What kinds of rocks do volcanoes make?

Written by mark slingo
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What kinds of rocks do volcanoes make?
Mauna Kea is a volcano in Hawaii. (Thinkstock Images/Comstock/Getty Images)

A volcano is a mountain on a tectonic plate boundary that builds on itself every time it erupts. When a volcano erupts it releases molten rock, called magma, out of its crater. The magma then cools to produce volcanic rock, which is termed as extrusive igneous rock. Igneous rocks can be classified into a few types depending on the cooling conditions.

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Obsidian

Obsidian is a naturally occurring volcanic glass. It has minimal crystal growth as it is a glass and as such can be used to make blades with very thin edges. The most common colour of obsidian rocks is black; however it can also be found in brown and green and more rarely blue, red, orange or yellow. The colours are thought to be caused by other elements being present in the cooling process. It has been used to make cutting tools for thousands of years and is still used in making scalpels today.

Pumice

Pumice is a rock formed from frothy lava. It has many air bubbles in it, which can be due the presence of water in the lava, causing rapid cooling with rapid depressurisation which creates the bubbles. Dry pumice can in fact float on water. The most common colours of pumice are tan and light grey but it can appear in other colours, such as white or black. It is used in the construction of breeze blocks and concrete, as an abrasive in items like polishes, and as a decorative stone.

Rhyolite

Rhyolite is a light-coloured, fine-grained, extrusive igneous rock. It contains quartz and feldspar minerals. It can often contain some gas bubbles frozen into the rock. It is usually light coloured: light grey, tan, reddish, greenish or brown. Although it is fine grained you can sometimes find larger crystals such as zircons embedded in the rock. Some types of rhyolite such as "Wonderstone" and "Hickoryite" are sought after as gem rocks. The fine grain in rhyolite also makes it an attractive rock to use in construction to incorporate colourful, pleasing accents.

Basalt

Basalt is a hard-stone igneous rock and one of the most common rock types in the world. It is formed by the slow cooling of lava with few air bubbles present. It has a variety of uses worldwide--for example in cobblestones and making statues. The strength of basalt makes it ideal for this purpose as it is very difficult to crush. Heating and extruding basalt allows the formation of "stone wool" for thermal insulation.

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