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What Is a Volcanic Rock?

Geisha A. Legazpi
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.

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.

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

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.

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

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.

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

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.

Frequently Asked Questions

What exactly is a volcanic rock?

Volcanic rock, also known as igneous rock, forms from the cooling and solidification of magma or lava. This process can occur either beneath the Earth's surface, resulting in intrusive rocks like granite, or on the surface following a volcanic eruption, leading to extrusive rocks such as basalt. The rapid cooling of lava yields fine-grained textures, while slower cooling magma produces coarser grains.

How are volcanic rocks classified?

Volcanic rocks are primarily classified based on their chemical composition and texture. They can be categorized into felsic, intermediate, mafic, and ultramafic, with felsic rocks being the richest in silica and ultramafic the least. Texturally, they range from glassy and fine-grained to coarse-grained and porphyritic, with large crystals embedded in a finer matrix.

What are some common uses of volcanic rocks?

Volcanic rocks have a wide array of uses. Basalt, a common extrusive volcanic rock, is used in construction due to its strength and durability. Pumice, known for its light weight and abrasive qualities, is often used in beauty products for exfoliation and in industrial settings as an abrasive material. Volcanic rocks are also important in landscaping and as soil amendments in agriculture.

Can volcanic rocks tell us about past volcanic activity?

Yes, volcanic rocks are key to understanding past volcanic activity. By studying their composition, texture, and layering, geologists can infer the type of eruption that occurred, the magma's source, and the conditions of the environment at the time of the eruption. This information helps in reconstructing the geological history of an area and in assessing future volcanic hazards.

Are there any famous landmarks formed by volcanic rocks?

Many famous landmarks are composed of volcanic rocks. The Giant's Causeway in Northern Ireland, with its distinctive basalt columns, is a UNESCO World Heritage Site. The majestic Mount Fuji in Japan is a stratovolcano made up of layers of lava and other volcanic materials. The rich geology of these sites attracts tourists and scientists alike.

How do volcanic rocks impact the environment?

Volcanic rocks can have both positive and negative impacts on the environment. They often create fertile soils that support diverse ecosystems and agriculture. However, during volcanic eruptions, the release of ash and gases can lead to environmental hazards such as air pollution and water contamination. Over time, weathered volcanic rocks contribute to soil formation and nutrient cycles.

Discussion Comments


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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    • Volcanic rock can be a good source of sulfur.
      By: berzina
      Volcanic rock can be a good source of sulfur.
    • Molten lava from an eruption eventually cools and forms volcanic rock.
      By: R. Sueswit Apriliant
      Molten lava from an eruption eventually cools and forms volcanic rock.
    • Pumice igneous rocks have a sponge-like appearance and a low density.
      By: Patricia Hofmeester
      Pumice igneous rocks have a sponge-like appearance and a low density.