Glossary of common geological terms

Simplified explanations of some commonly-used geological terms

Amphibolite:
A mafic metamorphic rock consisting mainly of the minerals hornblende and plagioclase

Andesite:
A fine-grained, mafic volcanic rock consisting of both mafic minerals and plagioclase, and also usually containing quartz

Banded iron formation:
A chemical sedimentary rock containing at least 15% iron of sedimentary origin, showing marked banding of iron-rich minerals and chert or quartz

Basalt:
A fine-grained, mafic volcanic rock consisting mainly of the minerals plagioclase, olivine, and pyroxene

Breccia:
A coarse-grained clastic rock composed of angular broken rock fragments held together by a mineral cement or in a fine-grained matrix; it differs from conglomerate in that the fragments have sharp edges and unworn corners

Calcarenite:
A limestone consisting mainly of detrital calcite particles of sand size

Calc-silicate rock:
A crystalline metamorphic rock formed mainly of calcium-bearing silicate minerals

Calcrete:
A calcareous duricrust

Chronostratigraphic:
A term used to describe a body of rock strata formed during a specific interval of geological time

Clastic:
Composed of broken fragments that have been derived from pre-existing rocks by weathering and erosion and transported some distance from their place of origin

Conglomerate:
A clastic sedimentary rock containing relatively large, rounded to subangular fragments (pebbles, cobbles or boulders), set in a fine-grained matrix of sand or silt

Dacite:
A fine-grained volcanic rock; the extrusive equivalent of granodiorite

Dolerite:
A mafic intrusive rock, similar in composition to, but finer-grained than, gabbro

Dolomite:
A sedimentary rock consisting mainly of the mineral dolomite (a calcium and magnesium carbonate)

Duricrust:
A general term for a hard crust on or near the surface of a soil formed in a semi-arid climate; e.g. calcrete, silcrete

Eon:
The longest unit of geological time, next in order of magnitude above an Era

Era:
A unit of geological time, next in order of magnitude below an Eon

Evaporite:
A non-clastic sedimentary rock composed mainly of minerals produced from a saline solution as a result of extensive or total evaporation; e.g. rock salt

Felsic rock:
Rock dominated by light-coloured minerals such as quartz and feldspar

Gabbro:
A coarse-grained, mafic intrusive rock composed mainly of the minerals plagioclase, pyroxene and olivine

Gneiss:
A metamorphic rock, commonly rich in quartz and feldspar, with a banded and foliated texture, formed at temperatures above about 550° Centigrade

Granite:
An intrusive rock dominated by quartz (10-50%)and alkali feldspar

Granodiorite:
An intrusive rock like granite but containing a high proportion of plagioclase feldspar and mafic minerals

Granulite:
A metamorphic rock, commonly granular in texture, which has cooled from temperatures above about 700° Centigrade

Intrusive rock:
Igneous rock crystallised from a magma deep in the earth’s crust

Komatiite:
Rocked formed of ultramafic lava, largely restricted to the Archaean of Western Australia

Laterite:
A highly weathered red subsoil rich in secondary oxides of iron and/or aluminium. Laterite develops in wet-tropical and warm to temperate regions and is a residual product of weathering

Limestone:
A sedimentary rock consisting mainly of the mineral calcite

Mafic rock:
Rock dominated by dark-coloured minerals such as pyroxene and hornblende

Magma:
Molten rock formed within the inner parts of the Earth which crystallises to form an igneous rock

Marble:
Metamorphosed limestone or dolomite

Metamorphic rock:
Metamorphism is a process that takes place when the heat and pressure deep in the Earth’s crust cause rocks to change their original mineral compositions and textures. The resulting metamorphic rocks can form from igneous, sedimentary or older metamorphic rocks

Metamorphic Complex:
The metamorphic rocks constituting a whole group closely related on a regional and/or stratigraphic basis

Mudstone:
A fine-grained sedimentary rock consisting mainly of clay mineral particles

Norite:
An intrusive rock similar to gabbro but containing an orthopyroxene (hypersthene) as the dominant mafic mineral

Orogeny:
A period of mountain-building

Period:
A geological time subdivision of an Era, during which the rocks of the corresponding System were formed

Peridotite:
A mafic igneous rock consisting mainly of the minerals of olivine and pyroxene

Phonolite:
A fine-grained alkaline volcanic rock, primarily composed of alkali feldspar and feldspathoid minerals

Phyllite:
A metamorphic rock, intermediate in grade between slate and mica schist

Pyroxenite:
An ultramafic intrusive rock, chiefly composed of the mineral pyroxene

Quartzite:
A crystalline rock formed mainly of quartz

Regolith:
The layer or mantle of fragmental and unconsolidated rock material, whether residual or transported, that covers the bedrock

Rhyolite:
A felsic volcanic rock; the extrusive equivalent of granite

Sandstone:
A clastic sedimentary rock composed of fragments of sand, set in a fine-grained matrix of silt or clay

Schist:
A crystalline metamorphic rock that can be readily split into thin flakes or slabs because of a strong foliation of the constituent minerals

Sedimentary rock:
Rock formed from the consolidation of sediments transported by water, wind or ice, or deposited by organisms

Serpentinite:
A rock consisting almost wholly of serpentine-group minerals, derived from the alteration of mafic minerals such as olivine and pyroxene

Shale:
A fine-grained sedimentary rock formed by the consolidation of clay or mud

Siltstone:
A sedimentary rock similar in composition to mudstone, but slightly coarser grained

Slate:
A compact, fine-grained metamorphic rock that splits into slabs and thin plates; generally formed from shale

Stratigraphic unit:
A body of adjacent rock strata recognised as a unit in the classification of a rock sequence with respect to any of the many attributes that rocks may possess

Stratotectonic:
A term used to describe tectonic evolution in relation to stratigraphy

Syenite:
An intrusive rock containing alkali feldspar, with minor plagioclase and mafic minerals and little quartz; similar to granite, but less quartz-rich

Supergroup:
An assemblage of related rock groups (which comprise a number of rock formations), having significant lithological features in common

Tillite:
A sedimentary rock formed from consolidation of glacial drift (till), which is deposited directly by and underneath a glacier, and consists of a heterogeneous mixture of clay, silt, sand, gravel and boulders

Tonalite:
An intrusive rock like granodiorite, but containing less quartz (20-60%) and plagioclase feldspar well in excess of alkali feldspar

Trachyte:
A fine-grained porphyritic volcanic rock, consisting mainly of alkali feldspar and minor mafic minerals; the extrusive equivalent of syenite

Trachybasalt:
A volcanic rock intermediate in composition between trachyte and basalt

Volcanic rock:
Igneous rock generated as a result of volcanic eruptions. Lava flows represent magma that has reached the earth’s surface

Ultramafic rock:
An igneous rock composed chiefly of mafic minerals, such as dunite, peridotite and pyroxenite