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Some igneous rocks form so quickly that their crystals do not have much time to grow. These are called extrusive igneous rocks because they are formed when lava flows out (or extrudes) onto Earth's surface.

Characteristics
Most of the crystals of these rocks are not large enough to be seen with the naked eye. Some extrusive igneous rocks do not even have crystals because they cooled too quickly for any to form. Some examples of these are obsidian, a rock that looks like glass, and pumice, a rock that looks just like the bubbly, volcanic foam from which it is formed.

How they are formed
Extrusive igneous rocks are formed in the way that most people think of igneous rocks being formed through volcanoes and lava flows (video). These rocks are also known as volcanic rocks. When lava (molten rock above the surface of the Earth) hits air or water, it cools very quickly and does not get a chance to form large crystals (video).

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Where they are found
These rocks are found where there are, or have previously been, active volcanoes. The Japanese and Hawaiian Islands are some examples.

Click here to view Intrusive Igneous Rocks

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This page was last updated on December 4, 2000
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