Discover How Rocks Are Formed!

What are the 3 basic types of rocks?

Just as any person can be put into one of two main categories of human being, all rocks can be put into one of three fundamentally different types of rocks. They are as follows:

Igneous Rocks (Fire Rocks)Igneous rocks started out far underground, where it is very hot and they were melted. Then they shot out of a volcano as lava, and cooled back into rocks as they fell back down to earth, or after they landed. Granite is a common igneous rock. Igneous rocks are crystalline solids which form directly from the cooling of magma. This is an exothermic process (it loses heat) and involves a phase change from the liquid to the solid state. The earth is made of igneous rock - at least at the surface where our planet is exposed to the coldness of space. Igneous rocks are given names based upon two things: composition (what they are made of) and texture (how big the crystals are).

How do composition and texture relate to igneous rocks?

Igneous rocks are crystalline solids which cool from magma: the liquid phase of solid rock. Magmas occur at depth in the crust, and are said to exist in "magma chambers," a rather loose term indicating an area where the temperature is great enough to melt the rock, and the pressure is low enough to allow the material to expand and exist in the liquid state. Many different types of igneous rocks can be produced. The key factors to use in determining which rock you have are the rock's texture and composition.

TextureTexture relates to how large the individual mineral grains are in the final, solid rock. In most cases, the resulting grain size depends on how quickly the magma cooled. In general, the slower the cooling, the larger the crystals in the final rock. Because of this, we assume that coarse grained igneous rocks are "intrusive," in that they cooled at depth in the crust where they were insulated by layers of rock and sediment. Fine grained rocks are called "extrusive" and are generally produced through volcanic eruptions.

Grain size can vary greatly, from extremely coarse grained rocks with crystals the size of your fist, down to glassy material which cooled so quickly that there are no mineral grains at all. Coarse grain varieties (with mineral grains large enough to see without a magnifying glass) are called phaneritic. Granite and gabbro are examples of phaneritic igneous rocks. Fine grained rocks, where the individual grains are too small to see, are called aphanitic. Basalt is an example. The most common glassy rock is obsidian. Obviously, there are innumerable intermediate stages to confuse the issue.

CompositionThe other factor is composition: the elements in the magma directly affect which minerals are formed when the magma cools. Again, we will describe the extremes, but there are countless intermediate compositions. (Composition relates to the mafic and felsic terms discussed in another question. If these terms are confusing, please refer to that discussion before continuing.)

The composition of igneous magmas is directly related to where the magma is formed. Magmas associated with crustal spreading are generally mafic, and produce basalt if the magma erupts at the surface, or gabbro if the magma never makes it out of the magma chamber. It is important to remember that basalt and gabbro are two different rocks based purely on textural differences - they are compositionally the same. Intermediate and felsic magmas are associated with crustal compression and subduction. In these areas, rock and sediment from the surface is subducted back into the crust, where it re-melts. This allows the differentiation process to continue, and the resulting magma is enriched in the lighter elements. Intermediate magmas produce diorite (intrusive) and andesite (extrusive). Felsic magmas, the final purified result of the differentiation process, lead to the formation of granite (intrusive) or rhyolite (extrusive).

Igneous Rock Classification

Texture vs. composition

Felsic Intermediate Mafic Ultramafic

Aphanitic

fine grain

Rhyolite Andesite BasaltConditions needed to produce ultramafic flows do not

exist in nature at this time.

 Intermediate  Dacite Diabase  

Phaneritic

coarse grain

Granite Diorite Gabbro Peridotite

Glassy Obsidian      

Frothy Pumice   Scoria  

It is important to note that there are many, many intermediate steps between these main divisions. Geology is a science full of "shades of gray," and the naming of igneous rocks is certainly no exception.

Elements & minerals common to various magmas Ultramafic magmasOlivine - Mg2SiO4 to Fe2SiO4Pyroxene - Ca(Mg,Fe,Al)(Al,Si)2O6 Mafic (basaltic) magmasOlivine - Mg2SiO4 to Fe2SiO4Pyroxene - Ca(Mg,Fe,Al)(Al,Si)2O6Plagioclase - CaAlSi3O8 to NaAlSi3O8 Intermediate magmasPlagioclase - CaAlSi3O8 to NaAlSi3O8Amphibole - NaCa2(Mg,Fe,Al)5(Si,Al)8O22(OH)2Muscovite/Biotite - KAl2(Si3Al)O10(OH)2Quartz - SiO2 Felsic (granitic) magmasPotash Feldspar - KAlSi3O8Quartz - SiO2Muscovite/Biotite - KAl2(Si3Al)O10(OH)2Amphibole - NaCa2(Mg,Fe,Al)5(Si,Al)8O22(OH)2

Igneous rocks are called fire rocks and are formed either underground or above ground. Underground, they are formed when the melted rock, called magma, deep within the earth becomes trapped in small pockets. As these pockets of magma cool slowly underground, the magma becomes igneous rocks.

Igneous rocks are also formed when volcanoes erupt, causing the magma to rise above the earth's surface. When magma appears above the earth, it is called lava. Igneous rocks are formed as the lava cools above ground.

Sedimentary Rocks (Sitting Rocks)

In most places on the surface, the igneous rocks which make up the majority of the crust are covered by a thin veneer of loose sediment, and the rock which is made as layers of this debris get compacted and cemented together. This is rock that was originally mud, or the shells of tiny sea creatures. The little bits of dust, or mud, or shells fall to the ground, or to the bottom of a pond or ocean, and get covered up by more layers of mud or shells. Eventually the pressure of all the stuff on top of them mashes them together into a hard rock. Limestone is a common sedimentary rock Sedimentary rocks are called secondary, because they are often the result of the accumulation of small pieces broken off of pre-existing rocks. There are three main types of sedimentary rocks:

Clastic: your basic sedimentary rock. Clastic sedimentary rocks are accumulations of clasts: little pieces of broken up rock which have piled up and been "lithified" by compaction and cementation.

Chemical: many of these form when standing water evaporates, leaving dissolved minerals behind. These are very common in arid lands, where seasonal "playa lakes" occur in closed depressions. Thick deposits of salt and gypsum can form due to repeated flooding and evaporation over long periods of time.

Organic: any accumulation of sedimentary debris caused by organic processes. Many animals use calcium for shells, bones, and teeth. These bits of calcium can pile up on the seafloor and accumulate into a thick enough layer to form an "organic" sedimentary rock.

For thousands, even millions of years, little pieces of our earth have been eroded--broken down and worn awayby wind and water. These little bits of our earth are washed downstream where they settle to the bottom of the rivers, lakes, and oceans. Layer after layer of eroded earth is deposited on top of each. These layers are pressed down more and more through time, until the bottom layers slowly turn into rock.

Metamorphic Rocks

The third kind of rock is made out of the first two kinds. We call these metamorphic, or "changed" rocks. Metamorphic rocks are not as common as the first two, but sometimes an igneous or a sedimentary rock meets up with a lot of heat or pressure and this makes crystals form in the rocks, or, if the rock already has crystals, it can make bigger crystals. This turns the rock into a new kind of rock. A good example is carbon turning into diamonds. The metamorphics get their name from "meta" (change) and "morph" (form). Any rock can become a metamorphic rock. All that is required is for the rock to be moved into an environment in which the minerals which make up the rock become unstable and out of equilibrium with the new environmental conditions. In most cases, this involves burial which leads to a rise in temperature and pressure. The metamorphic changes in the minerals always move in a direction designed to restore equilibrium. Common metamorphic rocks include slate, schist, gneiss, and marble.

Metamorphic rocks are rocks that have "morphed" into another kind of rock. These rocks were once igneous or sedimentary rocks. How do sedimentary and igneous rocks change? The rocks are under tons and tons of pressure, which fosters heat build up, and this causes them to change. If you exam metamorphic rock samples closely, you'll discover how flattened some of the grains in the rock are.

List of rock typesFrom Wikipedia, the free encyclopedia

This page is intended as a list of all unique rock types as recognized by petrologists. Names of non-rock types and archaic rock types are given as appendices.

Igneous

A sample of andesite (dark groundmass) with amygdaloidal vesicules filled with zeolite. Diameter of view is 8 cm.

Andesite - an Intermediate volcanic rock Anorthosite - an igneous ultramafic rock composed predominantly of plagioclase Aplite - a very fine grained intrusive igneous rock Basalt - a volcanic rock of mafic composition Adakite - a class of basaltic rocks containing relatively small amounts of the trace elements yttrium and

ytterbium Hawaiite - a class of basalts formed from Ocean Island (hot spot) magmatism Basanite - a volcanic rock of mafic composition; essentially a silica undersaturated basalt Boninite - a high-magnesian basalt dominated by pyroxene Carbonatite - a rare igneous rock composed of >50% carbonate minerals Charnockite - a rare type of granite containing pyroxene Enderbite - a variety of charnockite Dacite - a felsic to intermediate volcanic rock with high iron content Diabase or dolerite - an intrusive mafic rock forming dykes or sills Diorite - a coarse grained intermediate plutonic rock composed of plagioclase, pyroxene and/or amphibole Dunite - an ultramafic cumulate rock composed of olivine and accessories Essexite - a silica undersaturated mafic plutonic rock (essentially a foid-bearing gabbro) Foidolite - a plutonic igneous rock composed of >90% feldspathoid minerals Gabbro - a coarse grained plutonic rock composed of pyroxene and plagioclase basically equivalent to basalt 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 Harzburgite - a variety of peridotite; an ultramafic cumulate rock Hornblendite - a mafic or ultramafic cumulate rock dominated by >90% hornblende Hyaloclastite - a volcanic rock composed primarily of glasses and glassy tuff Icelandite - a volcanic rock Ignimbrite - a fragmental volcanic rock Ijolite - a very rare silica-undersaturated plutonic rock

Limey shale overlaid by limestone. Cumberland Plateau, Tennessee

Kimberlite - a rare ultramafic, ultrapotassic volcanic rock and a source of diamonds Komatiite - an ancient ultramafic volcanic rock 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 Monzogranite - a silica undersaturated granite with <5% normative quartz Monzonite - a plutonic rock with <5% normative quartz 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 Obsidian - a type of volcanic glass Pegmatite - an igneous rock (or metamorphic rock) with giant sized crystals Peridotite - a plutonic or cumulate ultramafic rock composed of >90% olivine Phonolite - a silica undersaturated volcanic rock; essentially similar to nepheline syenite Picrite - an olivine-bearing basalt

Quartzite

Porphyry - a rock, usually granitic, with a porphyritic texture 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

Quartz diorite - a diorite with >5% modal quartz Quartz monzonite - an intermediate plutonic rock, essentially a monzonite with 5-10% modal quartz 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 Scoria - an extremely vesicular mafic volcanic rock Sovite - a coarse grained carbonatite rock Syenite - a plutonic rock dominated by orthoclase feldspar; a type of granitoid 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 Trondhjemite - a form of tonalite where plagioclase-group feldspar is oligoclase Tuff - a fine grained volcanic rock formed from volcanic ash 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

Sedimentary

Argillite - a sedimentary rock composed primarily of clay-sized particles Arkose - a sedimentary rock similar to sandstone Banded iron formation - a fine grained chemical sedimentary rock composed of iron oxide minerals Breccia - a sedimentary or tectonic rock composed of fragments of other, broken rocks

Bituminous coal seam in West Virginia

Cataclasite - a rock formed by faulting Chalk - a sedimentary rock composed primarily of coccolith fossils 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

Dolomite crystals from Touissite, Morocco

Diatomite - a sedimentary rock formed from diatom fossils Dolomite or dolostone - a carbonate rock composed of the mineral dolomite +/- calcite Evaporite - a chemical sedimentary rock formed by accumulation of minerals after evaporation

Flint - a form of chert Greywacke - an immature sandstone with quartz, feldspar and rock fragments within a clay matrix Gritstone - essentially a coarse sandstone formed from small pebbles Itacolumite - porous, yellow sandstone Jaspillite - an iron-rich chemical sedimentary rock similar to chert or banded iron formation Laterite - a residual sedimentary rock formed from a parent rock under tropical conditions Lignite - a sedimentary rock composed of organic material; otherwise known as Brown Coal Limestone - a sedimentary rock composed primarily of carbonate minerals Marl - a limestone with a considerable proportion of silicate material Mudstone - a sedimentary rock composed of clay and muds Oil shale - a sedimentary rock composed dominantly of organic material Oolite - a chemical sedimentary limestone Sandstone - a clastic sedimentary rock defined by its grain size Shale - a clastic sedimentary rock defined by its grain size Siltstone - a clastic sedimentary rock defined by its grain size Travertine - a sedimentary rock containing calcite and iron oxides

Turbidite (Gorgoglione Flysch), Miocene, South Italy

Turbidite - a particular sequence of sedimentary rocks which form within the deep ocean environment Wackestone - a matrix-supported carbonate sedimentary

Metamorphic

Anthracite - a type of coal Amphibolite - a metamorphic rock composed primarily of amphibole

Blueschist - a metamorphic rock composed of sodic amphiboles with a distinct blue color

Banded gneiss with a dike of granite orthogneiss

Eclogite - an ultra-high grade metamorphosed basalt or gabbro; also a facies of metamorphic rocks Gneiss - a coarse grained metamorphic rock Gossan - the product of the weathering of a sulfide rock or ore body Granulite - a high grade metamorphic rock formed from basalt; also a facies of metamorphic rocks Greenschist - a generic term for a mafic metamorphic rock

dominated by green amphiboles Greenstone

Hornfels - a metamorphic rock formed by heating by an igneous rock

Marble

Marble - a metamorphosed limestone Migmatite - a high grade metamorphic rock verging upon melting into a magma Mylonite - a metamorphic rock formed by shearing Pelite - a metamorphic rock with a protolith of clay-rich (siltstone) sedimentary rock

Phyllite

Phyllite - a low grade metamorphic rock composed mostly of micaceous minerals Psammite - a metamorphic rock with a protolith of quartz-rich (sandstone) sedimentary rock Quartzite - a metamorphosed sandstone typically composed of >95% quartz

Manhattan Schist, from Southeastern New York

Schist - a low to medium grade metamorphic rock Serpentinite - a metamorphosed ultramafic rock dominated by serpentine minerals Skarn - a metasomatic rock

Slate

Slate - a low grade metamorphic rock formed from shale or silts Suevite - a rock formed by partial melting during a meteorite impact Talc carbonate - a metamorphosed ultramafic rock with talc as an essential constituent; similar to a serpentinite Soapstone - essentially a talc schist

Specific Varieties of Rocks

The following terms are used to describe rocks which are not petrographically or genetically distinct, but are defined according to various other criteria; most are specific classes of other rocks, or altered versions of existing rocks. Some archaic and vernacular terms for rocks are also included.

Adamellite - a variety of quartz monzonite Appinite - a group of varieties of lamprophyre, mostly rich in hornblende Aphanite - an aphanitic felsic volcanic rock which confounds identification via optical means Borolanite - a variety of nepheline syenite from Loch Borralan, Scotland Blue Granite - essentially larvikite, a monzonite Epidosite - a type of metasomatite; essentially altered basalt Felsite - an aphanitic felsic volcanic rock which confounds identification via optical means Flint - typically a form of chert, jasper, or tuff Ganister - a Cornish term for a palaeosol formed on sandstone Ijolite - a silica-undersaturated plutonic rock associated with nepheline syenites Jadeitite - a very rare rock formed by concentration of jadeite pyroxene; a form of serpentinite Jasperoid - a hematite-silica metasomatite analogous to a skarn Kenyte - a variety of phonolite, first found on Mount Kenya Vogesite - a variety of lamprophyre Larvikite - a variety of monzonite with microperthitic ternary feldspars from Larvik, Norway Litchfieldite - a metamorphosed nepheline syenite occurrence near Litchfield, Maine Luxullianite - a tourmaline-bearing granite with a peculiar texture, occurring at Luxulyan, Cornwall, England Mangerite - a hypersthene-bearing monzonite Minette - a variety of lamprophyre Novaculite - a chert formation found in Oklahoma, Arkansas and Texas Pyrolite - a chemical analogue considered to theoretically represent the earth's upper mantle Rapakivi granite - a granite which exhibits the peculiar rapakivi texture Rhomb porphyry - a type of latite with euhedral rhombic phenocrysts of feldspar Shonkinite - an archaic and informal term used to describe melitilic and kalsititic rocks; often used today Taconite - a term for banded iron formation primarily used in the United States of America Teschenite - essentially a silica undersaturated, analcime bearing gabbro Theralite - essentially a nepheline gabbro Variolite - devitrified glass