Igneous Rocks

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Magma that reaches the Earth's surface is called lava. Lava forms volcanic rocks. Volcanoes are created when hot magma rises, finds a weak spot in the crust and breaks through, flowing onto the Earth's surface. Some types of lava are thick like pudding, while others are thin and flow easily down the sides of volcanoes. Thick lava often contains a large amount of carbon dioxide or hydrogen sulphide gas, and when it erupts it sprays into the air and freezes instantly to form volcanic ash or bombs called pyroclasts.

For a much more comprehensive look at volcanoes check out the Volcanoes section of Exploring Our Dynamic Earth, coming soon! Here are a few important things to remember. All rocks are made of minerals, and all minerals are made of elements. Of the three main types of rocks, igneous rocks are the only ones that form from magma. Hot, molten rock that slowly cools kilometres beneath the Earth's surface forms plutonic rocks. If the magma erupts onto the Earth's surface it cools to form volcanic rocks. Perhaps the next time you see some igneous rocks you'll stop and take a closer look.

Most volcanoes occur near the boundaries of tectonic plates (the dozen or so plates that form the hard outer crust of the Earth). Volcanoes are especially common near subduction zones, which occur where one plate slides under another. As the plate is pulled down into the mantle it becomes heated. This results in blobs of molten rock that rise towards the surface to form a magma chamber. If the pressure in the magma chamber builds high enough, the magma will erupt, and a volcano is born.