Natural Resources

Glossary of Geological Terms

Glossary of Geological Terms



adit - A nearly horizonal tunnel for entering a mine from the surface.

algae - A varied group of plants that generally live in the sea or in fresh water; modern examples are pond scum and seaweed.

amphibole - An important group of silicate minerals containing iron, magnesium and calcium.

andesite - A dark-coloured, fine-grained, volcanic rock composed of feldspar and biotite, hornblende or pyroxene; the name is derived from the Andes Mountains; it is the volcanic equivalent of diorite.

anorthosite - A coarse-grained igneous intrusive rock consisting mainly of calcium-rich feldspar, known as labradorite.

anticline - (see antiform)

antiform - A fold in which the limbs dip away from the axis of the fold. If the oldest rocks are in the central core, it is called an Anticline.

aphanitic - Refers to grain size in an igneous rock in which the crystals are not distinguishable by the unaided eye.

archeocyathid - A simple, reef-building, marine organism that lived during the Cambrian period of geological time (i.e., 500-600 million years ago).

asbestos - A white to green, fibrous form of the mineral serpentine which is used widely in fire-retardant and heat- resistant applications; can be hazardous to one's health if dust is inhaled over a period of time.

augen - Eye-shaped mineral grains in a mylonite schist or gneiss; German for eye.


barachois - (see barasway)

barasway - A body of water separated from the sea by a bar or ridge of sand, gravel, etc., built up by waves and currents. Also known as a lagoon.

barite - A white, heavy mineral, the main ore of barium, used in paints, drilling muds and TV screens.

basalt - A dark-coloured, fine-grained volcanic rock composed of feldspar, pyroxene and sometimes olivine; it is the volcanic equivalent of diabase and gabbro.

batholith - A very large body of intrusive rock, usually granite.

baymouth bar - A narrow, usually submerged, ridge of sand and gravel deposited across the mouth of a bay. Commonly formed by the extension of spits and may connect two headlands, thus straightening the coast.

bed, bedding - Layers in sedimentary rocks formed during deposition and reflecting different compositions or grain sizes in the original sediment; originally horizontal in most cases (see cross-bedding).

bedrock - The continuous, solid rock that underlies soil and loose sediment everywhere; an exposure of bedrock is called an outcrop.

biotite - The black variety of the mica group of minerals containing potassium, aluminum, magnesium and iron; it splits easily into thin, translucent sheets.

boudinage - Layer, such as bed, dyke, etc., that has been pulled apart into sausage-like pieces.

brachiopod - A type of shelled animal with two unequal shell halves, which usually lived attached to the seafloor by a stalk.

breccia - A rock consisting of angular fragments in a finer grained matrix which may or may not be similar to the larger fragments; it may be formed from any rock type by fracturing (in a fault), explosion (volcanic), intrusion (intrusion breccia) or sedimentation (scree slope).


calcite - A common, white mineral composed of calcium carbonate, and the principal constituent of limestone.

carbonate - A general term used to describe a rock composed mainly of the carbonate minerals, calcite and dolomite.

chalcopyrite - A brassy yellow, metallic mineral composed of copper, iron and sulphur, and an important source of copper.

chlorite - A dark green, soft, flaky mineral similar to mica; it is common as an alteration or metamorphic mineral formed from ferromagnesium minerals.

chilled margins - Edge of dyke or other intrusive body where magma has cooled quickly in contact with relatively cold wall rock; crystals close to margin are much smaller than those farther inside the body.

cirque - A bowl-shaped depression formed at the head of glacial valleys by frost wedging and ice plucking.

cleavage - The tendency of a mineral or rock to break along a preferred direction in smooth parallel planes. In metamorphic rocks, this is also called foliation or schistosity (see schistosity).

columnar jointing - A system of polygonal fractures, resulting from cooling of molten magma, that splits a rock body into long prisms or columns. It is characteristic of lava flows and shallow intrusive igneous rocks.

conglomerate - A coarse-grained sedimentary rock composed of rounded fragments which may vary in size from pebbles to boulders.

consolidated - the condition of sediments when they have hardened into rock from their original, soft state.

contact - The surface separating two different rock types or bodies.

coral - A bottom-dwelling, reef-building marine invertebrate organism of the class Anthozoa. It produces a skeleton of calcium carbonate.

country rock - The rock intruded by and surrounding an igneous intrusion (see wall rock).

crag and tail - An elongate hill or ridge resulting from glaciation, having at the stoss end a steep, often precipitous, face or knob of ice smoothed bedrock (crag) obstructing the movement of the glacier and at the lee end a tapering, streamlined, gentle slope (tail) of till.

crinoid - A marine animal related to modern starfish and sea urchins; it lives attached to the seafloor by a stalk.

cross-bedding - Bedding that was inclined when originally deposited; it formed by variable current or wave action, or by wind (sand dunes).

crystal - Solid material bounded by natural flat surfaces that result from a regular internal arrangement of atoms; almost all minerals can form crystals but in most rocks the flat surfaces do not have room to develop.


delta - A large body of sediment deposited at the mouth of a river.

diabase - A dark-coloured intrusive rock, found as dykes or sills, composed of mainly feldspar, pyroxene and/or olivine; it is the shallow intrusive equivalent of basalt and gabbro.

dimension stone - Natural building stone that may be cut to specific size requirements. It includes granite, gabbro, anorthosite, marble, limestone, slate and sandstone. It can be rough or finished stone.

diorite - A grey intrusive igneous rock composed of mostly feldspar and biotite, pyroxene or amphibole; it forms dykes and small stocks; it is the intrusive equivalent of andesite.

dip - The angle between the horizontal and a sloping surface such as a bedding plane; dip is measured perpendicular to strike direction.

dolomite - A creamy-white mineral composed of calcium and magnesium carbonate and the principal component of dolostone.

dolostone - A sedimentary rock formed by precipitation from seawater; composed mainly of dolomite (calcium - magnesium carbonate).

dyke - a sheet of intrusive rock that cuts across the surrounding rock.


epidote - A greenish-yellow mineral consisting of calcium, aluminum, iron and silica; it is common in metamorphic rocks, especially as a coating on joint surfaces.

erosion - The processes that break up or wear down rocks or unconsolidated sediment and moves them from one place to another; the forces of erosion include water, ice, wind and gravity.

erratic - A large boulder carried by glacial ice to an area removed from its point of origin. It is left behind when the ice melts.

esker - A narrow, sinuous ridge of sand and gravel deposited by a stream flowing under a glacier or ice sheet.

extrusive rock - Rock formed from magma that has erupted explosively or has flowed onto the earth's surface from a volcano (see volcanic rock).


fault - A surface along which one body of rock has moved relative to another.

feldspar - A common group of rock-forming minerals composed of silicates of aluminum and one or more of potassium, sodium or calcium.

felsic - An acronym derived from feldspar and silica and used to describe an igneous rock containing light-coloured minerals, such as quartz and feldspar; also applied to those minerals. Rhyolite and granite are felsic rocks. Opposite of mafic.

felsite - A light-coloured, fine-grained extrusive or shallow-intrusive igneous rock with or without phenocrysts and composed chiefly of quartz and feldspar.

felsitic - A syn. of aphanitic.

ferromagnesium minerals - A variety of silicate minerals containing abundant iron and magnesium, e.g., olivine, pyroxene, amphibole and biotite.

fiord - A glaciated valley flooded by the sea to form a long, narrow, steep-walled inlet.

flow (volcanic) - Volcanic rock formed from lava that flowed out onto the earth's surface.

flysch - A thick marine sequence of sedimentary rocks characterized by thinly interbedded sandy shale, mudstone, sandstone and conglomerate; flysch is indicative of rapid erosion of a nearby newly formed mountain belt.

fold - A bend, buckle or flexure in a rock formed by compressive forces.

foliation - In metamorphic rocks, the layering of minerals caused by oriented growth of mineral grains in response to pressure (see cleavage).

formation - A distinctive body of rock that is given a name and separated from other rocks on a geological map (see Group).

fossil - Remains, traces or imprints of animals or plants that have been preserved in rocks; includes bones, shells, casts and tracks.

fossiliferous - Containing fossils.


gabbro - Dark-coloured, coarse-grained, intrusive igneous rock composed of calcium-rich feldspar, pyroxene and sometimes olivine; it is the deep intrusive equivalent of basalt and diabase.

garnet - A silicate mineral that usually occurs as small, rounded, red or pink crystals mainly in metamorphic rocks but is rarely also found in granite veins.

gastropod - Any mollusc belonging to the class Gastropoda characterized by a distinct head with eyes and tentacles and, in most, by a single calcareous shell that is closed at the apex, sometimes spiralled, not chambered, and generally asymmetrical, e.g., a snail.

glacier - A large mass of ice formed by the compaction of snow that survives from year to year without completely melting; glaciers vary in size from small valley glaciers to huge ice sheets of continental dimensions.

gneiss - A coarse-grained metamorphic rock having banding of alternate light and dark minerals (gneissosity).

gneissic, gneissose - Pertaining to features of a gneiss.

Gondwana - The ancient continental landmass, comprising the present South America, Africa, India, Australia and Antarctica, that split apart during Mesozoic time.

gossan - An iron-rich, rusty-coloured, weathering product formed over rocks commonly containing sulphide minerals, in particular iron sulphides.

graded bedding - A type of bedding in which each layer is characterized by a progressive decrease in grain size from the bottom to the top of the bed.

grain - Particles or discrete crystals which comprise a rock or sediment.

grain size - Size of the particles or crystals that constitute a rock or sediment; hence fine-, medium-, or coarse-grained.

granite - Light-coloured, coarse-grained felsic igneous intrusive rock composed of quartz, feldspar and ferromagnesium minerals such as mica and hornblende; it is the intrusive equivalent of rhyolite.

graptolite - An extinct animal, the fossil remains of which look like tiny, serrated black rods.

group - In geologic mapping, a unit that consists of two or more formations.

gypsum - A soft, white mineral consisting mainly of calcium sulphate, formed during the evaporation of sea water; it is used in the manufacture of plaster of paris and wallboard.


hanging valley - In a glaciated landscape, a tributary valley whose mouth is in the steep wall of the main valley high above its floor.

hematite - A grey, metallic or earthy iron oxide mineral, which powders to a red colour; the principal ore of iron; specular hematite is a shiny, hard variety of the mineral and is used for jewellery as "Alaska Black Diamond".

hornblende - A black or dark-green mineral that forms needle-shaped or fibrous crystals; it is the most common member of the amphibole group and occurs in both igneous and metamorphic rocks.


igneous rock - Rock that has crystallized from molten material, known as magma.

intrusion - A mass of igneous rock that, while it was liquid, intruded into older rock, then solidified.

intrusion breccia - A rock composed of angular pieces of country rock in a matrix of intrusive igneous rock.

intrusive rock - Igneous rock that forms intrusions; as opposed to volcanic or extrusive rock which is erupted from a volcano.


joint - A crack in a rock along which there has been no relative movement.


Laurasia - The ancient continent that split apart in Mesozoic time to form Europe, Asia, North America and Greenland.

Laurentia - The ancient North American continent that lay on the west side of the Iapetus (or proto-Atlantic) Ocean in Cambrian time, 500 million years ago.

lava - Liquid rock that comes out of a volcano and is extruded onto the earth's surface; also refers to the same material that has solidified by cooling.

limestone - A sedimentary rock type composed of mostly calcium carbonate, that formed by chemical precipitation from sea water or by accumulation of fossils with carbonate shells.


mafic (mineral) - A dark-coloured silicate mineral rich in iron and/or magnesium; ferromagnesium minerals, include olivine, pyroxene, amphibole and biotite. It is the opposite of silicic or felsic.

mafic (rock) - An igneous rock that is dark in colour containing more than 50 percent ferromagnesium minerals such as amphibole, pyroxene and olivine (e.g., basalt, gabbro).

magma - Liquid from which all igneous rock crystallizes, either as volcanic rock or intrusive rock.

magma chamber - A space within the earth occupied by molten rock (magma).

magnesite - A white mineral composed of magnesium carbonate and an important source of magnesium; magnesite is a common alteration mineral in ultramafic rocks.

magnetite - A black, magnetic, iron oxide mineral, and an important source of iron.

mantle - The part of the earth's interior between the crust and the core; composed mainly of ultramafic rocks.

marble - A metamorphic rock type formed by metamorphosism of limestone or dolostone.

matrix - The finer grained rock material filling the spaces between larger particles or crystals in a rock.

meander - A broad, looping bend in a river.

metamorphic rock - A rock converted from pre-existing rock in the earth's crust by changes in temperature and pressure or by chemical action of fluids.

mica - A group of silicate minerals that cleave perfectly into thin sheets (see biotite, muscovite).

migmatite - A mixture of igneous and metamorphic rocks in which thin dykes and stringers of granitic material interfinger with metamorphic rocks.

mine - Underground excavation and workings for the purpose of extracting minerals.

mineral - A naturally occurring substance that has a fixed chemical composition and a characteristic crystal form.

moraine - A mound or ridge of debris left by melting glaciers, composed of a mixture of rock fragments, gravel, sand and clay.

mud crack - A crack formed in a layer of mud or silt resulting from the contraction that accompanies drying; they are generally polygonal.

muscovite - A light-coloured variety of the mica group of minerals containing potassium and aluminum; muscovite splits easily into thin transparent sheets.

mylonite - A fine-grained metamorphic rock formed by extreme crushing and pulverizing along a fault zone.


oceanic crust - That part of the earth's crust that underlies the main ocean basins; it is composed of mafic igneous rocks covered by a thin veneer of sediments and is continually renewed at mid-ocean ridges and consumed at subduction zones (see ophiolite).

olivine - A green, iron-magnesium silicate mineral found particularly in basalt, gabbro and ultramafic rock: gem varieties are known as peridot.

ophiolite - A group of rocks originally formed as oceanic crust, consisting of ultramafic rock, gabbro, diabase, basalt and deep-sea sedimentary rocks.

ore - The naturally occurring materials from which minerals of economic value can be profitably extracted; also the mineral(s) thus extracted.

orogeny - A major period of mountain-building.

outcrop - An exposure of bedrock.

oxbow lake - A lake formed in the channel of an abandoned meander, i.e., the river forms a new channel, isolating the bend or meander.


pegmatite - A very coarse-grained igneous rock type, usually found as dykes or veins, and commonly composed of quartz, feldspar and mica.

peridotite - Dark-coloured, igneous rock, composed of mainly olivine and pyroxene (see serpentinite).

perched boulder - A large block of rock that has been left by melting ice in a prominent and relatively precarious position.

pillow lava - Rounded masses of volcanic rock, usually basalt, that have been formed by rapid cooling of lava under water.

plate tectonics - The theory that the earth's crust is composed of a number of individual plates that move in response to convection in the upper mantle. Margins of plates are sites of considerable geologic activity. Plate tectonics explains how continents drift and oceans open and close.

porphyritic - A texture in igneous rocks in which some crystals are distinctly larger than others.

porphyroblast - Large grains or crystals, commonly perfectly shaped, developed in schists during metamorphism.

porphyry - An igneous rock which has porphyritic texture.

plutonic rock - Igneous rock formed deep beneath the Earth's surface. These rocks generally cooled slowly therefore their crystals are large, i.e., coarse-grained.

pyrite - A metallic, pale yellow, cubic mineral composed of iron and sulphur, commonly known as "fool's gold".

pyroclastic - Pertaining to fragmental material formed by volcanic explosions.

pyroxene - A common, dark-coloured, silicate mineral which contains iron and magnesium and is a common constituent of gabbro, anorthosite and some ultramafic rocks.


quarry - An open pit from which rock such as slate, limestone or building stone is extracted.

quartz - An important rock-forming mineral that is glassy, white and relatively hard; it consists of silicon and oxygen.

quartzite - A white or brownish metamorphic rock consisting mainly of quartz, and formed by the metamorphism of quartz-rich sandstone.


raised beach - An ancient beach that is located above the present shoreline; it indicates that sea level has fallen and/or land has risen.

reef - A ridge of rocks, sand or gravel that stands above the level of the sea floor and reaches to the surface; many modern reefs consist of the hard skeletons of corals and fossil reefs have been formed by corals, archeocyathids and algae.

rhyolite - A generally light-coloured volcanic rock, high in quartz content; rhyolite is the extrusive equivalent of granite.

ripple marks - Small ridges produced on a surface of sand or mud by the movement of wind or water.

roìche moutonneÇe - A knob of bedrock that has been striated and rounded by glaciers, with a gentle slope facing the up-ice direction.


sandstone - A sedimentary rock composed of sand-sized particles.

schist - A medium- or coarse-grained metamorphic rock that is rich in mica or chlorite, and has a strong foliation, allowing it to split easily in one direction.

schistosity - The type of foliation that characterizes schist, resulting from the parallel arrangement of coarse-grained platy minerals, such as mica, chlorite and talc.

scree slope - A steep slope consisting of loose, broken rock fragments. Also called talus slope.

sediment - Unconsolidated material such as gravel, sand or mud that is transported and deposited by wind, water, ice or gravity.

sedimentary rock - Rock formed by accumulation and cementing of loose sediment (e.g., sandstone), the deposition of chemical compounds held in solution in water (e.g., limestone), or by the accumulation of animal or plant debris (e.g., coal).

serpentine - A greenish metamorphic mineral characterized by long, waxy or fibrous crystals; asbestos is one form of serpentine; usually associated with ultramafic rocks.

shale - A fine-grained, layered sedimentary rock formed from mud or clay.

sichelwannen - Curved grooves on rock surfaces formed by water that was under immense pressure at the base of a glacier.

silicate - A silica-rich igneous rock or magma, e.g., granite and rhyolite.

sill - A sheet of igneous rock that has intruded along the layering of sedimentary or metamorphic rock.

silt - Unconsolidated sediment that contains particles larger than those in clay but smaller than those in sand.

siltstone - A fine-grained sedimentary rock composed mostly of silt-size particles.

slate - A fine-grained, metamorphic rock formed from shale, that has excellent cleavage; it tends to split easily into thin layers, and is used for roofing and floors (see cleavage).

specular hematite (or specularite) - A type of hematite (iron oxide) that is very hard, has a metallic lustre and is known as "Alaska Black Diamond", used for jewellery.

sphalerite - A mineral composed of zinc, sulphur and some iron and the main source of zinc; colour ranges from yellow to black ("black jack") as iron content increases.

spit - A sandy bar projecting from the mainland into open water. Spits are formed by deposition of sediment moved by longshore currents.

stack - A pillar-shaped body of rock close to the shoreline that has been separated from the land by the erosion of intervening rocks.

staurolite - A brown, iron-rich silicate mineral that tends to form cross-shaped crystals; it is found in metamorphic rocks.

stock - A small, roughly circular intrusion of igneous rock, usually less than 100 square km.

stoss and lee - An arrangement of small hills or prominent rocks in glaciated terrain having gentle slopes on the stoss side and somewhat steeper, pitched slopes on the lee side, the reverse direction of crag and tail.

stratified - Layered or bedded; refers to sedimentary rock.

striations - Scratches formed on a rock surface by rock fragments frozen into the bottom of a moving glacier.

strike - The direction of a horizontal line on a plane (e.g., bedding, cleavage or fault plane); strike is perpendicular to dip direction (see dip).

stromatolite - A generally domal- shaped, laminated, calcareous sedimentary structure, formed in a shallow-water environment under the influence of a mat or assemblage of sediment-binding blue-green algae that trap fine, silty detritus and precipitate calcium carbonate and that commonly develop colonies.

syenite - An intrusive igneous rock consisting of mostly feldspar, with a little hornblende or biotite.

syncline - (see synform)

synform - A fold in which the limbs dip toward the axis of the fold. If the youngest rocks are in the central core of the fold, it is called a syncline


talus slope - See scree slope.

terrace - A level surface part of the way up a slope, and often made by waves or a flowing stream.

till - Unsorted (all sizes of particles mixed together) and unstratified loose material deposited by a glacier.

tillite - A rock formed by consolidation of glacial till.

tourmaline - A silicate mineral that contains boron and usually occurs as long, black, three-sided, striated crystals; tourmaline is common in granite pegmatite veins and some metamorphic rocks.

trilobite - An extinct marine animal found as fossil remains that is distantly related to crabs and lobsters; they had a segmented exterior skeleton that was divided into 3 main parts - head, body and tail, and which was shed periodically as the animal grew.

tuff - A fine-grained rock composed of volcanic ash.


ultramafic rock - An igneous rock composed of mainly olivine and pyroxene; it forms an important part of oceanic crust.

unconformity - A discontinuity in the succession of rocks, which represents a gap in time and deposition. A surface separating younger rocks from much older rocks.


vein - A thin, sheet-like body of igneous rock or of minerals such as quartz, calcite, barite, etc., deposited in a crevice or fracture in a rock.

vesicle - A small round hole formed in volcanic rock by a gas bubble trapped when the rock solidified.

volcanic rock - Igneous rock that solidified on surface following a volcanic eruption; extrusive rock or lava; generally cools quickly, therefore is fine grained.


wall rock - The rock enclosing a vein or mineral deposit; the rock intruded by and surrounding an igneous intrusion (see country rock).


younging direction - A term used to identify the top of a sedimentary sequence. Moving towards the top of a sedimentary sequence the material becomes younger or more recently deposited.

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