystals that develop from supersaturated solutions, often in small cracks, often resembling plant or trees.

Density : The ratio of the weight of a substance to its volume expressed in g/cm 3, and numerically equal to the specific gravity.

Detrital : Occurrence of minerals in gravels that came from a mineral deposit. (placer)

Diaphaneity : Showing light through its substance; transparent; translucent.

Dichroism : Possessing the property of showing two different colors when viewed from different angles.

Dike :  In the forming of rocks, when intruding sedimentary rocks in a vertical or nearly vertical position.

Dispersion..The separation of white light into its constituent colors by its refraction or diffraction.

Double Refraction : Ability of certain crystals to split incident light into two rays with different refractive indices.

Doublets : A common method of building up sufficient thickness to permit a gem to be used in a setting. A non gem mineral is cemented to the top or bottom of the gem material. (See Opals.)

Dripstone : stalagmites or stalactites

Druse : A crystal coated surface of rock.

Doctile : Able to be drawn into a wire.

Endogenous : Generated deep in the earth by volcanism or earthquakes.

Enhydro : A chalcedony or carnelian geode having the center cavity filled with water.

Epithermal vein :  Formed at shallow depths from ascending hot solutions.

Epizone : In regional metamorphism, the depth level nearest the surface to approx. 3.75 to 4.25 miles.

Extinction : Diminution in the intensity of radiation due to absorption by or scattering in the medium; also the stopping of incident X-rays by the outer layers of atoms in a crystal.

Extraordinary Ray : Crystals and minerals belonging to the hexagonal tetragonal systems in which the ray or refractive index varies according to its direction through the crystal.
Extrusive rock : Igneous rock that solidifies on the surface of the earth.

Facet : Man made flat part of a mineral, a planar surface.

Ferromagnetism : magnetic even in the absence of an external magnetic field.

Fire : Same as dispersion

Fluorescence : Temporary emission of radiations of different wavelength (color) by a substance struck by light waves usually long or short wave ultraviolet light.

Fluvial : Deposits from rivers.

Foliated : Made up of thin leaves, like mica.

Formulas : For weight estimation based on measurements in millimeters to 1/10. assuming well cut stones with little or no bulge factor. Some formulas for various gem cuts.

Fracture : A break with an uneven or irregular surface.

Fraunhofer Lines : A series of groups of dark lines in the spectrum of an object, visible using a spectroscope.

Friable :  Easily crumbled or pulverized.

Fumaroles : Spots in or near active volcanos where gases are expelled.

Gabgue : The minerals of no value in veins with ore minerals.

Gamma rays : Short wave electromagnetic waves.

Gangue : Minerals of no value associated in veins with ore minerals.

Geniculated :  Knee like intergrowths of crystals.

Geode : A stone having a cavity lined with crystals; the cavity in such a stone.

Girdle : The wide part of a cut gemstone

Gliding plane :  acrystal direction along which the atoms can slip a defined distance without destroying the coherence of the crystal.

Grain : (Troy system)..480 grains to the oz.

Habit : Characteristic crystal form.

Hacklt : A fracture characteristic of metals in rock, like gold and copper. (hackly)

Hardness : The resistance by a substance to actions which tend to modify its surface by scratching, abrasion, penetration.

Hemimorphic : Half formed crystals in which the faces that grow on one end are different in angle and position from the faces to be found on the other end.

Hydrothermal : Hot water or solution sometimes superheated

Idiochromatic : Minerals : in which the color is due to an essential constituent.

Igneous rock.. Rock formed by the solidification of magma.

Inclusions : Substances within a mineral, example, other minerals, gas bubbles, liquids, or other foreign objects.

Imitation Stones : Substances used to look like a genuine. ie: glass, plastic, etc.

Inclusions : Solid, gaseous, or liquid material of various types incorporated in a crystal during its formation and growth.

Intaglio : Incised carving; a sunken design, ie: cameo

Intarsia : Mosaic inlay work using colored stones. (and wood)

Ion : Electrically charged atom, radical or molecule.

Isomorphous  : Minerals in which two or more elements can replace each other to any extent without notably changing the appearance of the crystal.

Isotropic : Materials in which the optical character is the same in all directions.

Labradorescence : Patchy or diffuse iridescence due to the interference of light by reflection from parallel inclusions. Example Labradorite.

Light : Radiant energy which gives the sensation of sight. Velocity is 186,285 miles per second.

Litmus paper :  colored paper used to show whether a solution is acid or alkaline.

Lopolith.. A large, lenticular, sunken mass of igneous rock whose surfaces are concordant with the enclosing rocks.

Luster : A reflective property of mineral surfaces.

Magma : Molten silica containing volatile substances in solution, present beneath the surface in certain areas of the earth's crust.

Malleable : Can be flattened by pounding, as in metals.

Mammillary :  rounded mineral surface

Massive : Minerals not bounded by crystal faces.

Metamorphism : (geological)..Changes in rocks brought about by heat and pressure acting in the rocks just below the surface.

Meteorite : Solid object that comes from outer space and falls to earth.

Miaroles : Small cavities in granitic rocks created by volatile components.

Miller indices : Group of three digits used to designate crystal faces.

Mohs Scale : The relative scale of the hardness of minerals, from 1 to 10, the order of hardness with no significance to quantitative relationship.

Molecule : Two or more atoms in close relationship, The smallest quantity of an element or compound which is capable of independent existence.

Monochromatic Light : Light of one wavelength only.

Nicol Prism : Prism for producing polarized light, having split the light into two rays.

Nodule : A lump or node.

Opaque : Not transparent or translucent. (you can't see through it)

Optic Axis : The direction of single refraction in a double refractive mineral.

Optic sign : If the lower index of the refractive reading does not vary when the gemstone is rotated, it is uniaxial and + optic sign, if the higher index is steady and the lower index varies the optical sign is -.

Ore :  A mineral occurring in sufficient amounts to permit its recovery at a profit.

Organic : compounds of carbon, ie: coal, jet

Orient : Characteristic sheen and iridescence, as displayed in a good pearl. v Orientation :  Applied to crystals, this means visualizing the disposition of the principal directions within the crystal.

Orogenesis : A complex phenomena that leads to the formation of mountain ranges.

Outcrop : Where the bed rock is exposed.

Paleontology :  a division of geology that concerns itself with prehistoric life.

Paragenesis..The sequence in time in which a mineral crystallizes with respect to the other minerals.

Paramorph : A chemically identical crystal compared with the original, but the atoms have been rearranged so that they no longer conform to the original outline.

Parting : A smooth fracture that looks like a cleavage plane but takes place only on certain planes in the crystal, not between any set of atoms, like true cleavage.

Paste : Lead glass imitation stone.

Pegmatites : .A very coarse platonic rock, generally granitic in composition. Usually forming dikes that cut granite or the gneisses and schists that border granite masses. They are coarse because the liquid residue at the time of their crystallization contained a high percentage of water and other volatile elements that did not go into the makeup of the common minerals of granite, and were concentrated in the residue.

Petrography.. The branch of geology that deals with the description and classification of rocks.

Phaneritic rock.. An igneous rock in which all of the essential minerals can be seen by the unaided eye.

Phenocryst : Crystal that is set in a finer grained ground mass.

Phosphorescence : Luminescence emanating from substances that have been irradiated with ultraviolet light or X-rays, but persisting after the source of stimulation has stopped.

Piezoelectric : A substance that becomes electrically charged by pressure.

Pipe : In geology, the tubular ascending structures in volcanic rock.

Placer : Concentrations of heavy or resistant minerals that have been transported to rivers or other water areas.

Plastics : Synthetic resin, hard, usually molded.

Pleochroism : Dichroism and similar effects shown by some biaxial minerals in which three distinct colors or shades can be seen by using a dichroscope. (two at a time)

Plutonic rock : rock that has solidified deep underground.

Pneumatolysis : The action of gases of a magma on the surrounding rocks and on the solidifying molten mass itself. Point : (diamonds).. 1/100 of a carat, .01 ct.

Polarized Light : Light which vibrates in one direction or plane.

Polaroid : Class or plastic which passes light on one plane.

Potch : native material in or around a gemstone, a dead spot or area. ie: ironstone surrounding opal.

Radioactive : Emitting alpha, beta, or gamma rays.

Reconstructed Stones : Old method of fusing Stones made from chips. Used now to describe amber remelted and fused from otherwise unusable pieces.

Reflection : The throwing off or back, light from the surface.

Refraction : The bending of light passing through one medium to another. The changing of direction.

Refractive Index : The measurement of the amount of change in direction of light passing through one medium to another. The ratio of the velocity of the light in air to the velocity of light in the medium being measured.

Refractometer : Instrument used to determine the refractive index of various substances.

Rock : Stoney matter. Any mass of mineral matter forming an essential part of the earth's crust.

Sclerometer : Device for the quantitative measuring of hardness.

Sedimentary Rocks : formed originally of sediment, including shale and sandstone, composed of fragments of other rocks deposited after transportation from their sources, and including those formed by precipitation, as gypsum, or by calcareous secretions of animals as in certain limestones.

Selvage : The area bordering a vein.

Sheen : The iridescence of light reflected from the surface of a stone. ie: moonstone

Silk : Fine rod like crystals embedded in a crystal

Skarn : Contact rock of igneous silicate masses with limestone.

Specific Gravity : The weight of a substance compared to the weight of an equal volume of pure water at 4 degrees C.

Spectroscope : Instrument which resolves light into its component wavelengths by refraction through prisms or diffraction grating.

Spectrum : The band of light showing the succession of rainbow colors corresponding to the different wavelengths. The VISIBLE spectrum is the small portion in the vast spectrum of electromagnetic waves, which extend from very long waves, (now used by submarines for underwater communication) to very short waves (gamma) emitted by radioactive elements.

Stalactites : dripstone, stalagmite..the formation of a conelike structure that grows from the deposits of carbonate of lime build up.

Star : The effect of proper cutting of a gemstone as a cabochon when the gemstone contains long rod like crystals or fibers in parallel or cavities where these have formally been. The reflection, (using a single light source), from the surface produces lines in even numbers, ie: 2,4,6,8,12, with a common center.

Step cut : (trap cut)..Cut with edges are parallel to those the rectangular table.

Synthetic stones : Man made stones which have approx. the same composition and or crystal structure of the natural crystal which they represent. (See synthetic diamonds.)

Transparency : The degree to which light passes through a substance.

Twin Crystals : Two or more crystals which have grown together in a symmetrical fashion.

Ultraviolet Light : Wavelengths of light shorter than visible violet light.

Ultrabasic rock.. Any plutonic igneous rock with very low silica content.

Uniaxial : Minerals having single refraction, one optic axis.

Vacuolar texture .. texture typical of igneous rocks rich in cavities.

Vectorial properties .. properties of a mineral which vary with direction.

Vein : A more or less upright sheet deposit of minerals, cutting other rocks and formed from solutions rather than from a molten magma as in a dike.

Vicarious elements .. those that take each other's place in trace amounts in a mineral,s crystal structure. Vitreous.. Glasslike in appearance or texture.

Vug : An open cavity in rocks, often lined with crystals.

Xenolith .. rock enclosed in magma.

Xrays : Electromagnetic radiations of a shorter wavelength than visible light. Usually less than one Angstrom.

Geology Glossary Special- II

This glossary includes words commonly used to describe the nature of earthquakes, how they occur, and their effects, as well as a discussion of the instruments used to record earthquake motion. Each word or phrase that is in blue in the text is explained in this glossary.


A seismograph whose output is proportional to ground acceleration (in comparison to the usual seismograph whose output is proportional to ground velocity). Accelerographs are typically used as instruments designed to record very strong ground motion useful in engineering design; seismographs commonly record off scale in these circumstances. Normally, strong motion instruments do not record unless triggered by strong ground motion.


One of many earthquakes that often occur during the days to months after some larger earthquake (mainshock) has occurred. Aftershocks occur in the same general region as the mainshock and are believed to be the result of minor readjustments of stress at places in the fault zone.


The amplitude of a seismic wave is the amount the ground moves as the wave passes by. (As an illustration, the amplitude of an ocean wave is one-half the distance between the peak and trough of the wave. The amplitude of a seismic wave can be measured from the signal recorded on a seismogram.)

Aseismic creep

Movement along a fracture in the Earth that occurs without causing earthquakes. This movement is so slow that it is not recorded by ordinary seismographs.


A term sometimes applied to the convergence of two plates in which neither plate subducts. Instead, the edges of the plates crumple and are severely deformed.


The motion of a liquid driven by gravity and temperature differences in the material. In the Earth, where pressure and temperature are high, rocks can act like viscous fluids on a time scale of millions of years. Thus, scientists believe that convection is an important process in the rocks that make up the Earth.

Convergent boundary

The boundary between two plates that approach one another. The convergence may result in subduction if one plate yields by diving deep into the Earth, obduction if one plate is thrust over the other, or collision if the plates simply ram into each other and are deformed.


The Earth's central region, believed to be composed mostly of iron. The core has a radius of 3,477 kilometers and is surrounded by the Earth's mantle. At the center of the molten outer core is a solid inner core with a radius of 1,213 kilometers.


The release of stored clastic energy caused by sudden fracture and movement of rocks inside the Earth. Part of the energy released produces seismic waves, like P, S, and surface waves, that travel outward in all directions from the point of initial rupture. These waves shake the ground as they pass by. An earthquake is felt if the shaking is strong enough to cause ground accelerations exceeding approximately 1.0 centimeter/second' (Richter, 1958).


The location on the surface of the Earth directly above the focus, or place where an earthquake originates. An earthquake caused by a fault that offsets features on the Earth's surface may have an epicenter that does not lie on the trace of that fault on the surface. This occurs if the fault plane is not vertical and the earthquake occurs below the Earth's surface.


A break in the Earth along which movement occurs. Sudden movement along a fault produces earthquakes. Slow movement produces aseismic creep.
Fault plane solution
The calculation of the orientation, dip, and slip direction of a fault that produced the ground motion recorded at seismograph stations. Sometimes called a focal mechanism solution.


The place in the Earth where rock first breaks or slips at the time of an earthquake; also called the hypocenter. The focus is a single point on the surface of a ruptured fault. During a great earthquake, which might rupture a fault for hundreds of kilometers, one could be standing on the rupturing fault, yet be hundreds of kilometers from the focus.


A measure of the severity of shaking at a particular site. It is usually estimated from descriptions of damage to buildings and terrain. The intensity is often greatest near the earthquake epicenter. Today, the Modified Mercalli Scale is commonly used to rank the intensity from I to XII according to the kind and amount of damage produced. Before 1931 earthquake intensifies were often reported using the Rossi-Forel scale (Richter, 1958).

Kilometers and other metric units of measure:

Conversion formulae:

  • Millimeters x 0.039 = inches

  • Centimeters x 0.394 = inches

  • Meters x 3.28 = feet

  • Kilometers x 0.621 = statute miles

  • Square kilometers x 0.386 = square miles

  • Cubic kilometers x 0.240 = cubic miles


A process, in which, during ground shaking, some sandy, water-saturated soils can behave like liquids rather than solids.


A quantity characteristic of the total energy released by an earthquake, as contrasted with intensity, which describes its effects at a particular place. A number of earthquake magnitude scales exist, including local (or Richter) magnitude (ML), body wave magnitude (Mb), surface wave magnitude (Ms), moment magnitude (Mw), and coda magnitude (Mc). As a general rule, an increase of one magnitude unit corresponds to ten times greater ground motion, an increase of two magnitude units corresponds to 100 times greater ground motion, and so on in a logarithmic series. Commonly, earthquakes are recorded with magnitudes from 0 to 8, although occasionally large ones (M = 9) and very small ones (M = -I or -2) are also recorded. Nearby earthquakes with magnitudes as small as 2 to 3 are frequently felt. The actual ground motion for, say, a magnitude 5 earthquake is about 0.04 millimeters at a distance of 100 kilometers from the epicenter; it is 1.1 millimeters at a distance of 10 kilometers from the epicenter.


The largest in a series of earthquakes occurring closely in time and space. The mainshock may be preceded by foreshocks or followed by aftershocks.


A rock layer, about 2,894 kilometers thick, between the Earth's crust and core. Like the crust, the upper part of the mantle is relatively brittle. Together, the upper brittle part of the mantle and the crust form tectonic plates.

Modified Mercalli Intensity Scale

A scale for measuring ground shaking at a site, and whose values range from I (not felt) to XII (extreme damage to buildings and land surfaces).


The federal National Earthquake Hazard Reduction Program, enacted in 1977, to reduce potential losses from earthquakes by funding research in earthquake prediction and hazards and to guide the implementation of earthquake loss reduction programs.
Normal Fault

A normal fault can result from vertical motion of two adjacent blocks under horizontal tension. (It also occurs in rocks under compression if stress is unequal in different directions. In this case, the minimum and maximum compressive stresses must be applied horizontally and vertically respectively.) In a normal fault, the upper of the two adjacent blocks of rock slips relatively downward.

P (Primary) waves

Also called compressional or longitudinal waves, P waves are the fastest seismic waves produced by an earthquake. They oscillate the ground back and forth along the direction of wave travel, in much the same way as sound waves, which are also compressional, move the air back and forth as the waves travel from the sound source to a sound receiver.


Pieces of crust and brittle uppermost mantle, perhaps 100 kilometers thick and hundreds or thousands of kilometers wide, that cover the Earth's surface. The plates move very slowly over, or possibly with, a viscous layer in the mantle at rates of a few centimeters per year.

Plate boundaries

The edges of plates or the junction between plates. See also plates, convergent (both collision and subduction), spreading, and transform boundaries.

Plate tectonics

A widely accepted theory that relates most of the geologic features near the Earth's surface to the movement and interaction of relatively thin rock plates. The theory predicts that most earthquakes occur when plates move past each other.

Return times

Sometimes called the recurrence time or recurrence interval. The return time, or more properly the average return time, of an earthquake is the number of years between occurrences of an earthquake of a given magnitude in a particular area. For example, if the average time of an earthquake having magnitude greater than or equal to 7 is 100 years, then, on the average, such earthquakes will occur every 100 years. If such earthquakes occur randomly in time, there is always the chance that the actual time interval between the events will be less or greater than 100 years. Return time is best described in terms of probabilities. In the case of an earthquake having a 100-year average return time, there is about an 18 percent chance that such an earthquake will occur in the next 20 years and a 63 percent chance than it will occur in the next 100 years. On the other hand, there is a 14 percent chance that it will not occur in the next 200 years.

Reverse Fault

A rupture that results from vertical motion of two adjacent blocks caused by horizontal compression. Sometimes called a thrust fault. In a reverse fault, the upper of the two adjacent blocks moves relatively upward.

Richter Magnigtude Scale

An earthquake magnitude scale, more properly called local magnitude scale, based on measurements of the amplitude of earthquake waves recorded on a standard Wood-Anderson type seismograph at a distance of less than 600 kilometers from the epicenter (Richter, 1958).

S (Secondary or shear) waves

S waves oscillate the ground perpendicular to the direction of wave travel. They travel about 1.7 times slower than P waves. Because liquids will not sustain shear stresses, S waves will not travel through liquids like water, molten rock, or the Earth's outer core.


A standing wave in a closed body of water such as a lake or bay. It can be characterized as the sloshing of water in the enclosing basin. Seiches can be produced by seismic waves from earthquakes. The permanent tilting of lake basins caused by nearby fault motions has produced very energetic seiches.

Seismic waves

A vibrational disturbance in the Earth that travels at speeds of several kilometers per second. There are three main types of seismic waves in the earth: P (fastest), S (slower), and surface waves (slowest). Seismic waves are produced by earthquakes.


A graph showing the motion of the ground versus time.


A sensitive instrument that can detect, amplify, and record ground vibrations too small to be perceived by human beings.

Site response

Local vibratory response to seismic waves. Some sites experience more or less violent shaking than others, depending on factors such as the nature and thickness of unconsolidated sediments and/or the configuration of the underlying bedrock.
Strike-slip fault

Horizontal motion of one block relative to another along a fault plane. If one stands on one side of the fault and observes that an object on the other side moves to the right during an earthquake, the fault is called a right-lateral strike-slip fault (like California's San Andreas fault). If the object moves to the left, the fault is called a left-lateral strike-slip fault.

Subduction zone boundary

The region between converging plates, one of which dives beneath the other. The Cascadia subduction zone boundary is an example.

Subduction earthquake

A thrust-type earthquake caused by slip between converging plates in a subduction zone. Such earthquakes usually occur on the shallow part of the boundary and can exceed magnitude 8.

Surface waves

Seismic waves, slower than P or S waves, that propagate along the Earth's surface rather than through the deep interior. Two principal types of surface waves, Love and Rayleigh waves, are generated during an earthquake. Rayleigh waves cause both vertical and horizontal ground motion, and Love waves cause horizontal motion only. They both produce ground shaking at the Earth's surface but very little motion deep in the Earth. Because the amplitude of surface waves diminishes less rapidly with distance than the amplitude of P or S waves, surface waves are often the most important component of ground shaking far from the earthquake source.

Transform boundary

A boundary between plates where the relative motion is horizontal. The San Andreas fault is a transform boundary between the North America plate and the Pacific plate. The Blanco fracture zone is a transform boundary between the Juan de Fuca and the Pacific plates.


A tsunami is a series of very long wavelength ocean waves caused by the sudden displacement of water by earthquakes, landslides, or submarine slumps. Ordinarily, tsunamis are produced only by earthquakes exceeding magnitude 7.5. In the open ocean, tsunami waves travel at speeds of 600-800 kilometers/hour, but their wave heights are usually only a few centimeters. As they approach shallow water near a coast, tsunami waves travel more slowly, but their wave heights may increase to many meters, and thus they can become very destructive.

World-wide Standard Seismograph Network

A network of about 110 similarly calibrated seismograph stations that are distributed throughout the world. The network was originally established in the early 1960s, and its operation is now coordinated by the U.S. Geological Survey. Each station has six seismometers that measure vertical and horizontal ground motion in two frequency ranges.


What Is a Rock?

Everyone knows what a rock is, until you ask what it is exactly. After some thought and discussion, most people will agree that rocks are more or less hard solids, of natural origin, made of minerals. But all of those criteria have exceptions.

Rocks Are Natural

Not entirely. The longer humans stay on this planet, the more that concrete accumulates. Concrete is a mixture of sand and pebbles (aggregate) and a mineral glue (cement) of calcium silicate compounds. It is a synthetic conglomerate, and it acts just like the natural rock, turning up in riverbeds and on beaches. Some of it has entered the rock cycle to be discovered by future geologists.

Brick, too, is an artificial rock—in this case, an artificial form of massive slate.

Another human product that closely resembles rock is slag, the byproduct of metal smelting. Slag is a complex mixture of oxides that has many uses, such as in road building and concrete aggregate. It too has surely found its way into sedimentary rocks already.
Rocks Are Made of Minerals

Many are not. Minerals are inorganic compounds with chemical formulas and mineral names, like quartz or pyrite (see "What Is a Mineral?"). But what about coal? Coal is made of organic material, not minerals. The various types of stuff in coal are instead called macerals. Similarly, what about coquina, a rock made entirely of seashells? Shells are made of mineral matter, but they aren't minerals any more than teeth are.

Rocks Are Hard

Not necessarily. Some common rocks can be scratched with your fingernail: shale, soapstone, gypsum rock, peat. Others may be soft in the ground, but they harden once they spend time in the air (and vice versa). And there is an imperceptible gradation between consolidated rocks and unconsolidated sediments. Indeed, geologists name and map many formations that don't consist of rock at all. This is why geologists refer to work with igneous and metamorphic rocks as "hard-rock geology," opposed to "sedimentary petrology."

Rocks Are Solid

Most are complete solid. Many rocks include water in their pore spaces. Many geodes—hollow objects found in limestone country—hold water inside them like coconuts. And the fine lava threads called Pele's hair, and the fine open meshwork of exploded lava called reticulite, are barely solids.

Then there's the matter of temperature. Mercury is a liquid metal at room temperature (and down to 40 below zero), and petroleum is a fluid unless it's asphalt erupted into cold ocean water. And good old ice meets all the criteria of rockhood too, in permafrost and in glaciers.

Rocks like these are not controversial, but they have their own category: biogenic rocks. Perhaps concrete and slag could be added to that category too. Concrete would fit in with the others, being essentially sedimentary, but slag would probably be a biogenic igneous rock.

Finally we have the exception of obsidian. Obsidian is a rock glass, cooled so quickly that none of it has gathered into crystals. It is an undifferentiated mass of geological material, rather like slag but not as colorful. While obsidian has no minerals in it per se, it is unquestionably a rock.

Types of Rocks:

• Igneous: A tough, frozen melt with little texture or layering; mostly black, white and/or gray minerals; may look like lava.

• Sedimentary: Hardened sediment with layers (strata) of sandy or clayey stone; mostly brown to gray; may have fossils and water or wind marks.

• Metamorphic: Tough rock with layers (foliation) of light and dark minerals, often curved; various colors; often glittery from mica.

Next, check the rock's grain size and hardness. Then start in the left column of the appropriate table below and work your way across. Follow the links to pictures and more information. If you don't find a match, try another of the three big types.

Grain Size:

"Coarse" grains are visible to the naked eye (greater than about 0.1 millimeter), and the minerals can usually be identified using a magnifier; "fine" grains are smaller and usually cannot be identified with a magnifier.


Hardness (as measured with the Mohs scale) actually refers to minerals rather than rocks, so a rock may be crumbly yet consist of hard minerals. But in simple terms, "hard" rock scratches glass and steel, usually signifying the minerals quartz or feldspar (Mohs hardness 6-7 and up); "soft" rock does not scratch a steel knife but scratches fingernails (Mohs 3-5.5); "very soft" rock does not scratch fingernails (Mohs 1-2). Igneous rocks are usually hard.

Origin of Igneous Rocks

"Igneous" comes from the Latin for fire, and all igneous rocks began as hot, fluid material. This material may have been lava erupted at the Earth's surface, or magma (unerupted lava) at shallow depths, or magma in deep bodies (plutons). Rock formed of lava is called extrusive, rock from shallow magma is called intrusive and rock from deep magma is called plutonic.

Igneous rocks form in three main places: where lithospheric plates pull apart at mid-ocean ridges, where plates come together at subduction zones and where continental crust is pushed together, making it thicker and allowing it to heat to melting.

People commonly think of lava and magma as a liquid, like molten metal, but geologists find that magma is usually a mush — a liquid carrying a load of mineral crystals. Magma crystallizes into a collection of minerals, and some crystallize sooner than others. Not just that, but when they crystallize, they leave the remaining liquid with a changed chemical composition. Thus a body of magma, as it cools, evolves, and as it moves through the crust, interacting with other rocks, it evolves further. This makes igneous petrology a very complex field, and this article is only the barest outline.

Igneous Rock Textures

The three types of igneous rocks apart by their texture, starting with the size of the mineral grains. Extrusive rocks cool quickly (over periods of seconds to months) and have invisible or very small grains. Intrusive rocks cool more slowly (over thousands of years) and have small to medium-sized grains. Plutonic rocks cool over millions of years, deep underground, and can have grains as large as pebbles — even a meter across. Because they solidified from a fluid state, igneous rocks tend to have a uniform texture, without layers, and the mineral grains are packed together tightly. Think of the texture of a fruitcake, or the pattern of bubbles in a piece of bread, as similar examples.

In many igneous rocks, large mineral crystals "float" in a fine-grained groundmass. The large grains are called phenocrysts, and a rock with phenocrysts is called a porphyry; that is, it has a porphyritic texture. Phenocrysts are minerals that solidified earlier than the rest of the rock, and they are important clues to the rock's history. Some extrusive rocks have distinctive textures. Obsidian, formed when lava cools very quickly, has a glassy texture. Pumice and scoria are volcanic froth, puffed up by millions of gas bubbles. Tuff is a rock made entirely of volcanic ash, fallen from the air or avalanched down a volcano's sides. And pillow lava is a lumpy formation created by extruding lava underwater.

Basalt, Granite and Other Igneous Rock Types

The main minerals in igneous rocks are hard, primary ones: feldspar, quartz, amphiboles and pyroxenes (called "dark minerals" by geologists), and olivine along with the softer mineral mica.

The two best-known igneous rock types are basalt and granite, which differ in composition. Basalt is the dark, fine-grained stuff of many lava flows and magma intrusions. Its dark minerals are rich in magnesium (Mg) and iron (Fe), hence basalt is called a mafic rock. So basalt is mafic and either extrusive or intrusive. Granite is the light, coarse-grained rock formed at depth and exposed after deep erosion. It is rich in feldspar and quartz (silica) and hence is called a felsic rock. So granite is felsic and plutonic.

These two categories cover the great majority of igneous rocks. Ordinary people, even ordinary geologists, use the names freely. (Stone dealers call any plutonic rock at all "granite.") But igneous petrologists use many more names. They generally talk about basaltic and granitic rocks among themselves and out in the field, because it takes lab work to determine an exact rock type according to the official classifications. True granite and true basalt are narrow subsets of these categories.

But a few of the less common igneous rock types can be recognized by non-specialists. For instance a dark-colored plutonic mafic rock, the deep version of basalt, is called gabbro. A light-colored intrusive or extrusive felsic rock, the shallow version of granite, is called felsite or rhyolite. And there is a suite of ultramafic rocks with even more dark minerals and even less silica than basalt.

Where Igneous Rocks Are Found

The deep sea floor (the oceanic crust) is made of basaltic rocks, with ultramafic rocks underneath. Basalts are also erupted above the Earth's great subduction zones, either in volcanic island arcs or along the edges of continents. However, continental magmas tend to be less basaltic and more granitic.

The continents are the exclusive home of granitic rocks. Nearly everywhere on the continents, no matter what rocks are on the surface, you can drill down and reach granite eventually. In general, granitic rocks are less dense than basaltic rocks, and thus the continents actually float higher than the oceanic crust on top of the ultramafic rocks of the Earth's mantle. The behavior and histories of granitic rock bodies are among geology's deepest and most intricate mysteries.


The Four Agents of Regional Metamorphism

Heat and pressure usually work together, because both rise as you go deeper in the Earth. The clay minerals of sedimentary rocks, in particular, respond to high temperatures and pressures. Clays are surface minerals, which form as feldspar and mica break down in the conditions at the Earth's surface.

With heat and pressure they slowly return to mica and feldspar. Thus the sedimentary rock shale metamorphoses first into slate, then into phyllite, then schist. The mineral quartz does not change under high temperature and pressure, although it becomes more strongly cemented as the sedimentary rock sandstone turns to quartzite. Intermediate rocks that mix sand and clay — mudstones — metamorphose into gneiss. The sedimentary rock limestone recrystallizes and becomes marble.

Fluids are the most important agent of metamorphism. Every rock contains some water, but sedimentary rocks hold the most. First there is the water that was trapped in the sediment as it became rock. Second is the water that is liberated by clay minerals as they change back to feldspar and mica. This water can become so charged with dissolved materials that the resulting fluid is no less than a liquid mineral. It may be acidic or alkaline, full of silica (forming chalcedony) or full of sulfides or carbonates or metals, in endless variety. Fluids tend to wander away from their birthplaces, interacting with rocks elsewhere. That process, the interaction of rock with chemically active fluids, is called metasomatism.

Strain refers to any change in the shape of rocks due to the force of stress. As fluids form and move in buried rocks, new minerals grow with their grains oriented according to the direction of pressure. Where the strain makes the rock stretch (shear strain), these minerals form layers. In most metamorphic rocks the layers are made of mica. The presence of mineral layers is called foliation and is important to observe when identifying a metamorphic rock. As strain increases, the foliation becomes more intense, and the mineral sort themselves into thicker layers. That's what gives schist and gneiss their foliation.

Metamorphism can be so intense, with all four factors acting at their extreme range, that the foliation can be warped and stirred like taffy, and the result is called migmatite. With further metamorphism, rocks can be turned into something hard to tell from plutonic granites. These kinds of rocks give joy to experts because of what they say about deep-seated conditions during things like plate collisions. The rest of us can only admire the laboratory skills needed to make sense of such rocks.
What I've described is how regional metamorphism affects sedimentary rocks. Igneous rocks give rise to a different set of minerals and metamorphic rock types; these include serpentinite, blueschist, greenschist and other rarer species such as eclogite. If you're a mineral collector it's worth your while to learn about these, but they aren't found in most parts of the world.

Contact or Local Metamorphism

A lesser type of metamorphism, important in specific localities, is contact metamorphism. This usually occurs near igneous intrusions, where hot magma forces itself into sedimentary strata. The rocks next to the lava invasion are baked into hornfels, another subject for specialists. Lava can rip chunks of country rock off the channel wall and turn them into exotic minerals, too.

Underground coal fires can also cause mild contact metamorphism of the same degree as occurs when baking bricks.


Geologists know about thousands of minerals locked in rocks, but when rocks are exposed at the surface and weather away, less than 10 minerals remain. They are the ingredients of sediment, which in turn becomes sedimentary rock. When the mountains crumble to the sea, all of their rocks, whether igneous, sedimentary or metamorphic, break down. Physical or mechanical weathering reduces the rocks to small particles. These break down further by chemical weathering in water and oxygen. A very small number of minerals can resist indefinitely: zircon is one and native gold is another. Quartz resists for a very long time, which is why sand, being nearly pure quartz, is so persistent, but given enough time even quartz dissolves into silicic acid, H4SiO4.

But most of the silicate minerals produce solid residues after chemical weathering. Silicate residues are what make up the minerals of the Earth's land surface.

The olivine, pyroxenes and amphiboles of igneous or metamorphic rocks react with water and leave behind rusty iron hydroxides. These are an important ingredient in soils but uncommon as solid minerals. They also add brown and red colors to sedimentary rocks.

Feldspar, the most common silicate mineral group and the main home of aluminum in minerals, reacts with water too. Water pulls out silicon and other major cations (positive ions) except for aluminum. The feldspar minerals thus turn into hydrated aluminosilicates—that is, clays.

Amazing Clays

Clay minerals are not much to look at, but life on Earth depends on them. At the microscopic level, clays are tiny flakes, like mica but infinitely smaller. At the molecular level, clay is a sandwich made of sheets of silica (SiO4) tetrahedra and sheets of magnesium or aluminum hydroxide (Mg(OH)2 and Al(OH)3). Some clays are a proper three-layer sandwich, a Mg/Al layer between two silica layers, while others are open-face sandwiches of two layers.

What makes clays so valuable for life is that with their tiny particle size and open-faced construction, they have very large surface areas and can readily accept many substitute cations for their Si, Al and Mg atoms. Oxygen and hydrogen are available in abundance. From the viewpoint of microbes, clay minerals are like machine shops full of tools and power hookups. Indeed, even the building blocks of life—amino acids and other organic molecules—are enlivened by the energetic, catalytic environment of clays.

The Makings of Clastic Rocks

But back to sediments. With quartz and clay, the overwhelming majority of surface minerals, we have the ingredients of mud. Mud is what geologists call a sediment that is a mixture of particle sizes ranging from sand (visible) to clay (invisible), and the world's rivers steadily deliver mud to the sea and to large lakes and inland basins. That is where the clastic sedimentary rocks are born, sandstone and mudstone and shale in all their variety.

The Chemical Precipitates

When the mountains were crumbling, much of their mineral content dissolved. This material reenters the rock cycle in other ways than clay, precipitating out of solution to form other surface minerals.

Calcium is an important cation in igneous rock minerals, but it plays little part in the clay cycle. Instead calcium remains in water, where it affiliates with carbonate ion (CO3). When it becomes concentrated enough in seawater, calcium carbonate comes out of solution as calcite. Living organisms can extract it to build their calcite shells, which also become sediment.

Where sulfur is abundant, calcium combines with it as the mineral gypsum. In other settings, sulfur captures dissolved iron and precipitates as pyrite.

There is also sodium left over from the breakdown of the silicate minerals. That lingers in the sea until circumstances dry up the brine to a high concentration, when sodium joins chloride to yield solid salt, or halite. And what of the dissolved silicic acid? That precipitates underground, from deeply buried fluids, as the silica mineral chalcedony. Thus every part of the mountains finds a new place in the Earth.

Minerals, Gemstones & Mineral Resources

What Is a Mineral?

A mineral is any substance with all of four specific qualities.

1. Minerals Are Natural: substances that form without any human help.
2. Minerals Are Solid: substances that don't droop or melt or evaporate.
3. Minerals Are Inorganic: substances that aren't carbon compounds like those found in living things.
4. Minerals Are Crystalline: substances that have a distinct recipe and arrangement of atoms.

Unnatural Minerals

Until the 1990s, mineralogists could propose names for chemical compounds that formed during the breakdown of artificial substances, things found in places like industrial sludge pits and rusting cars (although iron rust is the same as the natural minerals hematite, magnetite and goethite). That loophole is now closed, but there are minerals on the books that aren't truly natural.

Soft Minerals

Traditionally, native mercury is considered a mineral, even though the metal is liquid at room temperature. At about 40 degrees below zero, mercury solidifies and forms crystals like other metals. So there are parts of Antarctica where mercury is unimpeachably a mineral.

For a less extreme example, consider the mineral ikaite, a hydrated calcium carbonate that forms only in cold water. It degrades into calcite and water above 8 degrees Celsius. It is significant in the polar regions, the ocean floor and other cold places, but you can't bring it into the lab except in a freezer.

Ice is a mineral, even though it isn't listed in the mineral field guide. But when ice collects in large enough bodies, it flows in its solid state€”that's what glaciers are. And salt (halite) behaves similarly, rising underground in broad domes and sometimes spilling out in salt glaciers. Indeed, all minerals, and the rocks they are part of, slowly deform given enough heat and pressure. That's what makes plate tectonics possible. So in a sense, no mineral is really solid except maybe diamond.

Other minerals that aren't quite solid are instead flexible. The mica minerals are the best-known example, but molybdenite is another. Its metallic flakes can be crumpled like aluminum foil. And of course the asbestos mineral chrysotile is stringy enough to weave into cloth.

Organic Minerals

The rule that minerals must be inorganic may be the strictest one. The substances that make up coal, for instance, are different kinds of hydrocarbon compounds derived from cell walls, wood, pollen and so on. These are called macerals instead of minerals (for more see Coal in a Nutshell). But if coal is squeezed hard enough for long enough, the carbon sheds all its other elements and becomes graphite. Even though it is of organic origin, graphite is a true mineral, carbon atoms arranged in sheets. Diamond, similarly, is carbon atoms arranged in a rigid framework. After some 4 billion years of life on Earth, it's safe to say that all the world's diamonds and graphite are of organic origin even if they aren't strictly speaking organic.

Amorphous Minerals

A few things fall short in crystallinity, hard as we try. Many minerals form crystals that are too small to see under the microscope. But even these can be shown to be crystalline at the nano-scale using the technique of X-ray powder diffraction, though, because X-rays are a super-short-wave type of light that can image extremely small things.

Having a crystal form means that the substance has a definite recipe, or chemical formula. It might be as simple as halite's (NaCl) or complex like, say, epidote (Ca2Al2(Fe3+,Al)(SiO4)(Si2O7)O(OH)), but if you were shrunk to an atom's size, you could tell what mineral you were seeing by its molecular makeup and arrangement.

But a few substances fail the X-ray test. They are truly glasses or colloids, with a fully random structure at the atomic scale. They are amorphous, scientific Latin for "formless." These get the honorary name mineraloid.

Mineraloids are a small club: strictly speaking it includes only opal and lechatelierite. Opal is a nearly random combination of silica (SiO2, the same as quartz) and water formed under near-surface conditions, while lechatelierite is a quartz glass formed by the shock of a meteorite impact or lightning striking the ground.

Other substances considered mineraloids include the gemstones jet and amber, which are respectively high-quality fossils of coal and tree resin. Pearl goes here too, although I disagree because by that logic, seashells should be included. The last mineraloid is rather like the rusty car I mentioned earlier: limonite is a mixture of iron oxides that, while it may assume the shape of a proper iron-oxide mineral, has no structure or order whatever.
10 Steps to Mineral Identification
The first thing to do is to observe and test your mineral. Use the largest piece you can find, and if you have several pieces, make sure sure that they are all the same mineral.

Examine your mineral for all of the following properties, writing down the answers. After that you'll be ready to take your information to the right place.

Step 1: Pick Your Mineral

Step 2: Luster

Luster is the way a mineral reflects light and the first key step in mineral identification. Look for luster on a fresh surface. The three major types of luster are metallic, glassy (vitreous) and dull. A luster between metallic and glassy is called adamantine, and a luster between glassy and dull is called resinous or waxy.

Step 3: Hardness

Use the 10-point Mohs hardness scale. The important hardnesses are between 2 and 7. For this you'll need your fingernail (hardness about 2), a coin (hardness 3), a knife or nail (hardness 5.5) and a few key minerals.

Step 4: Color

Color is important in mineral identification, but it can be a complicated subject. Experts use color all the time because they have learned the usual colors and the usual exceptions for common minerals. If you're a beginner, pay close attention to color but do not rely on it. First of all, be sure you aren't looking at a weathered or tarnished surface, and examine your specimen in good light.

Color is a fairly reliable indicator in the opaque and metallic minerals—for instance the blue of the opaque mineral lazurite or the brass-yellow of the metallic mineral pyrite.

In the translucent or transparent minerals, color is usually the result of a chemical impurity and should not be the only thing you use.

For instance, pure quartz is clear or white, but quartz can have many other colors.

Try to be precise with color. Is it a pale or deep shade? Does it resemble the color of another common object, like bricks or blueberries? Is it even or mottled? Is there one pure color or a range of shades?
10 Steps to Mineral Identification
The first thing to do is to observe and test your mineral. Use the largest piece you can find, and if you have several pieces, make sure sure that they are all the same mineral.

Examine your mineral for all of the following properties, writing down the answers. After that you'll be ready to take your information to the right place.

Step 1: Pick Your Mineral

Step 2: Luster

Luster is the way a mineral reflects light and the first key step in mineral identification. Look for luster on a fresh surface. The three major types of luster are metallic, glassy (vitreous) and dull. A luster between metallic and glassy is called adamantine, and a luster between glassy and dull is called resinous or waxy.

Step 3: Hardness

Use the 10-point Mhos hardness scale. The important hardness is between 2 and 7. For this you'll need your fingernail (hardness about 2), a coin (hardness 3), a knife or nail (hardness 5.5) and a few key minerals.

Step 4: Color

Color is important in mineral identification, but it can be a complicated subject. Experts use color all the time because they have learned the usual colors and the usual exceptions for common minerals. If you're a beginner, pay close attention to color but do not rely on it. First of all, be sure you aren't looking at a weathered or tarnished surface, and examine your specimen in good light.

Color is a fairly reliable indicator in the opaque and metallic minerals—for instance the blue of the opaque mineral lazurite or the brass-yellow of the metallic mineral pyrite.

In the translucent or transparent minerals, color is usually the result of a chemical impurity and should not be the only thing you use.

For instance, pure quartz is clear or white, but quartz can have many other colors.

Try to be precise with color. Is it a pale or deep shade? Does it resemble the color of another common object, like bricks or blueberries? Is it even or mottled? Is there one pure color or a range of shades?

Step 8: Magnetism

Magnetism is a distinctive property in a few minerals. Magnetite is the prime example, but a few other minerals may be weakly attracted by a magnet, notably chromite (a black oxide) and pyrrhotite (a bronze sulfide). Use a strong magnet. The magnets I use came from the corners of an old plastic shower curtain. Another way to test magnetism is to see if the specimen attracts a compass needle.

Step 9: Other Mineral Properties

Taste is definitive for halite (rock salt), of course, but a few other evaporite minerals also have distinctive tastes. Just touch your tongue to a fresh face of the mineral and be ready to spit—after all it's called taste, not flavor. Don't worry about taste if you don't live in an area with these minerals.

Fizz means the effervescent reaction of certain carbonate minerals to the acid test. For this test, vinegar will do. Heft is how heavy a mineral feels in the hand, an informal sense of density. Most rocks are about three times as dense as water, that is, they have a specific gravity of about 3. Make note of a mineral that is noticeably light or heavy for its size.

Step 10: Look It Up

Now you are ready for mineral identification. Once you have observed and noted these mineral properties, you can take your information to a book or to an online resource. Start with my table of the rock-forming minerals, because these are the most common and the ones you should learn first. Each mineral's name is linked to a good photograph and notes to help you confirm the identification.

If your mineral has metallic luster, go to my Minerals with Metallic Luster gallery to see the most likely minerals in this group. If your mineral is not one of these, try the sources in the Mineral Identification Guides category.


Gemstones and Precious Stones

Gemstones are the sexy minerals. If minerals are like different sorts of people, gemstones are the supermodels. If mineralogists are like zookeepers, who collect and classify all the different animals, gemologists are like butchers, who focus on the edible ones. Where the mineralogist asks "What variety of cow is this?" the gemologist asks "Where's the beef?"

Gemstone Fanciers versus Mineral Collectors

Just as beeves and cows are different names for the same thing, many gemstones have names that differ from their proper mineral names. Olivine is an important rock-forming mineral, for example, but as a gemstone it's called peridot. To keep the two sets of names straight, use the Gemstones to Minerals tables.

There are two ways of appreciating the mineral kingdom.

The collector of minerals loves their natural crystal form, chemical variety, fluorescence, rarity—the personality of minerals in themselves. If you're a mineral-collecting kind of person, you might find a place like www.emeralds.com appealing, which sells only uncut emerald crystals.

The fancier of gemstones is in love with what makes minerals sexy: their purity, color, size, optical effects and value—in a word, their beauty. The rest of this article is for fanciers.

Of course these categories overlap. That's why I have a big Gemstones category that gives you a peek over the fence from the mineral collector's side.

Digging Gemstones

I suppose there could be a third reaction to browsing all these jewels—"Where can I dig up my own?" There are gemstone mines all over the place. One place they're concentrated in is the Franklin district of North Carolina. One of Carly Wickell's favorites is the Sheffield Ruby Mine. But most mines are generally enriched—the old-fashioned term is salted—with extra stones. If you don't mind that, or if you're taking children with you, then these places are fine. To do better, join the rockhounds near you and follow them around.

The ultimate gemstone fanatic dreams of opening a mine. People have found valuable things in their own yards, after all. You might not have to move to Franklin. For a real-life example, read about Scott Klein's fresnoite mine deep in the California Coast Range.


There are three main minerals that form carbonates:

  • Calcite (CaCO3), which comes in high magnesium and low magnesium forms.

  • Aragonite (CaCO3), which has a different structure to calcite.

  • Dolomite (CaMg(CO3)2), a magnesium rich carbonate produced by diagenesis.

Only low magnesium calcite is stable at surface pressure and temperatures. It is therefore the most common mineral in ancient carbonates. However, most modern carbonates are composed of aragonite as this is the mineral that most biological organisms create to make their shells or skeletons. Examples of organisms that produce aragonite shells are bivalves (sea shells), gastropods (snails) and Halimeda (a green algae). Organisms that produce a calcite shell include brachipods (a rare type of sea shell) and ostrocods (a small crustacean).

Components :
Carbonates can be made of several components.

These are:

(1) Bioclasts.
(2) Ooids.
(3) Peloids.
(4) Intraclasts.
(5) Micrite.
(6) Sparite.


Bioclasts are fragments of dead sea creatures. These include shells and corals. The creatures precipitate the carbonate in order to produce some kind of structure.

Ooids are rounded grains formed by precipitation of calcite around a nucleus to produce concentric circles (Figure 3). They form in warm, shallow waters, with a strong tidal currents. Wave action may also contribute to their near-spherical shape.


Peloids are sand sized grains (100-150 micrometers) of micro-crystalline carbonate. They are generally rounded or sub-rounded. They originate from fecal pellets, algae and mud clasts. They are sometimes found clumped together, in a formation known as a grapestone.


Intraclasts are clast of other limestone that appear in younger limestones. They can be quite difficult to distinguish at times, as they may be made of a similar rock as that which encases it. For example, hardgrounds can from when sea water flows through carbonate sediment, lithifying it rapidly. Subsequently, the hardground may be broken up and incorporated into the surrounding sediment.


Micrite is microcystalline carbonate mud, with grains less than 4 micrometers.


Sparite is coarser than micrite, with a grain size of more than 4 micrometers and is crystalline. Both micrite and sparite form the matrix or cement in carbonate rocks.


Sedimentary rocks are made by the accumulation of particles of older rocks, either as clasts (chunks of rocks) or as mineral grains, chemically or biogenically precipitated. Clastic sedimentary rocks are principally classified on the basis of grain size and then further divided in terms of mineralogy. One of the most important things sedimentary rocks can tell us about is palaeoenvironments - ancient environments. This is done by looking at the sedimentary structures and the fossils contained within the rocks. They are also an important resource for oil, gas and coal. This article concentrates on clastic sedimentary rocks. The carbonate tutorial for more information on a chemically precipitated sedimentary rocks.


Classification of sedimentary rocks is based principally on grain size. Grain size is measured in millimetres and is the approximate diameter of a single grain. There are several aids for estimating grain size in the field as well as more sophisticated aids when using thin secitons of sedimentary rocks under a microscope. The table below gives the grain sizes and names of the common sedimentary rocks.

Classification of clastic sedimentary rocks based on grain size.
Diameter (mm) -Sediment Name -Rock Type
>256 -Boulder -Rudaceous -Conglomerate or Breccia
Between 256 and 64- Cobble
Between 2 and 64 -Pebble
Between 2 and 0.625 -Sand- Arenaceous Sandstone
Between 0.625 and 0.0039 -Silt- Argillaceous siltstone
<0.0039 -Clay- Claystone or mudstone

NB: A conglomerate has rounded clasts, a breccia has angular clasts.


  • Textures in sedimentary rocks depend on the type of grains making up the rock.

  • Roundness - the degree of rounding of a grain. Not to be confused with sphericity. Grains can be angular to well rounded. A well rounded grain has generally traveled further before deposition.

  • Sphericity - degree to which grain is a perfect sphere. Does NOT depend on roundness.

  • Sorting - the amount of different sized grains in a rock. Ranges from very poor to well sorted.

  • Matrix or cement - the finer grains in a rock (matrix)or a chemical precipitate (cement) holding the rock together. Common cements are calcite or quartz.

  • Competence - the "toughness" of a rock.

Other properties of a sedimentary rock are porosity and permeability. The ability to store fluid (e.g. oil, gas or water) is the porosity. The porosity is expressed as a percentage and depends on the amount of pore space in the rock. The ability to allow a fluid to pass through a rock is the permeability. Fluid can pass through using cracks, fissures or space between grains. A high porosity rock can have a low permeability if the pore space does not connect in three dimensions

The structures in a rock tell us a great deal about the palaeoenvironment. This is where one of the great sayings in geology comes in use:

"The present is the key to the past" - the law of uniformatarianism

This essentially means if we can understand what processes occur today, for example, the forming of ripples in a tidal mud, then these principles can be applied to the geological record. Below are some examples of sedimentary structures and what formed them.

A way-up structure tells us which way up the bed was originally deposited. Graded bedding usually occurs with the coarse grains at the bottom. If you find some graded bedding with coarse grains at the top, then the bed has probably been tectonically turned upside down (e.g. by folding).

Cross Bedding (or stratification). The entire dune as around a metre in height. These cross beds were formed in a shallow fluvial environment, which can determined using the relatively poor sorting of the sandstone.

Mudcracks formed from the drying out of mud and then preserved in the rock. The scale on the left shows centimetres and inches.

Ooid Formation

Introduction : Ooids are spherical or ellipsoid concretions of calcium carbonate, usually less than 2mm in diameter (Donahue, 1969; Tucker and Wright, 1990). There have been examples in the Neoprotozoic of ooids that are 16mm in diameter (Sumner, 1993), but all modern ooids are 2mm or less.

The interior of an ooid is usually composed of a nucleus, which is surrounded by a cortex of calcite or aragonite crystals that are arranged radially, tangentially or randomly. These crystals are arranged in concentric lamina. The nucleus can be a shell fragment, quartz grain or any other small fragment (including an aragonite/calcite amalgamation).

The formation of these objects has been speculated from the early 19th Century and ideas for their origin range from crinoid eggs, insect eggs to the present day explanation of precipitated layers of CaCO3 (Simone, 1981).

Recent ooids are forming today in places such as the Bahamas (Tucker and Wright, 1990; Newell et al., 1966) and Shark Bay, Australia and are all composed of aragonite.

Life Cycle

Ooids do not form continuously; instead they go through stages of growth and rest (Davis et al., 1978). Davies et al. (1978) describe the typical life cycle of a Bahamian ooid:

1. Suspension Growth Phase Nuclei introduced into a suitable location, with enough turbulence to keep them in suspension and water that is supersaturated in CaCO3, will induce a short lived inorganic precipitation of calcium carbonate on their surfaces. The precipitation is stopped by crystal poisoning, which is the addition of Mg2+ or H+ onto the surface. If the proto-ooids remain in this environment the outer coating will be lost due to attrition. This means the suspension phase is short lived, but may be repeated several times.

2. Temporary Resting Phase Coated nuclei resting in the marine environment will quickly equilibrate with the surrounding fluid. Removal of the 'poisonous' ions will reactivate the coated surface in such conditions. However, not all 'poisonous' ions are removed, so after several growth and temporary resting stages have been completed a third stage is required.

3. Sleeping Stage A new surface is required in order to form a new coating. This membrane is probably organic in origin. Experiments show that this takes 1-3 weeks to form. The membrane forms a new, stable substratum for new CaCO3 precipitation.

The timing of these stages means that an ooid spends only 5% of its time actually growing; the rest is spent 'sleeping' (Davis et al., 1978; Bathurst, 1967).


As can be seen from the life cycle, the following factors will have an affect ooid growth: (Monoghan and Lytle, 1956; Newell, 1960; Bathurst, 1967; Davis et al., 1978; Deelman, 1978; Heller, 1980; Simone, 1981; Sumner and Grotzinger, 1993):

1. Supersaturation of CaCO3
2. Nuclei
3. Agitation
4. Location
5. Water depth


The supersaturation of the seawater is of vital importance (Monoghan and Lytle, 1956). Monoghan and Lytle (1956) investigated the effect of CO3 concentration on the formation of ooids. They found that the concentration needed to be above 0.002 moles/litre and below 0.0167 moles/litre for ooids to form successfully. Below 0.002 moles/litre only aragonite needles or poor ooids formed. Above 0.0167 moles/litre the ooids formed an amorphous mass. Other authors have stressed the importance of supersaturation, but they give no quantitative information (Bathurst, 1967; Davies et al., 1978; Simone, 1981).


The type of nuclei affects the rate of growth and the size of each lamination (Davies et al., 1978). Organic coating on the nuclei give faster and longer precipitation, while using oxidised quartz show much slower and shorter precipitation. Davies et al. (1978) show their results as a change of pH (a negative pH change is assumed to indicate precipitation), rather than growth or precipitation rates.

The agitation an ooid undergoes must be enough to keep it in suspension for the growing phase followed by removal to a non-supersaturated fluid (the rest phase) (Newell, 1960; Davies et al., 1978; Heller, 1980).

Davies et al. (1978) conducted a study using two different speeds of water current to test this: 5cm/s and 10 cm/s. The ooids were kept in suspension by this water flow, and in other experiments involving horizontal shaking and tumbling motion formed, the ooids were non-existent or more like those formed in non-agitated water in the presence of organic compounds. In all cases of different nuclei the larger water current increase precipitation rates, but the time that precipitation changed depending on the nuclei type.

Agitation may also control ooid size (Sumner and Grotzinger, 1993). As the ooid grows the mass lost per impact with another object increases as the cube of the radius. The mass gained from growth is proportional to the square of the radius. Eventually, the mass loss will equal or exceed the mass gained, limiting the size of the ooid. Sumner and Grotzinger (1993) performed numerical modelling on ooid formation.

Their model gave a higher ooid radius in higher velocity flows, with a decrease that looks like an exponential or a power law with decreasing velocity (Sumner and Grotzinger, 1993, their fig 6). They did not include the impact of ooids to limit size.

The agitation can come from waves or tidal movements. Storms provide that mixing of ooids in the rest stage and those that can no longer precipitate.

There is some change in crystal orientation with the amount of agitation (Donahue, 1969). Ooids can form in quiet waters, but organic CaCO3 precipitation is needed for them to form (Suess and Fütterer, 1972). These ooids will show radial crystals. Ooids formed in agitated waters have crystals arranged tangentially. The change between suspension to bedload transport may also initiate this change (Deelman, 1978).

The location off ooid formation is important. They must be kept in the same area throughout the formation, in order that their life cycle can be completed (Simone, 1981).

Water Depth

Most ooids form in water less than 2m deep (Simone, 1981), but this may have more to do with wave agitation and tidal movements than water depth itself. Newell et al. (1960) surveyed sediment at various depths and calculated the % fraction of ooids in the sediment. All the sediments that are near 100% ooids are formed with 8m of the surface.

Diatoms as Palaeo-Environment Indicators

Introduction: Diatoms are microscopic, photosynthetic algae (which due to the yellow-brown chloroplasts they contain are sometimes referred to as golden algae). Comprising one of the most common types of phytoplankton, they are found in a diverse range of environments from freshwater to saline oceanic waters. It is estimated that 20-25% of all the organic carbon fixation on Earth; via photosynthesis, is attributable to diatoms - in large due to their great abundance.


  • Photosynthetic, unicellular algae containing pigments, but possessing no flagella or pseudopodia. Also capable of absorbing nutrients in addition to producing them.

  • Range in size from ¼mm to 2mm, but are generally ~40¼mm.

  • Secrete a frustule or test, composed of silica, which under favourable conditions can be preserved. Each frustule consists of two valves, which fit closely over the top of each other - somewhat analogous to a petri-dish.

  • The valve surface is often, but not always, ornamented with any combination of pits, pores, or striations (rib-like structures).

  • Always inhabit the photic zone. For this reason, benthic forms are never present on the floor of very deep lakes, for example Loch Ness.

  • Reproduce primarily via asexual cell division.


Diatoms are differentiated between by forms that are centric, i.e. circular, and pennate, i.e. having bilateral form. The word pennate usually pertains to feathers, wings, or feather-like structures however; its use with diatoms denotes bilateral form.

In addition, diatoms can be divided into solitary and colonial forms. Diatoms can be further sub-divided according to whether they possess a raphe (a median line or slot in the cell wall), a pseudoraphe, or completely lack a raphe.

As previously mentioned, diatoms are very abundant, largely existing wherever there is water. The study of extant diatom species, and particularly their ecologies, can provide useful information for the interpretation of palaeoenvironmental conditions.

Diatoms exhibit three major modes of existence:

  • Planktonic

  • Benthic (Lake/Sea/Ocean bed)

  • Macrophytic (Attached to plants)

Planktonic forms contain oil globules, which help to keep the diatom afloat in the water column. As a result, it is often easier to identify dead diatoms, in which the internal oil globules and chloroplasts have decayed away to reveal the valve ornamentation, than it is to identify living diatoms to species level.

All diatom species are highly sensitive to environmental changes, giving rise to very different assemblages under rather tight environmental constraints. For example, diatoms display varying assemblages according to pH, trophic status, and pollution levels.

Diatoms bloom seasonally, with different species blooming at different times of the year.


Where conditions are conducive, diatom remains will usually accumulate on lake/sea beds, and will often exhibit mixed assemblages, (i.e. consist of both benthic and planktonic forms, in addition to those brought in from tributary river/stream systems, and from soil in-wash). The best preservation conditions in terms of diatoms are those with any mixture of fine grained, anaerobic, and slightly acidic sediments.

Sampling is most frequently carried out by random core samples of a given area, as this preserves changes in the diatom assemblages over time. Where cores are sampled from beneath existing lakes, care should be taken to disturb the sediment-water interface as little as possible. Often a rich organic mud called gyttja, (typical of interglacial periods) will have accumulated, consisting of mainly faecal debris, animal and plant remains, along with some clastic component, (sand/silt/clay). will retain a record of the most recent diatom activity.

Uses Of Diatoms

In general diatoms can be used to trace a variety of environmental phenomena, from changes in sea level, (whether brought about by climate change or tectonic activity), breaches of coastal barriers, (as a result of storms and/or sea-level rise), to the evaporation of lakes, (increasing salinity determining diatom assemblages). Below is an outline of their most prevalent uses.

Indicators of Salinity

1. Marine

Some species are restricted to a very narrow range of salinities and are know as stenohaline species, others have no such restrictions and are known as cosmopolitan species. As a result, this causes zonation, which is particularly evident in estuaries, where a spectrum (and a gradient for such a spectrum) can be calculated from coastal to offshore species. This has applications in determining palaeo-fluvial environments, and sediment focusing.

2. Freshwater.

Some freshwater species will tolerate a little salt, and are known as halophilic, occurring in coastal lakes, or where the groundwater is rich in salts. However most freshwater species are stenohaline and will not tolerate salt.
Indicators of Productivity (Trophic Status)

There are several ways of deducing palaeotrophic status using diatoms:

1. Total Diatom Count - This is relatively simple, the more diatoms there are in your sample, the more productive a given body of water is.

2. Centric:Pennate Ratio - The more centrics there are in your sample, the more productive the environment is. (With the exception of a species called Cyclotella.)

3. Indicator Species - Certain species are typical of certain conditions, for example Stephanodiscus is typical of eutrophic (abundant nutrient) conditions, and Tabellaria of oligotrophic (very low nutrient) conditions.

4. Planktonic:Non-planktonic Ratio - Planktonic forms are more common in eutrophic lakes.

5. Diversity Indicators - A low overall diversity amongst diatoms indicates stressful conditions, for example extreme trophic status (hyper-oligotrophic or hyper-eutrophic). However this could also indicate a source of pollution etc.

Indicators of Palaeo-pH
This perhaps the most important and most widely used application of diatom studies.

Diatoms are highly sensitive to pH and can illustrate differences of as little as 0.1 pH units. To accomplish this species are classified as either:

  • Acidobiontic (Acid Living) pH < 7

  • Acidophilous (Acid Preferring) pH ‰ 7

  • Circumneutral pH = 7

  • Alkaliphilous (Alkali Preferring) pH ‰ 7

  • Alkalibiontic(Alkali Living) pH > 7

This method is highly dependent upon knowing the pH preference for all of the diatoms present, as the percentage of each of the above groups is measured and the ratios used to calculate a log index of the given population. With the use of some complicated mathematics this, in turn, can then be used to determine the palaeo-pH. Obviously, it is not always possible to know the preference of all of the species in your sample, and therefore this method can not always be applied.

Indicators of Palaeo-pH
This perhaps the most important and most widely used application of diatom studies.

  • Diatoms are highly sensitive to pH and can illustrate differences of as little as 0.1 pH units. To accomplish this species are classified as either:

  • Acidobiontic (Acid Living) pH < 7

  • Acidophilous (Acid Preferring) pH ‰ 7

  • Circumneutral pH = 7

  • Alkaliphilous (Alkali Preferring) pH ‰ 7

  • Alkalibiontic(Alkali Living) pH > 7

This method is highly dependant upon knowing the pH preference for all of the diatoms present, as the percentage of each of the above groups is measured and the ratios used to calculate a log index of the given population. With the use of some complicated mathematics this, in turn, can then be used to determine the palaeo-pH. Obviously, it is not always possible to know the preference of all of the species in your sample, and therefore this method can not always be applied.

Indicators of Palaeo-temperature

Diatoms are not very useful in determining changes in palaeo-temperature, due to the fact that the large majority of species will tolerate very wide ranges of temperature, typically from 0oC to 20oC.

That said, different assemblages are present when comparing warm and cold waters. However, this is almost certainly due to other overriding factors such as: incident solar radiation, water chemistry, pH, and nutrient availability.
Difficulties in Interpreting Diatom Samples

1. Not all diatoms present in a body of water may settle out, they can be lost via outflows, dissolve, be crushed or eaten. In the best case scenario your assemblage is simply incomplete, or comparatively low in overall abundance. In the worst case scenario the ratios of different diatoms may be completely skewed, (for example planktonic forms with their oil-filled globules may be more prone to out-washing).

2. Samples may contain diatoms washed in from outside your sample area, from soils, animal droppings, or tributaries. The sample becomes augmented, and in the worst case scenario may include indicator species contrary to the actual palaeo-environmental conditions.

3. There may be insufficient silica dissolved in the body of water for diatoms to produce robust, preservable frustules, resulting in a complete absence in your sample.

4. Taxonomy, especially in poorly preserved specimens, can often be difficult resulting in mis-identification and a chain of consequent errors.

5. The ecology is not well known for all species, causing problems and/or errors with interpretation.

Evolution of Birds

Introduction : Birds are phylogenetically considered to be members of the theropod dinosaurs; their closest non-avian relatives are the dromaeosaurid theropods. The first known fossil bird is Archaeopteryx, from the late Jurassic of Bavaria, Germany, which is represented by seven skeletons and a feather. There is no fossil evidence from before this time that has been proven to be of avian origin. The fossil record of modern birds began in the early Tertiary (Padian et al., 1998).

There are a lot of anatomical terms used to describe the evolution of birds, therefore diagrams showing the skeletons of a theropod dinosaur, Archaeopteryx and a modern bird are shown in figure one, in order to define most of the terms used.

The Thecodont Hypothesis

The thecodont hypothesis for the origin of birds is characterised by being a default option. This is because it is not due to positive correlation of characters and taxa but due to the negative association with other taxa. It was originally thought that theropods shared more features with birds than any other group. However, at the time there was no evidence for clavicles in theropods, which are the equivalent of the furcula or wishbone in birds. It was thought that clavicles could not have been lost and then re-evolved into furcula and so a more ancient ancestor for birds was sought for.

The candidate suggested was the thecodonts from which all other archosaurs are thought to have evolved. A problem is that the archosaurs are a "wastebasket" group containing all archosaurian reptiles that do not fit into dinosaurs, crocodiles or pterosaurs. Hence, they have no diagnostic characters of their own and are not a "good" phylogenetic group. This makes it difficult to compare them with other taxonomic groups (Padian et al., 1998).
The thecodont hypothesis for the origin of birds is characterised by being a default option. This is because it is not due to positive correlation of characters and taxa but due to the negative association with other taxa. It was originally thought that theropods shared more features with birds than any other group. However, at the time there was no evidence for clavicles in theropods, which are the equivalent of the furcula or wishbone in birds. It was thought that clavicles could not have been lost and then re-evolved into furcula and so a more ancient ancestor for birds was sought for. The candidate suggested was the thecodonts from which all other archosaurs are thought to have evolved. A problem is that the archosaurs are a "wastebasket" group containing all archosaurian reptiles that do not fit into dinosaurs, crocodiles or pterosaurs. Hence, they have no diagnostic characters of their own and are not a "good" phylogenetic group. This makes it difficult to compare them with other taxonomic groups (Padian et al., 1998).

The Crocodylomorph Hypothesis

Crocodylomorphs include crocodiles and some Triassic-Jurassic forms that are closely related but not true crocodiles. This hypothesis has fewer problems than the thecodont hypothesis, as crocodiles are a monophyletic group, which can be compared to other taxa. The synapomorphies of birds and crocodiles have been tested, but it has been found that most are general to the archosaurs. Even if these are accepted, there are only 15-20 synapomorphies compared to over 70 with the theropods (Padian et al., 1998).

The Theropod Hypothesis

Cladistic analysis supports this hypothesis and shows that the most closely related group (sister group) of theropods to the birds includes the dromaesaurids such as Deinonychus. There are numerous synapomorphies of the skeleton and skull that link birds and theropods. There have been found a series of skeletal changes in theropods that are considered to be avian. For example, basal theropod dinosaurs have lightly built bones and a foot reduced to three main toes, with the first held off the ground and the fifth lost. Moving through the theropod sequence towards birds, there is a reduction and loss of manual digits four and five, increasing lightness of the skeleton, and a reduction in the number, and partial interlocking, of the tail vertebrae. In coelurosaurs, which include birds, dromaeosaurids and other groups, the arms become longer and the first toe begins to rotate backwards behind the metatarsals. Fused clavicles (furcula) are apparently basal to Tetanurae (carnosaurs and coelurosaurs) and sternal plates are known in a variety of tetanurans. In the pelvis, the pubis and ischium begin to show a greater disparity in length. Finally, in the dromaeosaurids and Archaeopteryx, the pubis begins to point backwards instead of forwards, the anterior projection on the foot of the pubis is lost, the tail becomes even shorter and the hyperflexing wrist joint is present, which allows the action that is crucial to the flight stroke in birds. These features were passed to birds from their dinosaurian ancestors and not specifically evolved for an avian lifestyle (Padian et al., 1998).

The Evolution of Feathers

The first known feathers are from a small coelurosaurian dinosaur called Sinosauropteryx, which has a row of small fringed structures along its vertebral column. Therefore, feathers are not a synapomorphy of birds, they are shared by a broader group within the theropod dinosaurs. This shows that feathers did not evolve specifically for flight (Padian et al., 1998).

Hypotheses for the original function of feathers include insulation, display and camouflage. These are hypotheses are difficult to test, and it seems likely that feathers were used for several different purposes as they are now (Padian et al., 1998).

The Cursorial Theory

The cursorial theory is based on the evidence that Archaeopteryx was a strong, agile, biped, and from evidence that birds evolved from small, active, running predators. It has been suggested that feathers were used as nets to capture insects, but it has been shown that this action would cause the proto-bird to lose its balance by throwing off its angular momentum. As an alternative, it was suggested that insects were caught using the teeth with the arms held out laterally. An increased airfoil surface would increase lift and stability. This suggests that flight could have begun by running, leaping and sustaining short, leg-powered glides after prey or away from predators (Padian et al., 1998).

Criticisms of this include the problems of drag and needing to work against gravity. It is biomechanically easier to evolve flight from gliding than from the ground, although this does not indicate that it evolved this way. The other problem is the ground speed that would be required to reach a typical flight speed of 6-7 ms-1. Modern lizards have been reported to reach theses speeds, but it is not known whether proto-birds could reach such speeds. There has been some doubt as to whether selection could improve both the forelimb and hindlimb at the same. However, decoupling of the fore- and hindlimbs was achieved in the coelurosaurs and the tail and hindlimb were progressively decoupled in the earliest birds (Padian et al., 1998).

Any model also needs to account for the evolution of the flight stroke. It has been shown that the immediate sister groups of birds, Deinonychus and the other dromaeosaurs, already had the sideways-flexing wrist joint that in birds is essential to the production of thrust. In dinosaurs, this feature was used as a prey-seizing stroke (Padian et al., 1998).

It would have only taken a slight adjustment of the angle of attack of this predatory stroke to create a suitable vortex wake. By running, leaping and a few such strokes, extension of the time in the air, and eventually flight from the ground up could have evolved. This idea requires no features not already known from fossils. It has been suggested that advantage may have been taken of any ridge or incline and so the model does have an element of the arboreal theory (Padian et al., 1998).

Overall, the evidence seems to point to a modified version of the cursorial theory of bird. Running leaps were aided by wings outstretched for balance, the wings were expanded at the distal ends for increased stability. The leaps were gradually extended by short flapping motions that elaborated the down and forward motion already present in the sister groups of the first birds. Running and leaping may have been enhanced by "ridge-gliding" or jumping from small heights (Padian et al., 1998).

The Arboreal Theory

The arboreal theory is more intuitive in that flight evolving from an arboreal gliding stage would seem to be relatively easy. However, this theory has little support from comparative biology as it requires the ability to climb trees and to glide. Neither capacity seems to be present in Archaeopteryx or in theropod dinosaurs.

Archaeopteryx has none of the features of typical vertebrate gliders, nor is it aerodynamically designed to fly. It has been argued that the lateral grooves and curvature of the claws are an arboreal specialisation, but these have also been compared to those of ground-dwelling birds. The evidence for an arboreal lifestyle overall is weak. It should also be considered that large theropods such as Allosaurus and Tyrannosaurus have curved claws with deep lateral grooves, and they were obviously not arboreal. Also, palaeobotanical evidence shows an absence of large trees anywhere near the Solnhofen lagoons in which Archaeopteryx is preserved (Padian et al., 1998).

Flight Capabilities of Archaeopteryx

The general consensus is that Archaeopteryx was a weak flier. This is supported by two arguments. The first is that Archaeopteryx lacks evidence of a supracoracoideus system, which in birds is the tendon that powers the upstroke. Experiments have shown that pigeons with a severed supracoracoideus tendon can not take-off from ground level, can not maintain level flight and can not land safely. However, it is questionable whether Archaeopteryx can be compared with modern birds in this way (Padian et al., 1998).

The second argument is that the feathers of Archaeopteryx are asymmetrical, which suggests that it was capable of some flight. However, data shows that the asymmetry is less than that of modern fliers and gliders and this argument is only valid if it is assumed that fossil animals must look exactly like modern ones when performing the same function (Padian et al., 1998).

Generally it is thought that Archaeopteryx could glide as well as most modern birds and fly by flapping to some extent. The evidence for this is; 1) its wing planform and size are like those of some modern weakly flying birds, 2) the flight feathers are well-developed, 3) the sternum was a strong site of origin for flight muscles, and 4) its aerodynamic planform is unlike that of birds that only glide (Padian et al., 1998).
Flight Improvements After Archaeopteryx

Phylogenetic analysis has shown that many of the characteristics associated with the origin of flight were already present in non-avian theropod dinosaurs before birds evolved. Feathers evolved in non-avian coelurosaurs whose forelimbs were too short to bear functional wings. Archaeopteryx had a forelimb longer than the hindlimb, and flight feathers on the wings and long tail feathers. It is thought to have had a fully evolved flight stroke capable of generating thrust as well as lift. The presence of an alula in the early Cretaceous Eoalulavis shows that the wing mechanism that allowed flying at lower speeds and to manoeuvre like living birds evolved early in bird history. Iberomesornis, of the early Cretaceous, has features diagnostic of a perching ability (Padian et al., 1998).

What kinds of isolation can lead to the formation of a new species?


According to the biological species concept, populations are different species if gene flow between them is prevented by biological differences, known as reproductive barriers. If populations exchange genes they are conspecific, i.e. belong to the species, even if they differ greatly in morphology. If they are reproductively isolated, they are different species even if they are indistinguishable phenotypically. Therefore speciation arises from the evolution of biological barriers to gene flow (Futuyma, 1998).

The factors leading to reproductive isolation can be divided into two categories; prezygotic factors, which operate before fertilisation can occur; and postzygotic factors, which operate after fertilisation leading to partial or complete failure of crosses between the two forms. These are summarised below.
A. Prezygotic factors

1. Geographical isolation: forms are separated by land or water barriers that they are unable to cross.

2. Ecological isolation: the forms fail to meet because they live in different places within the same geographic region.

3. Temporal isolation: the forms are active at different seasons or times of day.

4. Behavioural isolation: the forms meet, but do not mate.

5. Mechanical isolation: copulation occurs, but no transfer of male gametes takes place.

6. Gametic incompatibility: gamete transfer occurs, but egg is not fertilised.

B. Postzygotic factors

1. Zygote dies: zygotic mortality soon after fertilisation.

2. F1 hybrids (first generation) have reduced viability (hybrid inviability).

3. F1 hybrids viable but have reduced fertility (hybrid sterility)

4. Hybrid breakdown: reduced viability or fertility in F2 (second generation) or backcross (F1 crossed with parents) generations.

Prezygotic Barriers

One or more of these may be operating within a given population at any time. The evolutionary functions of these mechanisms are the same, to limit or prevent gene flow between species. They may occur only partially; for example, behavioural isolation can be complete or females may only show a slight preference for males of their own species (Dobzhansky et al., 1977). 


Introduction to Metamorphic Petrology

Metamorphic petrology is the study of rocks which have been changed (metamorphosed) by heat and pressure. They are broadly categorized into regional and contact. Metamorphism is an extension of the process which forms sedimentary rocks from sediment: lithification. However, all types of rocks; igneous, sedimentary and metamorphic, can all be metamorphosed. During metamorphism no melting takes place. All the chemical reactions which take place occur in the solid-state.

Factors Controlling Characteristics
The characteristics of a metamorphic rock depend on the following factors:

1. Composition of parent rock
2. Temperature and Pressure of metamorphism
3. Fluid
4. Time

The composition of the parent rock does not usually change during metamorphism (if it does it is then called metasomatism). The changes are the due to the minerals changing. A basalt which has around 50% of silica will produce a metabasalt with 50% silica.

Temperature and pressure affect the rock in terms of the mineral assemblage which is stable at the pressure and temperature obtained. The minerals stable at the pressure and temperatures that metamorphic rocks reach are simulated in a lab. This allows geologists to look at a mineral assemblage and give a (good) estimate of the pressure and temperature that the sample was exposed to. This gives tectonic information which is useful in other branches of geology.

Fluid changes the chemical composition of the rock being metamorphosed and hence is called metasomatism. The addition of fluid can radically change the rock.

Time has an important role as a rock which is heated to an extreme temperature for a short (years) period of time will not be altered too much. A rock heated for a longer period of time (millions of years) will show changes.

The classification of metamorphic rocks is split into contact and regionally metamorphosed rocks. After this it is divided according to the "amount" of metamorphism that has taken place and/or on the mineral content.

Contact Metamorphism (based on mineral content)
Parent Rock -Metamorphic rock -Dominant Minerals- Characteristics
Limestone -Marble- Calcite- Interlocking grains. Fizzes in weak acid
Quartz -Sandstone- Quartzite- Quartz Sugary texture
Shale -Hornfels (Spotted Rock) -Micas- Dark colour

Regional Metamorphism (name based on degree of metamorphism)
Texture -Rock Name -Characteristics
Slatey -Slate -Splits easily into sheets
Between slate and schistose- Phylitte -Silky lustre, splits into wavy sheets
Schistose -Schist- Pearly looking. Silky to touch
Gneissic -Gneiss- Wavy, white and dark layers

Causes of Metamorphism

Caused by heating from an external source. Contact metamorphism occurs next to an igneous body. The degree of metamorphism decreases away from the body. This occurs at fairly shallow depths, as temperature not pressure is the dominating factor.


Regional metamorphism is caused by high pressure and temperatures usually during mountain building (oregenesis). The extremes of regionally metamorphic rocks are a high pressure, low temperature rock (called a blueschist) and a high pressure and very high temperature rock (called a granulite). If the rock is heated to the point of melting, but doesn't actually melt, it is called a migmatite.

Introduction to Igneous Petrology

Introduction : Igneous rocks are formed form the cooling of molten rock, magma. They are crystalline, which means they are made up of crystals joined together. There are many different types of igneous rocks but they fall into two (very) broad categories; intrusive and extrusive. Intrusive rocks are igneous rocks which form at depth. They cool slowly, taking tens of thousand of years to cool. They have large crystals, tens of millimetres in size. Extrusive rocks are those which have erupted from volcanoes. They have very small crystals, not visible to the naked eye, as they cooled quickly. Of course there is every grain size possible in between these two extremes.


The chemistry of igneous rocks is quite complicated. It depends on two things; evolution and silica saturation. In this tutorial we will concern ourselves with the effect of evolution only, the silica saturation will be assumed to be constant. Igneous rocks evolve as they cool. This process is called differentiation. The mechanism for this process is as follows:

1. Liquid rich in minerals A,B and C.
2. Remove mineral A as it crystallises at a higher temperature than B and C. Liquid is relatively enriched in minerals B and C.
3. Remove mineral B as it crystallises at a higher temperature than C. Liquid is now completely mineral C. The minerals are removed in order of Bowen's Reaction Series.

As you can see, if you remove olivine, the magma will become more enriched in pyroxenes etc. This process continues until only quartz is left. This leads us to the following, simple, identification .

Textures & Names

Igneous rocks have many textures which tell us about their cooling histories and/or chemistry. In general rocks which have cooled rapidly are fine grained, that is with grains which are not visible to the naked eye. Rocks which have cooled slowly have large grains, sometimes as large as several centimetres across.
Textures & Names

This size variation arises as grains grow around a nucleus of some sort, i.e a minute grain. The slower the cooling the more time grains have to grow and amalgamate. Grains which show their true shape are said to be euhedral. Grains which show no shape are anhedral. Using this information, the order of grain growth can be worked out. For example, a rock may have large euhedral quartz grains, which are surrounded by anhedral feldspar. The quartz grew first as it had space, the feldspar then grew around the quartz.

Other features seen are:
Porphyritic texture - large grains (phenocrysts) surrounded by much finer grains (groundamss). This implies that the large grains grew slowly at depth, the magma with the grains in it, then rose up in the crust, cooling much more quickly forming the fine grains (the matrix).

  • Exsolution - occurs within grains on certain minerals (pyroxenes and feldspars). This can give an indication of pressure and hence depth.

  • Xenoliths - bits of the rock into which the magma intruded

  • Cumulate layer - when a mineral grows which is denser than the magma, it will sink to the base of the chamber causing a cumulate layer. Minerals may form from liquid trapped between the grains - interstitial minerals.

  • The name given to an igneous rock depends on it's mineralogy (basic, intermediate or acidic) and the grain size. This is sumemrised in Table givem below.

Textures & Names
Igneous Rock Formations

Igneous rock bodies are either intrusive or extrusive. Extrusive bodies are lava flows. If these occur under water they form pillow lavas. On land they can form lava tubes, aa (pronounced ah-ah and looks blocky) or pahoehoe (which looks ropey).

Intrusive bodies form when magma is injected into existing rock layers. A dyke is a body which cuts across the country (host) rock. A sill is parallel to the bedding layers. The baked margin is an area in the country rock, in contact with the igneous body, which has been thermally metamorphosed. The chilled margin is the area in an igneous body, in contact with the country rock, which cooled quicker than the rest of the rock due to the temperature difference between the magma and the country rock. These features are not always visible. The scale of these bodies is from millimetres to tens or even hundreds of metres.

The largest of igneous bodies is a pluton or batholith. These are massive, hundreds of kilometres in size. The moors of Cornwall and Devon are outcrops of a massive batholith.

Structural Geology


Structural geology is the study of the features formed by geological processes. Features include faults, folds and dipping strata. Geologists can work out the order of events and see which events are related by taking fairly simple measurements and using simple methods.

Measurements and Techniques

The most obvious thing to do when trying to decipher the structural history of a formation is to describe it. One way of doing this is to measure the dip and strike. The dip is the amount a bed of rock is tipped from the horizontal. The strike is the direction which is ninety degrees from the dip, i.e. along the horizontal line on the bed.

The strike can be in two directions, hence the dip could be in one of two directions also. There is a convention for the strike to be the in the direction you are facing if the rocks are dipping to your right. Some geologists prefer to measure the dip direction, rather than strike, as it is slightly simpler. However, all maps use dip and strike, not dip direction.

This is complicated slightly by apparent dip. This is due to the fact that you are not always looking edge on (perpendicular) to the bed you are measuring. If you are looking at a bed at a slight angle, then you see the apparent dip. The true dip will be greater than the apparent dip, as it is the maximum amount of dip, so the apparent dip can appear to be anything from 0° to the maximum (true) dip.

In this diagram, the dip is 30°, with a strike north/south (0°/180°), the dip direction is 270°.
On a geological map, symbols are used for the dip and strike. The strike is represented by a bar, and the dip by a mark on the strike bar on the downdip direction with the dip written alongside, as shown on the map below left. A geological cross section can be drawn from the map showing the subsurface structure. Obviously, only features which can be seen on the surface can be represented. The cross-section below right is drawn using the values in the map alongside.

A technique which is used often is to plot values of dip and dip direction on a stereogram. A stereogram (or stereonet or hemispherical projection) is a way of representing 3-dimensional directions on a 2-dimensional surface. The net is a projection from the point onto the equator.

The points are placed all around the sphere representing 3D space. The points are projected down onto the equatorial plane on a line which meets up at the south pole


Folding of rocks is caused by the compression of rocks. This occurs slowly, over a long period of time. If this happened quickly, the rocks would break, and fault. This is due to the mechanical properties of rocks, namely it's plastic nature. If a rock is stretched slowly, then it will behave in a ductile fashion. If stretched quickly, the rock behaves in a brittle fashion.

Nomenclature used when describing folds

Hinge: Where curvature of the fold is at a maximum
Crest & Trough: Where fold surface reaches a minimum and maximum respectively
Limb: Beds between two hinges
Antiform & Synform: Convex upwards or convex downward folds respectively
Anticline & Syncline: Older or younger beds at the core respectively. Can be used in conjunction with antiform and synform, i.e. an antiformal syncline

Folds are classified by shape and the chronological order of rocks in them. The shape of a fold is described by the angle between the limbs, which are given the terms: gentle (120-180°), open (70-120°), close (30-70°), tight (5-30°) or isoclinal(0-5°).

The chronological order of the rocks in a fold are described by syncline and anticline, as described above.

Identification of Rocks

Identification of Igneous Rocks

Grain Size

Usual Color



Rock Type



glassy appearance

lava glass




many small bubbles

lava froth from sticky lava




many large bubbles

lava froth from fluid lava


fine or mixed


contains quartz

high-silica lava


fine or mixed


between felsite and basalt

medium-silica lava


fine or mixed


has no quartz

low-silica lava



any color

large grains in fine-grained matrix

large grains of feldspar,quartz, pyroxene or olivine




wide range of color and grain size

feldspar and quartz with minor mica, amphibole or pyroxene




like granite but without quartz

feldspar with minor mica, amphibole or pyroxene



medium to dark

little or no quartz

low-calcium plagioclase and dark minerals



medium to dark

no quartz; may have olivine

high-calcium plagioclase and dark minerals




dense; always has olivine

olivine with amphibole and/or pyroxene





mostly pyroxene with olivine and amphibole





at least 90% olivine


very coarse

any color

usually in small intrusive bodies

typically granitic



Chemical Sedimentary Rocks

These same ancient shallow seas sometimes allowed large areas to become isolated and begin drying up. In that setting, as the seawater grows more concentrated, minerals begin to come out of solution (precipitate), starting with calcite, then gypsum, then halite. The resulting rocks are certain limestones or dolomites, gypsum rock, and rock salt respectively. These rocks, called the evaporite sequence, are also part of the sedimentary clan. In some cases chert can also form by precipitation. This usually happens below the sediment surface, where different fluids can circulate and interact chemically.

Diagenesis: Underground Changes

All kinds of sedimentary rocks are subject to further changes during their stay underground. Fluids may penetrate them and change their chemistry; low temperatures and moderate pressures may change some of the minerals into other minerals. These processes, which are gentle and do not deform the rocks, are called diagenesis as opposed to metamorphosis (although there is no well-defined boundary between the two).

The most important types of diagenesis involve the formation of dolomite mineralization in limestones, the formation of petroleum and of higher grades of coal and the formation of many types of ore bodies. The industrially important zeolite minerals also form by diagenetic processes.

Sedimentary Rocks Are Stories

The beauty of sedimentary rocks is that their strata are full of clues to what the past world was like. Those clues might be fossils, marks left by water currents, mudcracks or more subtle features seen under the microscope or in the lab.

From these clues we know that most sedimentary rocks are of marine origin, usually forming in shallow seas. But some sedimentary rocks formed on land: clastic rocks made on the bottoms of large freshwater lakes or as accumulations of desert sand, organic rocks in peat bogs or lake beds, and evaporites in playas. These are called continental or terrigenous (land-formed) sedimentary rocks.

Sedimentary rocks are rich in geologic history of a special kind. While igneous and metamorphic rocks also have stories, they involve the deep Earth and require intensive work to decipher. But in sedimentary rocks you can recognize, in very direct ways, what the world was like in the geologic past.

Identification of Sedimentary Rocks


Grain Size



Rock Type



clean quartz

white to brown




quartz and feldspar

usually very coarse


hard or soft


mixed sediment with rock grains and clay

gray or dark and "dirty"


hard or soft


mixed rocks and sediment

round rocks in finer sediment matrix


hard or


mixed rocks and sediment

sharp pieces in finer sediment matrix




very fine sand; no clay

feels gritty on teeth





no fizzing with acid




clay minerals






black; burns with tarry smoke





fizzes with acid



coarse or fine


fizzing with acid unless powdered

Dolomite rock



fossil shells

mostly pieces


very soft



salt taste

Rock Salt

very soft



white or pink


Sedimentary rocks are the second great rock class. Whereas igneous rocks are born hot, sedimentary rocks are born cool at the Earth's surface, mostly under water. They usually consist of layers or strata, hence they are also called stratified rocks. Depending on what they're made of, sedimentary rocks fall into one of three types.

Identification of Metamorphic Rocks


Grain Size


Usual Color


Rock Type





"tink" when struck






shiny; crinkly foliation





mixed dark and light

wrinkled foliation; often has large crystals












distorted "melted" layers






mostly hornblende






shiny, mottled surface



fine or coarse



dull and opaque colors, found near intrusions





red and green

dense; garnet and pyroxene






calcite or dolomite by the test






quartz (no fizzing with acid)


Metamorphic rocks are the third great class of rocks. These are what happens when sedimentary and igneous rocks become changed, or metamorphosed, by conditions underground. The four main agents that metamorphose rocks are heat, pressure, fluids and strain. These agents can act and interact in an infinite variety of ways. As a result, most of the thousands of rare minerals known to science occur in metamorphic ("shape-changed") rocks. Metamorphism acts at two scales, the regional scale and the local scale.

Minerals to Gemstones and Vice Versa

Gemstones to Minerals











Lapis Lazuli







Microcline Feldspar

Mandarin Garnet





Orthoclase, Plagioclase, Albite, Microcline Feldspars




































Almandine-Pyrope Garnet













Chrome Diopside
























Demantoid Garnet











Oligoclase Feldspar










Pyrope, Almandine, Andradite, Spessartine, Grossularite, Uvarovite













Tsavorite Garnet



















Nephrite or Jadeite







Minerals to Gemstones








Chrysolite, Peridot





Almandine-Pyrope Garnet


Orthoclase Feldspar




Plagioclase Feldspar







Demantoid Garnet


Amethyst, Ametrine, Cairngorm, Citrine, Morion










Aquamarine, Beryl, Emerald, Goshenite, Heliodore, Morganite








Agate, Aventurine, Bloodstone, Carnelian, Chrysoprase, Heliotrope, Jasper, Onyx, Sard


Mandarin Garnet


Alexandrite, Chrysoberyl




Cordierite, Dichroite, Iolite


Pleonast, Rubicelle


Ruby, Sapphire


Hiddenite, Kunzite






Chrome Diopside, Violan




Hessonite, Tsavorite Garnet






Achroite, Dravite, Indigolite/Indicolite, Rubellite, Schorl, Verdelite


Lapis Lazuli






Garnet, Uvarovite

Microcline Feldspar

Amazonite, Moonstone







Oligoclase Feldspar





The field of geochemistry involves study of the chemical composition of the Earth and other planets, chemical processes and reactions that govern the composition of rocks and soils, and the cycles of matter and energy that transport the Earth's chemical components in time and space, and their interaction with the hydrosphere and the atmosphere.

The most important fields of geochemistry are:
1. Isotope geochemistry: Determination of the relative and absolute concentrations of the elements and their isotopes in the earth and on earth's surface.
2. Examination of the distribution and movements of elements in different parts of the earth (crust, mantle, hydrosphere etc.) and in minerals with the goal to determine the underlying system of distribution and movement.
3. Cosmochemistry: Analysis of the distribution of elements and their isotopes in the cosmos.
4. Biogeochemistry: Field of study focusing on the effect of life on the chemistry of the earth.
5. Organic geochemistry: A study of the role of processes and compounds that are derived from living or once-living organisms.
6. Regional, environmental and exploration geochemistry: Applications to environmental, hydrological and mineral exploration studies.

Victor Goldschmidt is considered by most to be the father of modern geochemistry and the ideas of the subject were formed by him in a series of publications from 1922 under the title ‘Geochemische Verteilungsgesetze der Elemente’.

The more common rock constituents are nearly all oxides; chlorine, sulfur and fluorine are the only important exceptions to this and their total amount in any rock is usually much less than 1%. F. W. Clarke has calculated that a little more than 47% of the Earth's crust consists of oxygen. It occurs principally in combination as oxides, of which the chief are silica, alumina, iron oxides, and various carbonates (calcium carbonate, magnesium carbonate, sodium carbonate, and potassium carbonate). The silica functions principally as an acid, forming silicates, and all the commonest minerals of igneous rocks are of this nature. From a computation based on 1672 analyses of numerous kinds of rocks Clarke arrived at the following as the average percentage composition: SiO2=59.71, Al2O3=15.41, Fe2O3=2.63, FeO=3.52, MgO=4.36, CaO=4.90, Na2O=3.55, K2O=2.80, H2O=1.52, TiO2=0.60, P2O5=0.22, total 99.22%). All the other constituents occur only in very small quantities, usually much less than 1%.

These oxides combine in a haphazard way. For example, potash (potassium carbonate) and soda (sodium carbonate) combine to produce feldspars. In some cases they may take other forms, such as nepheline, leucite, and muscovite, but in the great majority of instances they are found as feldspar. Phosphoric acid with lime (calcium carbonate) forms apatite. Titanium dioxide with ferrous oxide gives rise to ilmenite. Part of the lime forms lime feldspar. Magnesium carbonate and iron oxides with silica crystallize as olivine or enstatite, or with alumina and lime form the complex ferro-magnesian silicates of which the pyroxenes, amphiboles, and biotites are the chief. Any excess of silica above what is required to neutralize the bases will separate out as quartz; excess of alumina crystallizes as corundum. These must be regarded only as general tendencies. It is possible, by rock analysis, to say approximately what minerals the rock contains, but there are numerous exceptions to any rule.

Mineral constitution: Hence we may say that except in acid or siliceous rocks containing 66% of silica and over, quartz will not be abundant. In basic rocks (containing 20% of silica or less) it is rare and accidental. If magnesia and iron be above the average while silica is low, olivine may be expected; where silica is present in greater quantity over ferro-magnesian minerals, such as augite, hornblende, enstatite or biotite, occur rather than olivine. Unless potash is high and silica relatively low, leucite will not be present, for leucite does not occur with free quartz. Nepheline, likewise, is usually found in rocks with much soda and comparatively little silica. With high alcalis, soda-bearing pyroxenes and amphiboles may be present. The lower the percentage of silica and the alkalis, the greater is the prevalence of t lime felspar as contracted with soda or potash felspar. Clarke has calculated the relative abundance of the principal rock-forming minerals with the following results: Apatite=0.6, titanium minerals=1.5, quartz=12.0, felspars=59.5, biotite=3.8, hornblende and pyroxene=16.8, total=94.2%. This, however, can only be a rough approximation.

The other determining factor, namely the physical conditions attending consolidation, plays on the whole a smaller part, yet is by no means negligible, as a few instances will prove. Certain minerals are practically confined to deep-seated intrusive rocks, e.g., microcline, muscovite, diallage. Leucite is very rare in plutonic masses; many minerals have special peculiarities in microscopic character according to whether they crystallized in depth or near the surface, e.g., hypersthene, orthoclase, quartz. There are some curious instances of rocks having the same chemical composition, but consisting of entirely different minerals, e.g., the hornblendite of Gran, in Norway, which contains only hornblende, has the same composition as some of the camptonites of the same locality that contain felspar and hornblende of a different variety. In this connection we may repeat what has been said above about the corrosion of porphyritic minerals in igneous rocks. In rhyolites and trachytes, early crystals of hornblende and biotite may be found in great numbers partially converted into augite and magnetite. Hornblende and biotite were stable under the pressures and other conditions below the surface, but unstable at higher levels. In the ground-mass of these rocks, augite is almost universally present. But the plutonic representatives of the same magma, granite and syenite contain biotite and hornblende far more commonly than augite.

Acid, intermediate and basic igneous rocks : Those rocks that contain the most silica, and on crystallizing yield free quartz, form a group generally designated the "acid" rocks. Those again that contain least silica and most magnesia and iron, so that quartz is absent while olivine is usually abundant, form the "basic" group. The "intermediate" rocks include those characterized by the general absence of both quartz and olivine. An important subdivision of these contains a very high percentage of alkalis, especially soda, and consequently has minerals such as nepheline and leucite not common in other rocks. It is often separated from the others as the "alkali" or "soda" rocks, and there is a corresponding series of basic rocks. Lastly a small sub-group rich in olivine and without felspar has been called the "ultrabasic" rocks. They have very low percentages of silica but much iron and magnesia.

Except these last, practically all rocks contain felspars or felspathoid minerals. In the acid rocks the common felspars are orthoclase, perthite, microcline, and oligoclase—all having much silica and alkalis. In the basic rocks labradorite, anorthite and bytownite prevail, being rich in lime and poor in silica, potash and soda. Augite is the commonest ferro-magnesian of the basic rocks, but biotite and hornblende are on the whole more frequent in the acid.

Effusive type or Lavas Phonolite, Leucitophyre Tephrite and Basanite Nepheline-basalt, Leucite-basalt : This classification is based essentially on the mineralogical constitution of the igneous rocks. Any chemical distinctions between the different groups, though implied, are relegated to a subordinate position. It is admittedly artificial by it has grown up with the grown of the science and is still adopted as the basis on which more minute subdivisions are erected. The subdivisions are by no means of equal value. The syenites, for example, and the peridotites, are far less important than the granites, diorites and gabbros. Moreover, the effusive andesites do not always correspond to the plutonic diorites but partly also to the gabbros. As the different kinds of rock, regarded as aggregates of minerals, pass gradually into one another, transitional types are very common and are often so important as to receive special names. The quartz-syenites and nordmarkites may be interposed between granite and syenite, the tonalites and adamellites between granite and diorite, the monzoaites between syenite and diorite, norites and hyperites between diorite and gabbro, and so on..

Soil Chemistry

Soil chemistry is the study of the chemical characteristics of soil. Soil chemistry is affected by mineral composition, organic matter and environmental factors.

Until the late 1960s, soil chemistry focused primarily on chemical reactions in the soil that contribute to pedogenesis or that affect plant growth. Since then concerns have grown about environmental pollution, organic and inorganic soil contamination and potential ecological health and environmental health risks. Consequently, the emphasis in soil chemistry has shifted from pedology and agricultural soil science to an emphasis on environmental soil science.

Environmental soil chemistry : A knowledge of environmental soil chemistry is paramount to predicting the fate, mobility and potential toxicity of contaminants in the environment. The vast majority of environmental contaminants are initially released to the soil. Once a chemical is exposed to the soil environment a myriad of chemical reactions can occur that may increase or decrease contaminant toxicity. These reactions include adsorption/desorption, precipitation, polymerization, dissolution, complexation and oxidation/reduction. These reactions are often disregarded by scientists and engineers involved with environmental remediation. Understanding these processes enable us to better predict the fate and toxicity of contaminants and provide the knowledge to develop scientifically correct, and cost-effective remediation strategies.


  • Anion and cation exchange capacity

  • Soil pH

  • Mineral formation and transformation processes

  • Clay mineralogy

  • Sorption and precipitation reactions in soil

  • Oxidation-reduction reactions

  • Chemistry of problem soils