How Do Igneous Rocks Form?

Intrusive and Extrusive Igneous Rocks

Igneous rocks form when hot molten rock (either magma or lava) cools and solidifies.

Igneous rocks are one of the three main rock types.  The other two are sedimentary and metamorphic.

Igneous rocks can form above or below Earth's surface.

When they form deep below the surface as magma cools the process takes thousands to millions of years.  This type of rock is known as an intrusive igneous rock.

Intrusive rocks often have large crystals because they have plenty of time to grow.  An example of an intrusive igneous rock is granite.

Igneous rocks that form above Earth's surface often from lava which has been expelled from a volcano are known as extrusive igneous rocks.  An example of an extrusive igneous rock is basalt. 

When lava cools too fast for crystals to grow the rock that forms will be non crystalline.  Examples of non crystalline igneous rocks include obsidian and pumice.

The cooling rate of the lava and its chemical composition plays a significant role in the final texture and appearance of an extrusive igneous rock.

Article Picture

The extrusive igneous rock in the our photo is basalt. The image is clickable and redirects to the original photo.  Courtesy of Stan Celestian.