Learn About Volcanic Rocks (Extrusive Igneous Rocks)

High Angle View Of Lava
Doug Keown / EyeEm / Getty Images
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Massive Basalt, Western U.S.

Massive basalt
Gallery of Volcanic Rocks. Photo (c) Andrew Alden, licensed to About.com (fair use policy)

Igneous rocks -- those which originate from magma -- fall into two categories: Extrusive and intrusive. Extrusive rocks erupt from volcanoes or seafloor fissures, or they freeze at shallow depths. This means that they cool relatively quickly and under low pressures, therefore they are typically fine-grained and gassy. The other category is intrusive rocks, which solidify slowly at depth and do not release gases.

Some of these rocks are clastic, meaning they are composed of rock and mineral fragments, or clasts, rather than solidified melt. Technically, that makes them sedimentary rocks but these volcaniclastic rocks have many differences from other sedimentary rocks -- in their chemistry and the role of heat, especially. Geologists tend to lump them with the igneous rocks. Learn more about igneous rocks.

This basalt from a Columbia Plateau lava flow is fine-grained (aphanitic) and massive (without layers or structure). See the basalt gallery.

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Vesiculated Basalt, Hawaii

Vesiculated and porphyritic
Gallery of Volcanic Rocks. Photo (c) Andrew Alden, licensed to About.com (fair use policy)

This basalt cobble has gas bubbles (vesicles) and large grains (phenocrysts) of olivine that formed early in the lava's history. See the basalt photo gallery.

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Pahoehoe Lava

The taffylike form of lava
Gallery of Volcanic Rocks. Photo (c) Andrew Alden, licensed to About.com (fair use policy)

Pahoehoe is a texture found in highly fluid, gas-charged lava due to the deformation of flow. Pahoehoe is typical in a basaltic lava, low in silica.

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Andesite, Sutter Buttes, California

A typical chunk
Gallery of Volcanic Rocks. Photo (c) 2007 Andrew Alden, licensed to About.com (fair use policy)

Andesite (specimen from Sutter Buttes) is more siliceous and less fluid than basalt. The large, light phenocrysts are potassium feldspar. Andesite can also be red.

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Andesite from La Soufrière

From the beach of St. Vincent
Gallery of Volcanic Rocks. Photo (c) 2008 Andrew Alden, licensed to About.com (fair use policy)

La Soufrière volcano, on St. Vincent island in the Caribbean, erupts a porphyritic andesite lava with phenocrysts largely of plagioclase feldspar.

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Rhyolite, Salton Sea Field, California

A banded red lava
Gallery of Volcanic Rocks. Photo (c) 2007 Andrew Alden, licensed to About.com (fair use policy)

Rhyolite is a high-silica rock, the extrusive counterpart of granite. It is typically banded and, unlike this specimen, full of large crystals (phenocrysts).

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Rhyolite with Quartz Phenocrysts

Like sugar in taffy
Gallery of Volcanic Rocks. Photo (c) 2007 Andrew Alden, licensed to About.com (fair use policy)

Rhyolite (from Sutter Buttes, California) displays flow banding and large grains of quartz in the almost glassy groundmass. Rhyolite can also be black, gray or red.

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Obsidian

From the Napa Valley
Gallery of Volcanic Rocks. Photo (c) Andrew Alden, licensed to About.com (fair use policy)

Obsidian is a volcanic glass, high in silica and so viscous that crystals do not form as it cools. Learn more about obsidian in the obsidian gallery.

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Perlite

Hydrated lava glass
Gallery of Volcanic Rocks. Photo (c) 2008 Andrew Alden, licensed to About.com (fair use policy)

Obsidian or rhyolite flows that are rich in water often produce perlite, a lightweight, hydrated lava glass. Read more about it.

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Peperite, Scotland

Where fire meets water
Gallery of Volcanic Rocks. Photo courtesy Eddie Lynch of Flickr; all rights reserved

Peperite is a rock formed where magma meets water-saturated sediments at relatively shallow depths, such as in a maar. The lava tends to shatter, producing a breccia, and the sediment is vigorously disrupted. This example is from the Glencoe caldera complex in Scotland, exposed on the massif of Bidean nam Bian, where andesite magma invaded sediment that later became the Old Red Sandstone.

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Scoria, Cascade Range

Scoria, Cascade Range
Gallery of Volcanic Rocks. Photo (c) Andrew Alden, licensed to About.com (fair use policy)

This bit of basaltic lava was puffed up by escaping gases to create scoria. The specimen is a cinder cone in northeast California.

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Pumice, Alaska

Stone styrofoam
Gallery of Volcanic Rocks. Photo (c) Andrew Alden, licensed to About.com (fair use policy)

This piece of pumice drifted onto an Alaska beach, probably from an Aleutian volcano. It is as light as foam. The next photo shows it close up.

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Pumice Closeup

Look at those pores
Gallery of Volcanic Rocks. Photo (c) Andrew Alden, licensed to About.com (fair use policy)

This closeup of Alaskan pumice shows the small, equal-sized vesicles in this glassy rock. Crushing this feather-light rock releases a sulfuric smell.

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Reticulite

A lava loofah
Gallery of Volcanic Rocks. U.S. Geological Survey photo by J. D. Griggs

The ultimate form of scoria, in which all the gas bubbles have burst and only a fine mesh of lava threads remains, is called reticulite or thread-lace scoria.

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Pumice, Napa Valley

From continental lavas
Gallery of Volcanic Rocks. Photo (c) Andrew Alden, licensed to About.com (fair use policy)

Pumice is also a gas-charged, lightweight volcanic rock like scoria, but is lighter colored and higher in silica and comes from continental volcanic centers.

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Pumice, Coso Range

A red cinder
Gallery of Volcanic Rocks. Photo (c) Andrew Alden, licensed to About.com (fair use policy)

This pumice was erupted in eastern California about 1000 years ago. Red volcanic rocks are usually altered from their original black by superheated steam.

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Pumice, Oakland Hills

Might be altered
Gallery of Volcanic Rocks. Photo (c) Andrew Alden, licensed to About.com (fair use policy)

This pumice specimen is from eruptions of Miocene age in the Oakland Hills east of San Francisco. It might, alternatively, be an altered scoria.

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Ashfall Tuff

Lightweight and abrasive
Gallery of Volcanic Rocks. Photo (c) Andrew Alden, licensed to About.com (fair use policy)

Fine-grained volcanic ash fell upon the Napa Valley several million years ago, later hardening into this lightweight rock. Such ash is usually high in silica.

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Tuff from Green Valley

Tuff, Green Valley
Gallery of Volcanic Rocks. Photo (c) Andrew Alden, licensed to About.com (fair use policy)

Green Valley is east of the Napa Valley, and like it is largely set in rocks of the Sonoma Volcanics. Tuff forms from erupted ash.

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Tuff from Green Valley, California

Tuff can have larger particles
Gallery of Volcanic Rocks. Photo (c) Andrew Alden, licensed to About.com (fair use policy)

This piece of tuff from Green Valley shows a large clast among the finer ash particles. Tuff often has chunks of older rock as well as freshly erupted material.

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Lapilli Tuff

A mixed ashfall
Gallery of Volcanic Rocks. Photo (c) Andrew Alden, licensed to About.com (fair use policy)

Volcaniclastic rock with mixed particles of lapilli (2 to 64 mm) and ash.

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Lapilli Tuff Detail

Components of hard tuffrock
Gallery of Volcanic Rocks. Photo (c) Andrew Alden, licensed to About.com (fair use policy)

This lapilli tuff includes reddish grains of old scoria, fragments of country rock, stretched grains of fresh gassy lava, and fine ash.

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Tuff in Outcrop

A white roadcut
Gallery of Volcanic Rocks. Image courtesy Ministerio de Obras Públicas Republica de El Salvador

Tierra blanca tuff underlies the metropolitan region of El Salvador's capital, San Salvador. Tuff is formed by the accumulation of volcanic ash. 

Tuff is a sedimentary rock formed by volcanic activity. It 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 lava tends to fragment and explode into tiny pieces, which weather quickly afterward. After the ash falls, it may be reworked by rainfall and streams. That accounts for the crossbedding near the top of the lower part of the roadcut.

If tuff beds are thick enough, they can consolidate into a fairly strong, lightweight rock. In parts of San Salvador, the tierra blanca is thicker than 50 meters. Presumably, this roadcut is in such a place. A lot of old Italian stonework is made of tuff. In other places, the tuff must be carefully compacted before buildings can be constructed on it. Salvadoreans have learned this through centuries of rueful experience with major earthquakes. Residential and suburban buildings that short-change this step remain prone to landslides and washouts, whether from heavy rainfall or from the inevitable quakes, like that which struck the area on January 13,  2001.

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Lapillistone, Oakland Hills, California

Made of airfall lumps
Gallery of Volcanic Rocks. Photo (c) Andrew Alden, licensed to About.com (fair use policy)

Lapilli are volcanic pebbles (2 to 64 mm), in this case, "ash hailstones" formed in the air. Here they accumulated and became lapillistone. Get the wallpaper version.

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Bomb

A large to very large piece of lava
Gallery of Volcanic Rocks. Photo courtesy Gerard Tripp, all rights reserved

A bomb is an erupted particle of lava -- a pyroclast -- that is larger than lapilli (greater than 64 mm) and that was not solid when it erupts. This bomb is on Krakatau.

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Pillow Lava

Fresh ones on the seafloor
Gallery of Volcanic Rocks. National Undersea Research Program photo

Pillow lavas may be the world's most common extrusive igneous formation, but they only form on the deep sea floor. 

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Volcanic Breccia

Example from underground
Gallery of Volcanic Rocks From stop 12 of the California Subduction tour. Photo (c) 2006 Andrew Alden, licensed to About.com (fair use policy)

Breccia, like conglomerate, consists of pieces of mixed size, but the large pieces are broken. This breccia is in volcanic rock was later altered.

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Alden, Andrew. "Learn About Volcanic Rocks (Extrusive Igneous Rocks)." ThoughtCo, Sep. 27, 2017, thoughtco.com/volcanic-extrusive-rock-types-4123253. Alden, Andrew. (2017, September 27). Learn About Volcanic Rocks (Extrusive Igneous Rocks). Retrieved from https://www.thoughtco.com/volcanic-extrusive-rock-types-4123253 Alden, Andrew. "Learn About Volcanic Rocks (Extrusive Igneous Rocks)." ThoughtCo. https://www.thoughtco.com/volcanic-extrusive-rock-types-4123253 (accessed May 21, 2018).