What is a volcanic rock?

Volcanic rock can be a good source of sulfur.

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

The molten lava from an eruption eventually cools and forms volcanic rock.

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

Igneous pumice rocks have a spongy appearance and low density.

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

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

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

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