Types of Igneous Rocks

Rocks in a stream
Getty Images

Igneous rocks are those that form via the process of melting and cooling. If they erupt from volcanoes onto the surface as lava, they are called extrusive rocks. By contrast, Intrusive rocks are formed from magma that cools underground. If the intrusive rock cooled underground but near the surface, it is called subvolcanic or hypabyssal, and often has visible, but tiny mineral grains. If the rock cools very slowly deep underground, it is called plutonic and typically has large mineral grains.

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Andesite

Named for the Andes
State of New South Wales Department of Education and Training

Andesite is an extrusive igneous rock that is higher in silica than basalt and lower than rhyolite or felsite.

Click the photo to see the full-size version. In general, color is a good clue to the silica content of extrusive igneous rocks, with basalt being dark and felsite being light. Although geologists would do a chemical analysis before identifying andesite in a published paper, in the field they readily call a gray or medium-red extrusive igneous rock andesite. Andesite gets its name from the Andes mountains of South America, where arc volcanic rocks mix basaltic magma with granitic crustal rocks, yielding lavas with intermediate compositions. Andesite is less fluid than basalt and erupts with more violence​ because its dissolved gases cannot escape as easily. Andesite is considered the extrusive equivalent of diorite.

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Anorthosite

A peculiar feldspathic end-member
Andrew Alden/Flickr

Anorthosite is an uncommon intrusive igneous rock consisting almost entirely of plagioclase feldspar. This is from New York's Adirondack Mountains.

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Basalt

Makes up the oceanic crust
Andrew Alden/Flickr

Basalt is an extrusive or intrusive rock that makes up most of the world's oceanic crust. This specimen erupted from Kilauea volcano in 1960.

Basalt is fine grained so the individual minerals are not visible, but they include pyroxene, plagioclase feldspar, and olivine. These minerals are visible in the coarse-grained, plutonic version of basalt called gabbro.

This specimen shows bubbles made by carbon dioxide and water vapor that came out of the molten rock as it approached the surface. During its long period of storage beneath the volcano, green grains of olivine came out of solution as well. The bubbles, or vesicles, and the grains, or phenocrysts, represent two different events in the history of this basalt.

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Diorite

Black and white
State of New South Wales Department of Education and Training

Diorite is a plutonic rock that is between granite and gabbro in composition. It consists mostly of white plagioclase feldspar and black hornblende. 

Unlike granite, diorite has no or very little quartz or alkali feldspar. Unlike gabbro, diorite contains sodic—not calcic—plagioclase. Typically, sodic plagioclase is the bright white variety albite, giving diorite a high-relief look. If a dioritic rock erupted from a volcano (that is, if it is extrusive), it cools into andesite lava.

In the field, geologists may call a black-and-white rock diorite, but true diorite is not very common. With a little quartz, diorite becomes quartz diorite, and with more quartz it becomes tonalite. With more alkali feldspar, diorite becomes monzonite. With more of both minerals, diorite becomes granodiorite. This is clearer if you view the classification triangle.

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Dunite

All-olivine magma
Andrew Alden/Flickr

Dunite is a rare rock, a peridotite that is at least 90% olivine. It's named for Dun Mountain in New Zealand. This is a dunite xenolith in an Arizona basalt.

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Felsite

Light lavas
Aram Dulyan/Flickr

Felsite is a general name for light-colored extrusive igneous rocks. Ignore the dark dendritic growths on this specimen's surface.

Felsite is fine-grained but not glassy, and it may or may not have phenocrysts (large mineral grains). It is high in silica or felsic, typically consisting of the minerals quartz, plagioclase feldspar, and alkali feldspar. Felsite is usually called the extrusive equivalent of granite. A common felsitic rock is rhyolite, which typically has phenocrysts and signs of having flowed. Felsite should not be confused with tuff, a rock made up of compacted volcanic ash that can also be light colored.

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Gabbro

A plutonic basalt
State of New South Wales Department of Education and Training

Gabbro is a dark-colored igneous rock that is considered to be the plutonic equivalent of basalt.

Unlike granite, gabbro is low in silica and has no quartz. Also, gabbro has no alkali feldspar, only plagioclase feldspar with a high calcium content. The other dark minerals may include amphibole, pyroxene, and sometimes biotite, olivine, magnetite, ilmenite, and apatite.

Gabbro is named after a town in Italy's Tuscany region. You can get away with calling almost any dark, coarse-grained igneous rock gabbro, but true gabbro is a narrowly defined subset of dark plutonic rocks.

Gabbro makes up most of the deep part of the oceanic crust, where melts of basaltic composition cool very slowly to create large mineral grains. That makes gabbro a key sign of an ophiolite, a large body of oceanic crust that ends up on land. Gabbro is also found with other plutonic rocks in batholiths when bodies of rising magma are low in silica.

Igneous petrologists are careful about their terminology for gabbro and similar rocks, in which "gabbroid," "gabbroic," and "gabbro" have distinct meanings.

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Granite

Type rock of continents

Andrew Alden

Granite is a type of igneous rock that consists of quartz (gray), plagioclase feldspar (white), and alkali feldspar (beige), plus dark minerals such as biotite and hornblende. 

"Granite" is used by the public as a catchall name for any light-colored, coarse-grained igneous rock. The geologist examines these in the field and calls them granitoids pending laboratory tests. The key to true granite is that it contains sizable amounts of quartz and both kinds of feldspar.

This granite specimen comes from the Salinian block of central California, a chunk of ancient crust carried up from southern California along the San Andreas fault.

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Granodiorite

An in-between rock type
Andrew Alden/Flickr

Granodiorite is a plutonic rock composed of black biotite, dark-gray hornblende, off-white plagioclase, and translucent gray quartz.

Granodiorite differs from diorite by the presence of quartz, and the predominance of plagioclase over alkali feldspar distinguishes it from granite. Although it isn't true granite, granodiorite is one of the granitoid rocks. Rusty colors reflect weathering of rare grains of pyrite, which releases iron. The random orientation of grains shows that this is a plutonic rock.

This specimen is from southeastern New Hampshire. Click the photo for a larger version.

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Kimberlite

Igneous Rock
Andrew Alden/Flickr

Kimberlite, an ultramafic volcanic rock, is quite rare but much sought after because it is the ore of diamonds.

This type of igneous rock originates when lava erupts very rapidly from deep in the Earth's mantle, leaving behind a narrow pipe of this greenish brecciated rock. The rock is of ultramafic composition—very high in iron and magnesium—and is largely composed of olivine crystals in a groundmass consisting of various mixtures of serpentine, carbonate minerals, diopside, and phlogopite. Diamonds and many other ultra-high pressure minerals are present in greater or lesser amounts. It also contains xenoliths, samples of rocks gathered along the way.

Kimberlite pipes (which are also called kimberlites) are scattered by the hundreds in the most ancient continental areas, the cratons. Most are a few hundred meters across, so they can be hard to find. Once found, many of them become diamond mines. South Africa seems to have the most, and kimberlite gets its name from the Kimberley mining district in that country. This specimen, however, is from Kansas and contains no diamonds. It's not very precious, just very interesting.

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Komatiite

Rare and ancient ultramafic lava
GeoRanger/Wikimedia Commons

Komatiite (ko-MOTTY-ite) is a rare and ancient ultramafic lava, the extrusive version of peridotite.

Komatiite is named for a locality on the Komati River of South Africa. It consists largely of olivine, making it the same composition as peridotite. Unlike the deep-seated, coarse-grained peridotite, it shows clear signs of having been erupted. It is thought that only extremely high temperatures can melt rock of that composition, and most komatiite is of Archean age, in line with the assumption that Earth's mantle was much hotter three billion years ago than today. However, the youngest komatiite is from Gorgona Island off the coast of Colombia and dates from about 60 million years ago. There is another school that argues for the influence of water in allowing young komatiites to form at lower temperatures than usually thought. Of course, this would throw into doubt the usual argument that komatiites must be extremely hot.

Komatiite is extremely rich in magnesium and low in silica. Nearly all examples known are metamorphosed, and we must infer its original composition through careful petrological study. One distinctive feature of some komatiites is spinifex texture, in which the rock is crisscrossed with long, thin olivine crystals. Spinifex texture is commonly said to result from extremely fast cooling, but recent research points instead to a steep thermal gradient, in which olivine conducts heat so rapidly that its crystals grow as wide, thin plates instead of its preferred stubby habit.

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Latite

Extrusive monzonite

Andrew Alden/Flickr

Latite is commonly called the extrusive equivalent of monzonite, but it's complicated. Like basalt, latite has little to no quartz but a lot more alkali feldspar.

Latite is defined at least two different ways. If crystals are visible enough to allow an identification by modal minerals (using the QAP diagram), latite is defined as a volcanic rock with almost no quartz and roughly equal amounts of alkali and plagioclase feldspars. If this procedure is too difficult, latite is also defined from chemical analysis using the TAS diagram. On that diagram, latite is a high-potassium trachyandesite, in which K2O exceeds Na2O minus 2. (A low-K trachyandesite is called benmoreite.)

This specimen is from Stanislaus Table Mountain, California (a well-known example of inverted topography), the locality where latite was originally defined by F. L. Ransome in 1898. He detailed the confusing variety of volcanic rocks that were neither basalt nor andesite but something intermediate, and he proposed the name latite after the Latium district of Italy, where other volcanologists had long studied similar rocks. Ever since then, latite has been a subject for professionals rather than amateurs. It is commonly pronounced "LAY-tite" with a long A, but from its origin it should be pronounced "LAT-tite" with a short A.

In the field, it is impossible to distinguish latite from basalt or andesite. This specimen has large crystals (phenocrysts) of plagioclase and smaller phenocrysts of pyroxene.

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Obsidian

Volcanic glass
Andrew Alden/Flickr

Obsidian is an extrusive rock, which means it is lava that cooled without forming crystals, hence its glassy texture.

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Pegmatite

Big-grained granites
Andrew Alden/Flickr

Pegmatite is a plutonic rock with exceptionally large crystals. It forms at a late stage in the solidification of granite bodies.

Click the photo to see it at full size. Pegmatite is a rock type based purely on grain size. Generally, pegmatite is defined as a rock bearing abundant interlocking crystals at least 3 centimeters long. Most pegmatite bodies consist largely of quartz and feldspar and are associated with granitic rocks.

Pegmatite bodies are thought to form predominantly in granites during their final stage of solidification. The final fraction of mineral material is high in water and often contains elements such as fluorine or lithium. This fluid is forced to the edge of the granite pluton and forms thick veins or pods. The fluid apparently solidifies rapidly at relatively high temperatures, under conditions that favor a few very large crystals rather than many small ones. The largest crystal ever found was in a pegmatite, a spodumene grain some 14 meters long.

Pegmatites are sought out by mineral collectors and gemstone miners not only for their large crystals but for their examples of rare minerals. The pegmatite in this ornamental boulder near Denver, Colorado, features large books of biotite and blocks of alkali feldspar.

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Peridotite

Typical of the mantle
Andrew Alden/Flickr

Peridotite is the plutonic rock beneath the Earth's crust located in the upper part of the mantle. This type of igneous rock is named for peridot, the gemstone variety of olivine.

Peridotite (per-RID-a-tite) is very low in silicon and high in iron and magnesium, a combination called ultramafic. It doesn't have enough silicon to make the minerals feldspar or quartz, only mafic minerals like olivine and pyroxene. These dark and heavy minerals make peridotite much denser than most rocks.

Where lithospheric plates pull apart along the mid-ocean ridges, the release of pressure on the peridotite mantle allows it to partially melt. That melted portion, richer in silicon and aluminum, rises to the surface as basalt.

This peridotite boulder is partially altered to serpentine minerals, but it has visible grains of pyroxene sparkling in it as well as serpentine veins. Most peridotite is metamorphosed into serpentinite during the processes of plate tectonics, but sometimes it survives to appear in subduction-zone rocks like the rocks of Shell Beach, California.

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Perlite

Stone styrofoam
Andrew Alden/Flickr

Perlite is an extrusive rock that forms when a high-silica lava has a high water content. It is an important industrial material.

This type of igneous rock forms when a body of rhyolite or obsidian, for one reason or another, has a relatively large amount of water. Perlite often has a perlitic texture, typified by concentric fractures around closely spaced centers and a light color with a bit of pearlescent shine to it. It tends to be lightweight and strong, making it an easy-to-use building material. Even more useful is what happens when perlite is roasted at around 900 degrees Celcius, just to its softening point—it expands like popcorn into a fluffy white material, a sort of mineral "Styrofoam."

Expanded perlite is used as insulation, in lightweight concrete, as an additive in soil (such as an ingredient in potting mix), and in many industrial roles where any combination of toughness, chemical resistance, low weight, abrasiveness, and insulation is needed.

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Porphyry

A style not a composition
Andrew Alden/Flickr

Porphyry ("PORE-fer-ee") is a name used for any igneous rock with conspicuous larger grains—phenocrysts—floating in a fine-grained groundmass.

Geologists use the term porphyry only with a word in front of it describing the composition of the groundmass. This image, for instance, shows an andesite porphyry. The fine-grained part is andesite and the phenocrysts are light alkali feldspar and dark biotite. Geologists also may call this an andesite with porphyritic texture. That is, "porphyry" refers to a texture, not a composition, just as "satin" refers to a type of fabric rather than the fiber it's made from.

A porphyry may be an intrusive or extrusive igneous rock.

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Pumice

A fluffy stone
Andrew Alden/Flickr

Pumice is basically lava froth, an extrusive rock frozen as its dissolved gases come out of solution. It looks solid but often floats on water.

This pumice specimen is from the Oakland Hills in northern California and reflects the high-silica (felsic) magmas that form when subducted marine crust mixes with granitic continental crust. Pumice may look solid, but it's full of small pores and spaces and weighs very little. Pumice is easily crushed and used for abrasive grit or soil amendments.

Pumice is much like scoria in that both are frothy, lightweight volcanic rocks, but the bubbles in pumice are small and regular and its composition is more felsic. Also, pumice is generally glassy, whereas scoria is a more typical volcanic rock with microscopic crystals.

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Pyroxenite

Black deep seafloor
Andrew Alden/Flickr

Pyroxenite is a plutonic rock that consists of dark minerals in the pyroxene group plus a little olivine or amphibole.

Pyroxenite belongs to the ultramafic group, meaning that it consists almost entirely of dark minerals rich in iron and magnesium. Specifically, its silicate minerals are mostly pyroxenes rather than other mafic minerals such as olivine and amphibole. In the field, pyroxene crystals display a stubby shape and square cross-section while amphiboles have a lozenge-shaped cross-section.

This type of igneous rock is often associated with its ultramafic cousin peridotite. Rocks like these originate deep under the seafloor, underneath the basalt that makes up the upper oceanic crust. They occur on land where slabs of oceanic crust become attached to continents, called subduction zones.

Identifying this specimen, from the Feather River Ultramafics of the Sierra Nevada, was largely a process of elimination. It attracts a magnet, probably due to fine-grained magnetite, but the visible minerals are translucent with a strong cleavage. The locality contained ultramafics. Greenish olivine and black hornblende are absent, and the hardness of 5.5 also ruled out these minerals as well as the feldspars. Without large crystals, a blowpipe and chemicals for simple lab tests, or the ability to make thin sections, this is sometimes as far as the amateur can go.

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Quartz Monzonite

Quartz-poor granite
Andrew Alden/Flickr

Quartz monzonite is a plutonic rock that, like granite, consists of quartz and the two types of feldspar. It has much less quartz than granite.

Click the photo for the full-size version. Quartz monzonite is one of the granitoids, a series of quartz-bearing plutonic rocks that commonly must be taken to the laboratory for a firm identification.

This quartz monzonite is part of the Cima Dome in the Mojave Desert of California. The pink mineral is alkali feldspar, the milky white mineral is plagioclase feldspar, and the gray glassy mineral is quartz. The minor black minerals are mostly hornblende and biotite.

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Rhyolite

Stiff stuff
Andrew Alden/Flickr

Rhyolite is a high-silica volcanic rock that is chemically the same as granite but is extrusive rather than plutonic. 

Click the photo for the full-size version. Rhyolite lava is too stiff and viscous to grow crystals except for isolated phenocrysts. The presence of phenocrysts means that rhyolite has a porphyritic texture. This rhyolite specimen, from the Sutter Buttes of northern California, has visible phenocrysts of quartz.

Rhyolite is often pink or gray and has a glassy groundmass. This is a less typical white example. Being high in silica, rhyolite originates from a stiff lava and tends to have a banded appearance. Indeed, "rhyolite" means "flowstone" in Greek.

This type of igneous rock is typically found in continental settings where magmas have incorporated granitic rocks from the crust as they rise from the mantle. It tends to make lava domes when it erupts.

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Scoria

Close to pumice
Andrew Alden/Flickr

Scoria, like pumice, is a lightweight extrusive rock. This type of igneous rock has large, distinct gas bubbles and a darker color.

Another name for scoria is volcanic cinders, and the landscaping product commonly called "lava rock" is scoria — as is the cinder mix widely used on running tracks.

Scoria is more often a product of basaltic, low-silica lavas than of felsic, high-silica lavas. This is because basalt is usually more fluid than felsite, allowing bubbles to grow larger before the rock freezes. Scoria often forms as a frothy crust on lava flows that crumble off as the flow moves. It also is blown out of the crater during eruptions. Unlike pumice, scoria usually has broken, connected bubbles and does not float in water.

This example of scoria is from a cinder cone in northeastern California at the edge of the Cascade Range.

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Syenite

Strong and dull
NASA

Syenite is a plutonic rock consisting chiefly of potassium feldspar with a subordinate amount of plagioclase feldspar and little or no quartz.

The dark, mafic minerals in syenite tend to be amphibole minerals like hornblende. Being a plutonic rock, syenite has large crystals from its slow, underground cooling. An extrusive rock of the same composition as syenite is called trachyte.

Syenite is an ancient name derived from the city of Syene (now Aswan) in Egypt, where a distinctive local stone was used for many of the monuments there. However, the stone of Syene is not a syenite, but rather a dark granite or granodiorite with conspicuous reddish feldspar phenocrysts.

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Tonalite

Much quartzier than diorite
Andrew Alden/Flickr

Tonalite is a widespread but uncommon plutonic rock, a granitoid without alkali feldspar that may also be called plagiogranite and trondjhemite.

The granitoids all center around granite, a fairly equal mixture of quartz, alkali feldspar, and plagioclase feldspar. As you remove alkali feldspar from proper granite, it becomes granodiorite and then tonalite (mostly plagioclase with less than 10% K-feldspar). Recognizing tonalite takes a close look with a magnifier to be sure that alkali feldspar is truly absent and quartz is abundant. Most tonalite also has abundant dark minerals, but this example is almost white (leucocratic), making it a plagiogranite. Trondhjemite is a plagiogranite whose dark mineral is biotite. This specimen's dark mineral is pyroxene, so it's plain old tonalite.

An extrusive rock with the composition of tonalite is classified as dacite. Tonalite gets its name from the Tonales Pass in the Italian Alps, near Monte Adamello, where it was first described along with quartz monzonite (once known as adamellite).

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Troctolite

Troutstone
Andrew Alden/Flickr

Troctolite is a variety of gabbro consisting of plagioclase and olivine without pyroxene. 

Gabbro is a coarse-grained mixture of highly calcic plagioclase and the dark iron-magnesium minerals olivine and/or pyroxene (augite). Different blends in the basic gabbroid mix have their own special names, and troctolite is the one in which olivine dominates the dark minerals. (The pyroxene-dominated gabbroids are either true gabbro or norite, depending on whether the pyroxene is clino- or orthopyroxene.) The gray-white bands are plagioclase with isolated dark-green olivine crystals. The darker bands are mostly olivine with a little pyroxene and magnetite. Around the edges, the olivine has weathered to a dull orange-brown color.

Troctolite typically has a speckled look, and it's also known as troutstone or the German equivalent, forellenstein. "Troctolite" is scientific Greek for troutstone, so this rock type has three different identical names. This specimen is from the Stokes Mountain pluton in the southern Sierra Nevada and is about 120 million years old.

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Tuff

A volcaniclastic rock
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Tuff is technically a sedimentary rock formed by the accumulation of volcanic ash plus pumice or scoria.

Tuff is so closely associated with volcanism that it is usually discussed along with types of igneous rocks. Tuff tends to form when erupting lavas are stiff and high in silica, which holds the volcanic gases in bubbles rather than letting them escape. The brittle lava is readily shattered into jagged pieces, collectively called tephra (TEFF-ra) or volcanic ash. Fallen tephra may be reworked by rainfall and streams. Tuff is a rock of great variety and tells the geologist a lot about conditions during the eruptions that gave birth to it.

If tuff beds are thick enough or hot enough, they can consolidate into a fairly strong rock. The city of Rome's buildings, both ancient and modern, are commonly made of tuff blocks from the local bedrock. In other places, tuff may be fragile and must be carefully compacted before buildings can be constructed with it. Residential and suburban buildings that shortchange this step remain prone to landslides and washouts, whether from heavy rainfall or from the inevitable earthquakes.