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Glossary of Terms

We have tried to give you a brief definition of all the technical terms used in the website. If we have missed any, or if you wish to find out more, then try entering the word in “Google” or another Search Engine. Don’t forget to come back!
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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.
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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.
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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 ( 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.
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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.                                                                                                                                                                                                                                        Back to Top



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).
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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.
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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.
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Hydrolysis: weathering process where acidic rain water slowly reacts with many common rock-forming minerals to form clay and dissolved salts.
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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).
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Joints: cracks (usually vertical) in rock caused by shrinkage or release of pressure as rocks above are eroded away.
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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.
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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.
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Normal fault: a fault (usually vertical or steep-angled) where rock has moved down the fault surface as a result of tension forces.
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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.
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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, 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!
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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.                                                                                                                                                                                                                                                                                            Back to Top



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.
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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.
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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!                                                                                                                                                                                                                                    Back to Top



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.                                                                                                                                                                                                                                       Back to Top



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.
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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...                                                                                                                                                                                                                            Back to Top
Bottom curves