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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 magma deep in the Eearth'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
Suture:
A suture in structural geology is a major fault zone through an orogen or mountain range. Sutures separate terranes: tectonic units which have different plate tectonic, metamorphic and paleogeographic histories.
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
Ultramafic rock:
An igneous rock composed chiefly of mafic minerals, such as dunite, peridotite and pyroxenite.
Volcanic rock:
Igneous rock generated as a result of volcanic eruptions. Lava flows represent magma that has reached the Earth's surface.