Rocks

Geologists classify rocks in three groups, according to the major Earth processes that formed them. The three rock groups are igneous, sedimentary, and metamorphic rocks. 

Igneous
Igneous rocks are formed when magma (molten rock deep within the earth) cools and hardens. Sometimes the magma cools inside the earth, and other times it erupts onto the surface from volcanoes (in this case, it is called lava). When lava cools very quickly, no crystals form and the rock looks shiny and glasslike. Sometimes gas bubbles are trapped in the rock during the cooling process, leaving tiny holes and spaces in the rock.

Examples of this rock type include basalt and obsidian.

Sedimentary
Sedimentary rocks are formed from particles of sand, shells, pebbles, and other fragments of material. Together, all these particles are called sediment. Gradually, the sediment accumulates in layers and over a long period of time hardens into rock. Generally, sedimentary rock is fairly soft and may break apart or crumble easily. You can often see sand, pebbles, or stones in the rock, and it is usually the only type that contains fossils.

Examples of this rock type include conglomerate and limestone.

Metamorphic
Metamorphic rocks are formed under the surface of the earth from the metamorphosis (change) that occurs due to intense heat and pressure (squeezing). The rocks that result from these processes often have ribbonlike layers and may have shiny crystals, formed by minerals growing slowly over time, on their surface.

Examples of this rock type include gneiss and marble.

Rock-forming and rock-destroying processes have been active for billions of years. Today, in the Guadalupe Mountains of western Texas, one can stand on limestone, a sedimentary rock, that was a coral reef in a tropical sea about 250 million years ago. In Vermont's Green Mountains one can see schist, a metamorphic rock, that was once mud in a shallow sea. Half Dome in Yosemite Valley, Calif., which now stands nearly 8,800 feet above sea level, is composed of quartz monzonite, an igneous rock that solidified several thousand feet within the Earth. In a simple rock collection of a few dozen samples, one can capture an enormous sweep of the history of our planet and the processes that formed it.

Names of Rocks

A

Adakite - a class of basaltic rocks containing relatively small amounts of the trace elements yttrium and ytterbium
Andesite - an Intermediate volcanic rock
Anorthosite - an igneous ultramafic rock composed predominantly of plagioclase
Anthracite - a type of coal
Aplite - a very fine grained intrusive igneous rock
Argillite - a sedimentary rock composed primarily of clay-sized particles
Arkose - a sedimentary rock similar to sandstone

 B
Banded iron formation - a fine grained chemical sedimentary rock composed of iron oxide minerals
Basalt - a volcanic rock of mafic composition
Basanite - a volcanic rock of mafic composition; essentially a silica undersaturated basalt
Blueschist - a metamorphic rock composed of sodic amphiboles with a distinct blue colour
Boninite - a high-magnesian basalt dominated by pyroxene
Breccia - a sedimentary or tectonic rock composed of fragments of other, broken rocks

 C

Bituminous coal seam in West VirginiaCarbonatite - a rare igneous rock composed of >50% carbonate minerals
Cataclasite - a rock formed by faulting
Chalk - a sedimentary rock composed primarily of coccolith fossils
Charnockite - a rare type of granite containing pyroxene
Enderbite - a variety of charnockite
Chert - a fine grained chemical sedimentary rock composed of silica
Claystone - a sedimentary rock formed from clay
Coal - a sedimentary rock formed from organic matter
Conglomerate - a sedimentary rock composed of large rounded fragments of other rocks
Diamictite - a poorly sorted conglomerate
Coquina - a sedimentary carbonate rock formed by accumulation of abundant shell fossils and fragments

 D

Dolomite crystals from Touissite, MoroccoDacite - a felsic to intermediate volcanic rock with high iron content
Diabase or dolerite - an intrusive mafic rock forming dykes or sills
Diatomite - a sedimentary rock fromed from diatom fossils
Diorite - a coarse grained intermediate plutonic rock composed of plagioclase, pyroxene and/or amphibole
Dolomite or dolostone - a carbonate rock composed of the mineral dolomite +/- calcite
Dunite - an ultramafic cumulate rock composed of olivine and accessories

E
Eclogite - an ultra-high grade metamorphosed basalt or gabbro; also a facies of metamorphic rocks
Essexite - a silica undersaturated mafic plutonic rock (essentially a foid-bearing gabbro)
Evaporite - a chemical sedimentary rock formed by accumulation of minerals after evaporation

F
Flint - a form of chert
Foidolite - a plutonic igneous rock composed of >90% feldspathoid minerals

 G
Gabbro - a coarse grained plutonic rock composed of pyroxene and plagioclase basically equivalent to basalt
Gneiss - a coarse grained metamorphic rock
Gossan - the product of the weathering of a sulfide rock or ore body
Granite - a coarse grained plutonic rock composed of orthoclase, plagioclase and quartz
Granodiorite - a granitic plutonic rock with plagioclase > orthoclase
Granophyre - a subvolcanic intrusive rock of granitic composition
Granulite - a high grade metamorphic rock formed from basalt; also a facies of metamorphic rocks
Greywacke - an immature sandstone with quartz, feldspar and rock fragments within a clay matrix
Greenschist - a generic term for a mafic metamorphic rock dominated by green amphiboles
Greenstone
Gritstone - essentially a coarse sandstone formed from small pebbles

 H
Harzburgite - a variety of peridotite; an ultramafic cumulate rock

Hawaiite - a class of basalts formed from Ocean Island (hot spot) magmatism
Hornblendite - a mafic or ultramafic cumulate rock dominated by >90% hornblende
Hornfels - a metamorphic rock formed by heating by an igneous rock
Hyaloclastite - a volcanic rock composed primarily of glasses and glassy tuff

 I
Icelandite - a volcanic rock
Ignimbrite - a fragmental volcanic rock
Ijolite - a very rare silica-undersaturated plutonic rock

 J
Jaspillite - an iron-rich chemical sedimentary rock similar to chert or banded iron formation

 K
Kimberlite - a rare ultramafic, ultrapotassic volcanic rock and a source of diamonds
Komatiite - an ancient ultramafic volcanic rock

 L
Lamproite - an ultrapotassic volcanic rock
Lamprophyre - an ultramafic, ultrapotassic intrusive rock dominated by mafic phenocrysts in a feldspar groundmass
Latite - a silica undersaturated form of andesite
Lherzolite - an ultramafic rock, essentially a peridotite
Lignite - a sedimentary rock composed of organic material; otherwise known as Brown Coal
Limestone - a sedimentary rock composed primarily of carbonate minerals

 M
Marble - a metamorphosed limestone
Marl - a limestone with a considerable proportion of silicate material
Migmatite - a high grade metamorphic rock verging upon melting into a magma
Monzogranite - a silica undersaturated granite with <5% normative quartz
Monzonite - a plutonic rock with <5% normative quartz
Mudstone - a sedimentary rock composed of clay and muds
Mylonite - a metamorphic rock formed by shearing

N
Nepheline syenite - a silica undersaturated plutonic rock with nepheline replacing orthoclase
Nephelinite - a silica undersaturated plutonic rock with >90% nepheline
Norite - a hypersthene bearing gabbro

 O

Oil shale - a sedimentary rock composed dominantly of organic material
Oolite - a chemical sedimentary limestone

 P
Pegmatite - a metamorphic or igneous rock with giant sized crystals
Pelite - a metamorphic rock with a protolith of clay-rich (siltstone) sedimentary rock
Peridotite - a plutonic or cumulate ultramafic rock composed of >90% olivine
Phonolite - a silica undersaturated volcanic rock; essentially similar to nepheline syenite
Phyllite - a low grade metamorphic rock composed mostly of micaceous minerals
Picrite - an olivine-bearing basalt
Psammite - a metamorphic rock with a protolith of quartz-rich (sandstone) sedimentary rock
Pseudotachylite - a glass formed by melting within a fault via friction
Pumice - a fine grained, extremely vesicular volcanic rock
Pyroxenite - a coarse grained plutonic rock composed of >90% pyroxene

Q

Quartzite - a metamorphosed sandstone typically composed of >95% quartz
Quartz diorite - a diorite with >5% modal quartz
Quartz monzonite - an intermediate plutonic rock, essentially a monzonite with 5-10% modal quartz

 R

Rhyodacite - a felsic volcanic rock which is intermediate between a rhyolite and a dacite
Rhyolite - a felsic volcanic rock
Comendite - a peralkaline rhyolite
Pantellerite - an alkaline rhyolite-rhyodacite with amphibole phenocrysts

 S
Sandstone - a clastic sedimentary rock defined by its grain size
Schist - a low to medium grade metamorphic rock
Scoria - an extremely vesicular mafic volcanic rock
Serpentinite - a metamorphosed ultramafic rock dominated by serpentine minerals
Shale - a clastic sedimentary rock defined by its grain size
Siltstone - a clastic sedimentary rock defined by its grain size
Skarn - a metasomatic rock
SlateSlate - a low grade metamorphic rock formed from shale or silts
Sovite - a coarse grained carbonatite rock
Suevite - a rock formed by partial melting during a meteorite impact
Syenite - a plutonic rock dominated by orthoclase feldspar; a type of granitoid

 T
Talc carbonate - a metamorphosed ultramafic rock with talc as an essential constituent; similar to a serpentinite
Tachylyte - essentially a basaltic glass
Tephrite - a silica undersaturated volcanic rock; can be a generic term
Tonalite - a plagioclase-dominant granitoid
Trachyandesite - an alkaline intermediate volcanic rock
Benmoreite - sodic trachyandesite
Basaltic trachyandesite
Mugearite - sodic basaltic trachyandesite
Shoshonite - potassic basaltic trachyandesite
Trachyte - a silica undersaturated volcanic rock; essentially a feldspathoid-bearing rhyolite
Troctolite - a plutonic ultramafic rock containing olivine, pyroxene and plagioclase
Turbidite (Gorgoglione Flysch), Miocene, South ItalyTrondhjemite - a form of tonalite where plagioclase-group feldspar is oligoclase
Tuff - a fine grained volcanic rock formed from volcanic ash
Turbidite - a particular sequence of sedimentary rocks which form within the deep ocean environment

 U

 V

W
Wackestone - a matrix-supported carbonate sedimentary rock
Websterite - a type of pyroxenite, composed of clinoproxene and orthopyroxene
Wehrlite - an ultramafic plutonic or cumulate rock, a type of peridotite, composed of olivine and clinopyroxene

X

 Y

 Z

Credit: USGS, University of Wisconsin-Stevens Point Karen A. Lemke