What Is a Volcanic Rock?

Geisha A. Legazpi

A volcanic rock is a fine-grained type of igneous rock whose matrix usually consists of glass and tiny crystals. Through the rapid cooling of molten lava such that no crystallization of its quartz and feldspar contents occurs, the natural glass variety called obsidian is formed. The variety that has large crystals deposited in smaller, fine-grained crystals is called porphyry. Among the volcanic rock type, the most common are basalt, andesite, and rhyolite. Also called volcanics, volcanic rocks can be found in many parts of the world, including the Ring of Fire that surrounds the Pacific Ocean.

Volcanic rock can be a good source of sulfur.
Volcanic rock can be a good source of sulfur.

Igneous rocks are one of the three main groups in standard rock classification; the other two are sedimentary rocks and metamorphic rocks. The cooling and crystallization of magma result in the formation of igneous rocks. Occurring as molten or partially molten, magma is a mixture of rock particles and crystals, as well as liquid and gas deposits within the Earth. The key factors in subdividing igneous rocks into volcanic or extrusive, subvolcanic, pyroclastic, and plutonic or intrusive are particle texture and size, mechanism of formation, mineral contents, and chemical compounds present. Direct evidence that a rock is of the igneous type is when it is formed directly from the cooling of materials that flow from volcanoes.

Molten lava from an eruption eventually cools and forms volcanic rock.
Molten lava from an eruption eventually cools and forms volcanic rock.

Volcanic rocks have grains that are less than 0.04 inches (1 mm) in size. Obsidian, a usually black and occasionally red or brown volcanic rock variety, contains a large amount of silica but low in volatile contents. It can be used as a material for sharp tools and weapons, although its shiny luster makes it a semiprecious gem. Porphyry, which contains crystals of different sizes, occurs in dikes and sills. Rocks that are of volcanic origin are good sources of minerals such as sulfur and mercury, and may also be used as construction materials, especially for building of roads.

Basalt, a hard, dense, and dark volcanic rock, is the most abundant and composed of a mixture of compounds such as silicon dioxide and aluminum oxide that form the silicate mineral contents of the rock. Andesite, whose color ranges from gray to black, is a volcanic rock that may resemble basalt, but contains more minerals such as silica, potassium, and sodium. This volcanic rock, however, contains less magnesium, iron, and calcium than basalt. The light-colored rhyolite has very high silica contents, which is why its viscosity may become very high; it also contains quartz and feldspar, but iron and magnesium are seldom present.

Other examples of volcanic rocks include pumice, scoria, and dacite. The light-colored and high-silica pumice is used for abrasives and insulators. Scoria is a dark volcanic rock that is used for insulators as well, while dacite contains quartz and feldspar, with colors ranging from light gray to black.

Pumice igneous rocks have a sponge-like appearance and a low density.
Pumice igneous rocks have a sponge-like appearance and a low density.

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If you have any friends or family members who collect rocks, volcanic rocks make nice gifts. After all, no rock collection is complete without this type of rock, and rock collectors will appreciate their beauty, history and unique qualities that make them special.


Large pieces of volcanic rock make great additions to an outdoor rock garden. The shiny, attractive appearance of these rocks enhance the decor of any garden. They also look great around objects such as mailboxes, fence posts, and light poles.

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