Glossary of Terms

 

Source : geolsoc.org.uk

 

A

Aa: blocky lava (Hawaiian for "ouch, this hurts" when you walk on it barefoot!)

Abrasion: wearing down of rock sufaces by mineral and rock grains in transport.

Abyssal Plain: deep ocean floor (usually 2 to 6km deep), generally flat apart from occasional volcanic mountains and mid-ocean ridges.

Algae: plants that mostly grow in the sea. Most are microscopic organisms that float around in the surface water (plankton).

Alignment (of minerals): mineral grains tend to line up in the same direction in metamorphic rocks, often leading to foliation.

Alluvial Fan: fan-shaped deposit formed when a river or flash flood slows down and rapidly dumps its load of sediment.

Andesite: Igneous rock, fine-grained, grey-coloured, formed from a fairly viscous type of lava.

Angular: word used to describe sediment grains that have not been much rounded off in transport.

Anticline: A rock fold that bulges upward in the middle.

"Ash" - volcanic: small fragments of rock and pumice thrown out by explosive eruptions from a volcano; often deposited over huge areas.

Attrition: wearing down and rounding of pebbles and rock grains in transport.
                                           
 

B

Basalt: igneous rock, fine-grained, almost black in colour, formed from a fairly runny type of lava (e.g. in Hawaii).

Batholith: A large intrusion of igneous rock, usually granite, formed deep beneath the surface so the rock cooled very slowly.

Beach: sediment (pebbles, sand, mud) deposited by the sea along the shoreline.

Bedding, bedding planes: layering formed in sedimentary rocks as the rate of sediment deposition varies. Layer boundaries are called bedding planes.

Bed-load: gravel & sand carried along a river bed by traction and saltation.

Biological weathering: breakdown of rock (see weathering) by the action of bacteria, plants and animals.

Boulder clay: mixture of clay, pebbles and boulders deposited by ice sheets.

"Bombs" - volcanic: lumps of rock and/or lava thrown out of a volcano during an eruption.

Burial: covering over of sediment by more and more layers, leading to compaction and cementation of grains.
                                           
 

C

Calcite: A common mineral which fizzes with dilute acid and is the main component of limestone. Chemically known as calcium carbonate.

Calcium Carbonate: chemical name for the mineral calcite.

Cementation:, cement sediment grains are stuck together (cemented) by minerals deposited from solution in groundwater, turning the sediment into rock.

Chalk: a white limestone almost entirely made of microscopic calcite platelets (coccoliths) produced by planktonic algae.

Chemical Weathering: breakdown of rock (see weathering) by chemical reaction with air, water and dissolved acids.

Clay: microscopic mineral grains formed by chemical weathering of rocks - the main component of all squishy mud!

Coal: sedimentary rock made up of compressed and carbonised plant material. An important “fossil fuel”

Coarse-grained: a rock whose grains (crystals or sediment particles) are roughly pea-sized or larger.

Coccoliths: microscopic calcite platelets formed by microscopic algae. Main component of chalk.

Columnar joints: cracks that form long pillars (columns), often six-sided, due to contraction as some lava flows cool down (e.g.Giant's Causeway).

Compaction: process where sediment grains become squashed closer together during burial.

Compression: process where rocks are squashed by the weight of overlying rock, or squeezed by huge tectonic forces, often resulting in folds.

Conglomerate: a rock formed mainly of pebbles or boulders that have become cemented together.

Continental Shelf: region of relatively shallow sea (usually less than 200m deep) around the edge of a continent.

Contraction: shrinkage resulting from cooling (e.g.in igneous rocks).

Contact Metamorphism: metamorphism resulting from high temperatures close to an intrusion of hot magma.

Coral: simple marine creatures that build up structures of calcium carbonate, sometimes forming reefs (e.g.Great Barrier Reef, Australia).

Crater: Hollow or depression, usually at the top of a volcano, but sometimes caused by meteor impacts.

Creep (sediment grains): slow, sliding movement of small pebbles along desert surfaces due to impacts of wind-borne sand grains.

Cross-bedding: layers formed on an angle (usually around 20 degrees to horizontal) as sediment is deposited by flowing water or wind.

Crust (of the Earth): outermost layer (average 20 km thick) of Earth composed of rocks similar to those seen at the surface

Crystal: a mineral solid with a regular atonic structure, often having a regular shape and splitting along flat, shiny surfaces.

Crystalline (rock): rock made up of interlocking crystals. Igneous and metamorphic rocks are almost all crystalline, as are salt deposits.

Crystallisation: process where solid crystals form as molten rock (magma or lava) cools, or a solution (such as sea-water) evaporates.
                                           
 

D

Deformation: where foces acting on rocks change their shape through folding, faulting and re-crystallisation.

Delta: a river mouth where a river splits into many channels as it deposits sediment on meeting a lake or the sea (e.g.Mississippi).

Deposition: process of settling out of sediment grains from water or wind (usually as flow slows down) or ice (as it melts).

Desert: a region with very low rainfall. Most deserts are hot, like the Sahara, but there are cold deserts, too.

Dyke: an intrusion of igneous rock, formed when magma flows toward the surface through cracks or faults, cutting through the rock layers.                                                                                                                                                                                                                                        

 

E

Earthquakes: ground shaking caused by the energy that is released when rock on either side of a fault suddenly moves.

Effusive eruption: a volcanic eruption where hot, runny lava is produced, rather than violent explsions of ash and gas.

Estuary: Tidal part of a river towards its mouth (e.g.Thames estuary, UK).

Erosion: breakdown and removal of rock material by flowing water, wind, or moving ice. Not to be confused with weathering!

Eruption (volcanic): escape of molten rock (magma) at the Earth's surface. A series of eruptions (over many years) gradually builds up a volcano.

Evaporation: drying out of sea water (usually) in warm, dry climates, leaving behind deposits of crystalised mineral salts.

Exfoliation: a weathering process where surface layers of rock peel away rather like dead skin!

Expansion: a growth in volume which happens when most solids get hotter, but also when water freezes to form ice.

Explosive eruption: a volcanic eruption where viscous, gas-rich magmas burst out as explosions of pyroclastic material ("bombs", pumice and "ash").

Extrusive (igneous): rocks formed by eruptions at the Earth's surface (see volcanic).
                                           
 

F

Faults: deep cracks caused by movement of rock during earthquakes. Different rock types are often seen on each side of a fault.

Feldspar: a common mineral in igneous and some metamorphic rocks.

Fine-grained: a rock in which the grains (crystals or sediment particles) are too small to see easily without a magnifying glass.

Flash flood: Sudden flood of water, often heavily-laden with sediment, produced by sudden storms, especially in hot desert regions.

Floodplain: a flat valley floor in which a river meanders, and which may be flooded from time to time after heavy rain.

Folds: bending of rock layers caused by compression of rocks, usually as part of mountain-building when tectonic plates collide.

Foliated: a "squashed" texture common in metamorphic rocks (e.g. schist) where crystals of platy minerals like mica lie in the same direction.

Fossil: any trace of past life preserved in a rock (includes animal tracks & burrowsas well as shells, skeltons and impressions of soft flesh).

Fragmental (clastic): made up of sediment grains (fragments or clasts) produced by erosion of more ancient rocks, and/or fossil fragments (e.g. shells).

Freeze-thaw: weathering process where water seeps into cracks, freezes and expands, gradually wedging open cracks until the rock breaks.
                                           
 

G

Gabbro: dark-coloured, coarse-grained, igneous rock; cooled slowly in large intrusions (same chemical composition as basalt).

Glacier, glacial erosion: a slowly-moving "river" of ice which rapidly erodes deep valleys because of the rocks embedded in the base and sides of the glacier.

Gneiss: coarse-grained, metamorphic rock often showing a "banded" texture due to separation of pale and dark-coloured minerals.

Grains (in rock): mineral crystals or sediment particles that make up all rocks.

Granite: light grey or pinkish, coarse-grained, igneous rock, cooled slowly in large intrusions (same chemical composition as pumice).

Groundwater: water trapped in gaps between the grains of rocks (usually sedimentary rocks) beneath the surface. See also porosity.
                                           
 

H

Hydrolysis: weathering process where acidic rain water slowly reacts with many common rock-forming minerals to form clay and dissolved salts.
                                           
 

I

Ice sheet: a mass of ice covering a large land area (as in Greenland or Antactica at the present day).

Igneous, igneous rock: Formed from magma , either erupted from a volcano or cooled below ground in an intrusion. (latin: ignis = fire).

Interlocking: a rock texture where grains (mineral crystals in igneous and metamorphic rocks) lock together like a mosaic.

Intrusion (igneous): a body of igneous rock that cooled and crystallised from magma deep underground (see also sills, dykes and batholiths).

Invertebrates: animals without backbones (worms, shellfish, insects etc).
                                           
 

J

Joints: cracks (usually vertical) in rock caused by shrinkage or release of pressure as rocks above are eroded away.
                                           
 

L

Lagoon: an area of shallow water enclosed by sand banks or reefs. Tropical lagoons may also have small "patch reefs" growing within them.

Lahar: a mudflow formed of water and volcanic ash. A major hazard in parts of Japan, the Philippines etc.

Landslide: general term for mass-movement of rock material down a slope (includes rock falls, landslips, mudflows etc).

Landslip: mass-movement where rock material (often clay) slips down slope along a defined slip surface (often gently curved).

Lava: molten rock (e.g.basalt) erupted from a volcano. See also magma (which is not quite the same thing!)

Limestone: sedimentary rock composed largely of calcium carbonate (fizzes with dilute acid), usually formed from remains of living organisms.

Lithosphere: outer layer of Earth (uppermost mantle and crust) that behaves as a number of rigid, moving "plates". See also plate tectonics.

Longshore drift: movement of sediment along a shoreline by wave action.
                                           
 

M

Magma: molten rock with dissolved volcanic gases, beneath the Earth's surface.

Mantle: layer of hot, dense rock deep beneath the surface (from about 25 to 2500 km down). Hot enough to flow slowly - see plate tectonics.

Mass-movement:any downhill movement of rock or soil material under gravity.

Marble: metamorphic rock formed from limestone - also fizzes with dilute acid.

Meanders (river): natural bends formed as a river flows across its floodplain.

Medium-grained: a rock in which the grains (crystals or sediment particles) are large enough to see unaided, but smaller than pea-sized.

Metamorphic aureole: region around an intrusion where hot magma has "baked" and altered the surrounding rocks. See also metamorphism.

Metamorphic rock: a rock which has re-crystallised due to heat and/or pressure. Examples: marble, slate, schist. See also metamorphism.

Metamorphism: process of changing rocks (minerals and texture) through heat and/or pressure (greek: meta = shange, morphos = appearance).

Mica: mineral with a shiny, flaky appearance. Common in granite and metamorphic rocks such as schist and gneiss.

Mineral: naturally-occurring chemical compound (e.g.calcium carbonate), often with a regular crystal structure. The "building blocks" of rocks.

Moraine: rock material carried by a glacier; dumped where the glacier melts.

Mudflow: mass-movement of soil and water formed after heavy rain. Mudflows can move large rocks, cars, demolish houses. See also lahar.

Mudstone: a relatively soft rock formed largely of fine clay. Compressed, flaky mudstone is often called shale.
                                           
 

N

Normal fault: a fault (usually vertical or steep-angled) where rock has moved down the fault surface as a result of tension forces.
                                           
 

O

Oolith: round grain of calcium carbonate formed by rolling action of waves in very shallow, warm sea-water. See also oolitic limestone.

Oolitic limestone: a limestone with round grains (a mm or so across) called ooliths, formed by chemical precipitation in warm, shallow sea water.

Oxidation: a weathering process where iron-rich minerals (usually) react with air to form rusty-brown oxides.
                                           
 

P

Pahoehoe: (Hawaiian) - a "ropy" surface formed as cooling lava forms a "skin" which rucks up into folds and twists as the lave flows beneath it.

Pebbles: rock fragments (often rounded) of around 1 to 10 cm in size.

Petroleum: crude oil (along with petroleum gas) occupying pore spaces in some sedimentary rocks. A valuable “fossil fuel”.

Physical Weathering: mechanical breakdown of rock material at the surface, e.g.by changes in temperature.

Pillow lava: lava that has been erupted beneath the sea, forming characteristic "pillow" shapes as it comes into contact with cold water.

Plankton: tiny living organisms (plants and animats) that float in the surface waters of the sea, drifting with the currents.

Plate, Plate Tectonics: plate tectonics describes the slow motion of rigid "plates" of the lithosphere due to movement (convection) of the mantle beneath.

Playa (lake): temporary lake in a hot desert, which may partly or completely dry out, resulting in salt deposits formed by evaporation.

Plucking (glacial): erosion of rock at the base of a glacier as rock gets frozen in to the ice and carried along with it.

Porous, porosity: rock (usually sedimentary) with spaces between the grains, usually filled by groundwater. Porosity measures amount of pore space.

Pumice: very light, "frothy" igneous rock formed when gas-rich magma erupts, especially during explosive eruptions of viscous magma.

Pyroclastic:Fragments of rock and volcanic ash thrown out of a volcano (greek: pyros = fire and klastos = fragment).

Pyroclastic flow: dense flow of hot gas, volcanic ash and rock fragments erupted from a volcano, that can move downhill at speeds of over 100 mph!
                                           
 

Q

Quartz: hard, resistant mineral (grey/white/glassy) in granite, many metamorphic rocks and sandstones. Silicon dioxide (SiO2).

Quartzite: A very hard, usually almost white, sandstone mad up of quartz grains cemented together by SiO2.                                                                                                                                                                                                                                                                                            

 

R

Re-crystallisation: change of mineral form (e.g. clay => mica) usually due to chemical reactions in rocks during metamorphism. No melting involved.

Reef: rocky mound or bank, close to sea surface. Many reefs are built up by living organisms such as corals and sponges.

Regional metamorphism: metamorphism of rocks over large areas, usually resulting from both heat and pressure during plate collision and mountain-building.

Reverse fault: a fault where rock has moved up the fault surface against the rock on the other side as a result of tectonic compression.

Rhyolite: fine-grained, igneous rock formed from a very viscous type of lava produced by some volcanoes.

Rift Valley: a large valley between two sets of opposite-facing Normal faults, resulting from tension forces in the rocks. (e.g. E.African Rift).

Ripple marks: marks left on sand surfaces by the action of waves (symmetrical ripples) or currents (asymmetric ripples).

Rock: any solid portion of the Earth - rocks are made up of many constituent minerals, such as quartz and calcite.

Rock fall: mass-movement of rock from a cliff - exactly what it says!

Rock salt: salt (sodium chloride), deposited from sea water as it evaporated in a hot climate, and now forming layers of sedimentary rock.

Rounded: word used to describe mineral grains and rock fragments that have become worn down by attrition during transport.
                                           
 

S

Salt: a (metallic) chemical compound that can dissolve in water. The most common natural salt in sea water is sodium chloride, NaCl.

Saltation: bouncing motion of sand grains carried along by wind or by flowing water (latin: saltare = to leap).

Sand: sediment particles from 0.1 to 2mm diameter. Most sand grains are made of quartz, a very hard and chemically resistand mineral.

Sandstone: medium-grained, sedimentary rock composed mainly of sand grains that have been cemented together.

Schist: metamorphic rock which has a shiny, foliated, medium-grained texture, often containing much mica.

Sea-stack: a pillar of rock, cut off from land by wave erosion. Examples: Old Man of Hoy (Orkney), The Needles (Isle of Wight).

Sediment: material deposited by water, wind or ice. Includes pebbles, sand, mud, organic remains (e.g.shells) and salts left by evaporation.

Sedimentary rock: any rock made up of sediment grains. Examples: mudstone, sandstone, limestone, rock salt, coal.

Shale: Mudstone that has been compressed to form a fine-grained, flaky, dark-coloured sedimentary rock.

Sill: an intrusion of igneous rock along the bedding planes of the surrounding rock layers (rather than cutting through them - see dyke).

Slate: metamorphic rock formed by compression and heating of mudstone. Much used for roofing as it splits easily into sheets.

Soil creep: gradual downhill mass-movement of soil on steep hillsides.

Solution (load): mineral salts, dissolved in water and carried by rivers to the sea.

Solution (weathering): chemical weathering process where water (usually slightly acidic) dissolves away rock material, especially limestone.

Strata: layers of rock formed by deposition of sediment (and sometimes lava and pyroclastic material).

Strike-slip fault: fault where one region of rock moves horizontally past another. Examples: Great
Glen fault, Scotland, San Andreas fault, California.

Suspension (load): fine particles (especially clay) carried along by flowing water or wind; they eventually settle out in calmer conditions.

Syncline: a rock fold that bulges downward ("sinks") in the middle.
                                           
 

T

Tectonic (forces): forces that stretch or squash the Earth's crust, and lead to folds, faults and mountain-building (greek: tekton = builder).

Tectonic plates: rigid portions of the lithosphere that are constantly moving, exerting huge forces in the Earth's crust. See plate tectonics.

Tension: forces that pull rocks apart, resulting in normal faults and rift valleys.

Texture (of rocks): describes grain sizes (coarse/medium/fine) , shapes (rounded/angular) and relationships (e.g.crystalline/fragmental) in rocks.

Thrust fault: a shallow-sloping reverse fault where a (often large) region of rock has slid up and over another.

Traction (load): rolling and sliding of gravel, pebbles and boulders along a river bed (sometimes sea-bed) by flowing water.

Transport: movement of sediment by water, wind or glacier ice.

Turbidity currents: a mixture of water and sediment that forms a dense fluid which can flow down to the deep ocean floor at surprising speed!                                                                                                                                                                                                                                    

 

U

Unconformity: a boundary where one set of rock layers cuts across another, representing a "gap" in geological time when rocks were worn away.

Uplift: when a region is literally lifted up as the crust is squeezed by tectonic forces or, sometimes, following the melting of ice-sheets.                                                                                                                                                                                                                                       

 

V

Vent :Opening through which lava and rock fragments erupt, usually within a volcanic crater.

Vesicles: bubbles formed by volcanic gases in lava, that become "frozen in" as the lava cools.

Viscous, viscosity: "viscous" (high viscosity) refers to magmas that are thick like treacle (only more so) and move very slowly. Low viscosity = runny.

Vocano, volcanic: cone-shaped (sometimes!) mountain formed by eruptions of lava and/or pyroclastics. Volcanic means "from a volcano".

Volcanic ash: fragments of rock and pumice thrown out of volcanoes by explosive eruptions, The finest particles are carried long distances by winds.

Volcanic gases: gases, like water vapour and carbon dioxide, are dissolved in magma below ground, but are released at the surface as pressure drops.
                                           
 

W

Wave-cut platform: a flat area of rock eroded by waves between the high and low tide marks.

Weathering: slow breakdown of rock at the Earth's surface, due to climatic and biological precesses. See also Physical, Chemical, Biological...