Some igneous rocks form from volcanic lava.
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Courtesy of USGS
Igneous Rocks
Igneous rocks form when molten rock cools and solidifies. Molten rock is called magma when it is below the Earth’s surface and lava when it is above.
There are two types of igneous rocks, depending on where the rock is formed.
Igneous rocks that form below the Earth’s surface are called intrusive igneous rocks (or plutonic). The word “plutonic” comes from Pluto, the name for the Greek god of the underworld. They form when magma enters a pocket or chamber underground that is relatively cool and solidifies into crystals as it cools very slowly.
Igneous rocks that form above the Earth’s surface are called extrusive igneous rocks. These rocks, also called volcanic rocks, form when lava cools at or above the Earth’s surface at places like volcanoes. They form when lava cools very quickly as it is thrust into the comparatively cool temperatures above ground.
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