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Extrusive Rocks: Definition & Examples

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Instructor: Amy Lange

Amy has taught university-level earth science courses and has a PhD in Geology.

This lesson covers one of the types of igneous rocks, called extrusive rocks. By the end of this lesson, you should know the definition and some ways to identify extrusive rocks yourself.

Definition of Extrusive Rocks

Kilauea on the big island of Hawaii has been erupting continuously for the past 30 years. Perhaps you've seen either video or pictures of the lava oozing out of the earth and into the sea. You might have also seen images from the recent and much more explosive eruptions at volcanoes in Indonesia and Iceland. Both the lava oozing out of the earth in Hawaii and the rocks blown into the sky in Indonesia are forms of extrusive rocks.

Extrusive rocks are igneous rocks that crystallize on the earth's surface. Keep in mind igneous rocks are rocks that crystallize from liquid magma. The other type of igneous rock is intrusive rock. Intrusive rocks crystallize below the earth's surface and are covered in greater detail in another lesson.

A good way to remember the difference between intrusive and extrusive rocks is extrusive rocks crystallize on the exterior of the volcano.

An important reminder is that liquid rock below the surface is termed magma. When this liquid rock erupts onto the surface, it is termed lava. Both of these terms can refer to the same batch of liquid rock, but it has a different name depending on where it is located.

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  • 0:01 Definition of Extrusive Rocks
  • 1:16 How to Spot an Extrusive Rock
  • 3:40 Lesson Sumary
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How to Spot an Extrusive Rock

Even if you didn't personally see the lava crystallize yourself, there are still many clues that can help us identify an extrusive rock.

The first clue is crystal size. Crystals take time to grow. Once it erupts on the surface, lava cools down very quickly because the air is much colder than beneath the volcano. Often, the lava cools down so quickly that there is only enough time for tiny crystals to form.

granite and basalt

This rock (pictured above) is unique because it shows the contact between intrusive and extrusive rock types. The lighter left side is an intrusive rock (granite). The right side is an extrusive rock (basalt). The crystals on the extrusive side are so small you cannot see them without a microscope.

Sometimes, the rock will cool so quickly that there is not enough time for any crystals to form. Obsidian is an extrusive volcanic rock that cools so quickly that it forms a glass rather than forming individual crystals.

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