What are igneous rocks?

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Igneous Rocks

Obsidian tools

Igneous means "fiery", and igneous rocks are rocks that were made by heating them up in a fire. Usually that fire is the big fire inside the earth, under the earth's crust. Inside the earth, radioactivity creates very high temperatures of about 1000 degrees Celsius (that's about 1800 degrees Fahrenheit). Because it's so hot, any rocks that are down in that core are liquids, not hard as they would be on the surface of the Earth.

But sometimes these liquid rocks shoot up to the surface when volcanoes suddenly erupt. Then tons of liquid rock rockets out of the top of the volcano and lands on the surface. When the liquid rock cools down, it becomes igneous rock.

There are a lot of different kinds of igneous rock, depending on what kind of molecules are involved, and how fast the liquid rock cooled down. Some of the better-known kinds of igneous rock that comes from volcanoes are pumice, lava, and obsidian. When lava cools, it often turns into basalt.

Other igneous rocks formed slowly, deep in the earth. Some examples are quartz and granite. About a quarter of the rocks on earth are igneous rocks. Nearly all of the rest are sedimentary.

Learn by doing: finding different kinds of rocks
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Copyright 2012-2015 Karen Carr, Portland State University. This page last updated September 2015.

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