Rock types

Rock type is usually defined as a particular kind of rock having a specific set of characteristics1. Rock types are specific assemblages of minerals (most rocks are composed of minerals). Rocks are much more vaguely defined than minerals. Even rocks within one rock type may have a highly variable composition.

Still, there is a fundamentally sound reason to talk about such a vague concept as a rock type because these assemblages occur again and again in many different locations. Rock type is like a biome (desert, savannah, rain forest). One particular desert may greatly differ from another (one is sandy and another rocky) but they both share something that is common (low precipitation). Exactly the same is true with rock types — one granite may be white and another red but they both have a similar composition (major minerals are feldspar and quartz).

Here is a list of major rock types recognized by geologists. Immense number of rock types have been defined (their definitions often overlapping) and many rock classifications exist. However, there seems to be a fair number of terms and classification principles that have successfully stood the test of time.

IGNEOUS ROCKS

It all starts with igneous rocks. The Earth is believed to have been entirely molten in its early stages of development. Hence, all other rock types are derivatives of igneous rocks.

ANDESITE is a volcanic rock with a so-called average composition. It is more felsic than basalt and more mafic than dacite. Andesite is a common rock of subduction zones volcanoes. It is generally dark gray and may contain phenocrysts of plagioclase (usually andesine), biotite, and hornblende.

ANORTHOSITE is a plutonic rock which is composed of plagioclase (>90%). Anorthosite is a fascinating and enigmatic rock type. Some anorthosites display very beautiful display of colors known as labradorescence and are therefore valuable ornamental stones. But anorthosites are interesting for many other reasons as well. They seem to have formed during unique episode of the ancient Earth more than a billion years ago. Anorthosites are associated with valuable mineral resources and last but not least, the same rock type is very widespread on the Moon. The highlands of the Moon which we see as brighter areas are largely composed of anorthosite which is, however, quite different in appearance from its terrestrial counterparts. Read more…

BASALT is a very common dark-colored volcanic rock composed of calcic plagioclase (usually labradorite), clinopyroxene (augite) and iron ore (titaniferous magnetite). Basalt may also contain olivine, quartz, hornblende, nepheline, orthopyroxene, etc. Basalt is a volcanic equivalent of gabbro. Read more…

BASANITE is a volcanic rock. It is similar to basalt but contains less silica and usually more alkali metals. Basanite may contain olivine, pyroxene, and feldspathoid phenocrysts.
00259 carbonatite CARBONATITE is an igneous rock that is predominantly composed of carbonate minerals. Both volcanic and plutonic carbonatites are known.

DIABASE is a dark-colored igneous rock. It is compositionally equivalent to gabbro and basalt but texturally between them. Diabase is a common rock type. It occurs mostly in shallow intrusions (dikes and sills) of basaltic composition. Diabase grades to basalt when it solidifies rapidly and to gabbro when more time is given to the crystals to grow. Read more…

DIORITE is a plutonic rock with sodium-rich plagioclase and hornblende.
Dunite xenolith in basaltic lava DUNITE is an ultramafic plutonic rock that is composed almost exclusively of olivine (>90%). Dunite is a sub-type of peridotite. Read more…

GABBRO is a coarse-grained and usually dark-colored igneous rock. Gabbro is an intrusive rock. It means that it formed as magma cooled slowly in the crust. Igneous rocks with similar composition are basalt (extrusive equivalent of gabbro) and diabase (the same rock type could be named dolerite or microgabbro instead). Read more…
Granite GRANITE is a very common plutonic rock with K-feldspar and quartz as the main constituents.
Granodiorite GRANODIORITE is a granitoid (quartz-rich plutonic rock) in which the dominant feldspar is plagioclase.
Harzburgite HARZBURGITE is what remains when basalt is taken out of the mantle. Harzburgite is a sub-type of peridotite. Harzburgite is composed of orthopyroxene and olivine, it is one of the ultramafic rocks and belongs into the peridotite group. These are rocks that are abundant in the mantle but scarce on the surface. Read more…

HAWAIITE is a sodic trachybasalt. Read more…

IGNIMBRITE is a fiery rock. It is a pyroclastic rock formed by very hot ground-hugging cloud of volcanic ash, blocks, and gases known as pyroclastic flow or pyroclastic density current. Ignimbrite is synonymous with flood tuff, welded tuff, ash-flow tuff and pyroclastic flow deposit3. Read more…
Limburgite LIMBURGITE is a variety of basanite occurring in Limberg, Germany. Read more…

MONZONITE is a plutonic rock intermediate in composition between syenite and diorite. Monzonite contains approximately equal amount of K-feldspar and plagioclase. Read more…
Obsidian OBSIDIAN is a volcanic glass — a rock type that contains no crystals. Read more…
Tourmaline pegmatite PEGMATITE is an exceptionally coarse-grained plutonic rock. Most pegmatites have a granitic composition. Read more…
Peridotite PERIDOTITE is the principal rock type in the Mantle. Unaltered peridotite is usually dark green in color but most peridotites we see in the surface are altered to a metamorphic rock type serpentinite. There are several sub-types of peridotite: harzburgite, dunite, lherzolite, etc. Read more…

PICRITE is a mafic igneous rock similar to basalt but with more olivine.
Rhyolite quartz porphyry PORPHYRY is an igneous rock characterized by porphyritic texture. Porphyritic texture is a very common texture in igneous rocks in which larger crystals (phenocrysts) are embedded in a fine-grained groundmass. Read more…

PUMICE is a lava froth — an extremely vesicular and light-weight volcanic rock with a variable composition. Read more…

PYROXENITE is a plutonic rock that is composed almost exclusively (over 90%) of pyroxene minerals.

RAPAKIVI is a variety of granite with K-feldspar phenocrysts that are mantled with plagioclase. Read more…

RHYOLITE is a felsic volcanic rock. It is an extrusive analogue of granite.

SCORIA is a highly vesicular mafic dark-colored volcanic rock. Read more…
Syenite (igneous rock type) SYENITE is a plutonic rock that contains little or no quartz and in which K-feldspar dominates over plagioclase. Read more…

TEPHRITE is a volcanic rock resembling basalt but it is rich in feldspathoid minerals. This makes it very similar to basanite but tephrite contains very little olivine.
Tonalite TONALITE is a granitoid in which almost all the feldspar (>90%) is plagioclase. Trondhjemite and plagiogranite are varieties of tonalite.
Trachyte TRACHYTE is a feldspar-rich rock. It is a volcanic equivalent of syenite. Read more…
Tuff in Auvergne TUFF is a volcanic analogue of sandstone. Read more…

SEDIMENTARY ROCKS

Sedimentary rocks are products of the consolidation of loose sediments which are in most part bits and pieces of other disintegrated rocks. There are three main types of sedimentary rocks: mudstone, sandstone, limestone. Sure, this is very simplistic approach but volumetrically almost all sedimentary rocks fall under these categories if we give them relatively broad definition e.g. sandstone also includes siltstones and conglomerates, and limestone includes all sedimentary carbonate rocks. There are notable exception that do not fit into this classification scheme like evaporites, coal, and chert but volumetrically they are clearly less important. In the list below, of course, these exceptions are not neglected.

ARKOSE is a feldspar-rich (25%) sandstone. Read more…

BAUXITE is an aluminum-rich sedimentary rock. It is a principal ore of aluminum. Aluminum in bauxite is hosted by aluminum hydroxide minerals, mostly gibbsite. The major impurities are iron oxides and hydroxides (which give reddish color to most bauxites) and clay minerals. Bauxite is a weathering product of aluminum-bearing rocks. Read more…

BOG IRON is a type of iron ore composed of limonite with impurities like clay or plant debris. Read more…
Breccia BRECCIA is a shattered rock that is composed of highly angular fragments held together by mineral cement.

CHERT is a hard and compact sedimentary rock, consisting dominantly of very small quartz crystals. Read more…

COAL is a combustible rock containing more than 50% by weight of carbonaceous material. Coal in most cases is a lithified peat.
Conglomerate from Cyprus CONGLOMERATE is a lithified gravel (rounded rock fragments or mineral grains larger than 2 mm in size). Read more…
Coquinite COQUINA is a detrital limestone consisting of shells or shell fragments. Read more…

DIATOMITE is a light-colored sedimentary rock composed chiefly of siliceous shells (frustules) of diatoms. Read more…

DOLOMITE or dolostone is a carbonate rock containing more than 50% by weight of the mineral dolomite. Read more…

GYPSUM as a rock type is an aggregate of gypsum crystals.
Laterite LATERITE is a reddish weathering product of basaltic (mostly) rocks. Read more…
Crinoid limestone LIMESTONE is a sedimentary rock that is chiefly composed of calcium carbonate. Read more…

OIL SHALE is a rock that yields oil (shale oil) upon heating without oxygen. Read more…
Ooid sand OOLITE is a rock made up of small, generally rounded grains with internal growth bands that are named ooids. Most oolites are limestones (ooids are composed of calcium carbonate). Read more…

PHOSPHORITE is a sedimentary rock containing significant amount of phosphate material.

SANDSTONE is a lithified sand. Read more…

SHALE is a laminated rock consisting of mostly clay minerals. Shale is a lithified mud.

SILTSTONE is similar to sandstone but it is composed of very fine dusty grains.
Travertine breccia TRAVERTINE is a chemically precipitated limestone. Stalactites and other speleothems in karst caves are made of this rock type.

UMBER is a marine sediment associated with black smokers. Read more…

METAMORPHIC ROCKS

Metamorphic rocks are derived from other pre-existing rocks by mineralogical and/or structural changes. Metamorphism takes place in the solid state and at elevated pressure and temperature generally at depth in the crust1.

AMPHIBOLITE is a metamorphic rock composed of amphiboles and plagioclase. Read more…

ANTHRACITE is a type of coal with a highest rank. Coal rank measures the carbon content which is in correlation with calorific value and metamorphic grade. Anthracite has a very high carbon (over 90%) and low volatile content (below 5%). Read more…
Glaucophane schist BLUESCHIST is a bluish metamorphic rock containing amphibole glaucophane. Blueschist forms at high pressure and low temperature (subducting slab of lithosphere). Its protoliths are basaltic rocks.
CHARNOCKITE is an orthopyroxene-bearing unhydrous metamorphic rock. Read more…
ECLOGITE is a beautiful rock consisting of bright green pyroxene omphacite and red garnet. Eclogite forms at high-pressure conditions. Its protolith is a basaltic rock. Read more…

EPIDOSITE is a hydrothermally altered basaltic rock consisting of epidote and quartz. Read more…

GNEISS is a banded coarse-grained metamorphic rock that formed deep in the crust beneath forming mountain ranges. Read more…
GREISEN is a pneumatolytically altered granitic rock. Greisen contains lots of quartz and mica plus several exotic minerals like cassiterite, tourmaline, topaz, etc. Read more…

HORNFELS is a fine-grained compact rock rock formed by contact metamorphism (low pressure, high temperature).
Marble from Karelia MARBLE is a metamorphosed (recrystallized) carbonate rock (limestone, dolomite rock). Marble is mostly composed of calcium carbonate.
Migmatite from Norway MIGMATITE is an inhomogenous rock consisting of dark-colored metamorphic rock and light-colored igneous leucosomes. Igneous or igneous-looking component of the rock is either the result or partial melting or material intruded into the rock.

QUARTZITE is a metamorphic rock consisting largely or entirely of quartz2. In vast majority of cases, quartzite is a metamorphosed sandstone. Read more…

SCHIST is a strongly foliated rock rich in platy or elongated minerals. Schist forms during regional metamorphism. Composition may be highly variable.

SERPENTINITE is a metamorphosed ultramafic rock. It is composed of serpentine minerals. Read more…

SHUNGITE is a metamorphosed oil shale consisting of structureless carbon. Read more…

References

1. Jackson, J. A. (1997). Glossary of Geology, 4th Edition. American Geological Institute.
2. Barth, T. F. W. (2007). Quartzite. In: McGraw Hill Encyclopedia of Science & Technology, 10th Edition. McGraw-Hill. Volume 14. 722.
3. Tilling, Robert I. (2007). Ignimbrite. In: McGraw Hill Encyclopedia of Science & Technology, 10th Edition. McGraw-Hill. Volume 9. 20-21.