Igneous Rocks

Some igneous rocks form from volcanic lava. Click on image for full size (26K JPG ) Courtesy of USGS

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.

Some igneous rocks form from volcanic lava. Click on image for full size (26K JPG) Courtesy of USGS

Igneous rocks form when molten rock cools and becomes solid. Molten rock is called magma when it is below the Earth’s surface and lava when it is above.

Igneous rocks are divided into two groups, based on where the rock forms.

Igneous rocks that form below the Earth’s surface are called intrusive igneous rocks (or plutonic). They form when magma enters an underground chamber, cools very slowly, and forms rocks full of large crystals.

Igneous rocks that form above the Earth’s surface are called extrusive igneous rocks. These rocks, also called volcanic rocks, form when lava cools quickly at or above the Earth’s surface.

Some igneous rocks form from volcanic lava. Click on image for full size (26K JPG) Courtesy of USGS

Igneous rocks form when molten rock cools and becomes solid. Molten rock is called magma when it is below the Earth’s surface and lava when it is above.

Sometimes magma pushes up through cracks to the surface of the Earth. It cools and becomes a volcanic rock. Basalt is the most common type of volcanic rock. It makes up most of the rocks on the ocean floor.

Other times, magma cools when it’s deep underground. This forms a different type of igneous rock called a plutonic rock. Plutonic rocks usually have large crystals. Granite is a type of plutonic rock that you are likely to find.