Volcanic and Extrusive Igneous Rocks

Volcano with clouds swirling above ominously.

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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 rather than solidified melt. Technically, that makes them sedimentary rocks. However, 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

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Massive Basalt

Large chunk of basalt rock.

James St. John/Flickr/CC BY 2.0

This basalt from a former lava flow is fine-grained (aphanitic) and massive (without layers or structure).

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Vesiculated Basalt

Vesicular basalt chunk with U.S. quarter placed on top.

Jstuby at en.wikipedia/Wikimedia Commons/Public Domain

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

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Pahoehoe Lava

Pahoehoe lava flow solidifying into rock.

J.D. Griggs/Wikimedia Commons/Public Domain

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

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Andesite

Large piece of andesite rock.

James St. John/Flickr/CC BY 2.0

Andesite 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

Large piece of andesite rock on a gray background.
Andesite rock from the Soufriere Hills volcano.

James St. John/Flickr/CC BY 2.0

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

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Rhyolite

Large rhyolite rock against a white background.

James St. John/Flickr/CC BY 2.0

Rhyolite is a high-silica rock, the extrusive counterpart of granite. It is typically banded and, unlike this specimen, full of large crystals (phenocrysts). Red volcanic rocks are usually altered from their original black by superheated steam.

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Rhyolite with Quartz Phenocrysts

Close up of a rhyolite rock with a coin for scale.

Andrew Alden

Rhyolite 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

Hunk of obsidian on a white background.

Amcyrus2012/Wikimedia Commons/CC BY 4.0

Obsidian is a volcanic glass, high in silica and so viscous that crystals do not form as it cools.

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Perlite

Perlite rock on a white background.

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

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Peperite

Chunk of peperite rock on gravel.

Ashley Dace/Wikimedia Commons/CC BY 2.0

Peperite is a rock formed where magma meets water-saturated sediments at relatively shallow depths, such as in a maar (a broad, shallow volcanic crater). The lava tends to shatter, producing a breccia, and the sediment is vigorously disrupted.

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Scoria

Scoria rock on a white background.

“Jonathan Zander (Digon3)"/Wikimedia Commons/CC BY 3.0

This bit of basaltic lava was puffed up by escaping gases to create scoria.

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Reticulite

Reticulite rock close up with scale marker.

J.D. Griggs, USGS/Wikimedia Commons/Public Domain

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

Large pumice stone among other rocks.

Norbert Nagel, Mörfelden-Walldorf, Germany/Wikimedia Commons/CC BY 3.0

Pumice is also a gas-charged, lightweight volcanic rock like scoria, but it is lighter in color and higher in silica. Pumice comes from continental volcanic centers. Crushing this feather-light rock releases a sulfuric smell.

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Ashfall Tuff

Large ashfall tuff rock.

James St. John/Flickr/CC BY 2.0

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. Tuff forms from erupted ash. Tuff often has chunks of older rock, as well as freshly-erupted material.

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Tuff Detail

Ettringer tuff detail.

Roll-Stone/Wikimedia/Public Domain

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

Bishop tuff, a rocky cliff in the daylight.

Roy A. Bailey/Wikimedia Commons/Public Domain

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. 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. 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 earthquakes, like that which struck the area in 2001.

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Lapillistone

Large lapillus rock on a white background.

James St. John/Flickr/CC BY 2.0

Lapilli are volcanic pebbles (2 to 64 mm in size) or "ash hailstones" formed in the air. Sometimes, they accumulate and became lapillistone.

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Bomb

Volcanic bomb on the ground.

National Park Service photo/Wikimedia Commons/Public Domain

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

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Pillow Lava

Pillow lavas under the water.

OAR/National Undersea Research Program (NURP)/Wikimedia Commons/Public Domain

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

Large chunk of volcanic breccia sitting on the grass.

Daniel Mayer/Wikimedia Commons/CC BY 3.0

Breccia, like conglomerate, consists of pieces of mixed size, but the large pieces are broken.