Amy has taught university-level earth science courses and has a PhD in Geology.
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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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.
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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.
Some extrusive igneous rocks will have bigger crystals set within a finer matrix. These rocks are called porphyritic. Don't be confused by this because these are still extrusive rocks, despite having some large crystals. In the case of these rocks, the bigger crystals actually formed deeper within the volcano, where they had enough time to grow to a larger size. However, these crystals were then carried to the surface by a liquid magma, where they erupted and the rest of the magma cooled. Because the majority of the lava cooled on the surface, these porphyritic rocks are extrusive.
Another clue to the extrusive nature of a rock can be the presence of vesicles. Vesicles are sphere-shaped cavities in extrusive rocks, which form when lava crystallizes with bubbles of gas trapped inside. Have you ever noticed that ice cubes often have small bubbles of air trapped inside? Vesicles in rocks are formed much the same way. Instead of trapping air, like bubbles in ice, the lava traps gases from the volcano during cooling. But just as not all ice has air bubbles, not all extrusive rocks have vesicles. So while you can use vesicles to determine a rock is extrusive, the absence of vesicles doesn't mean that the rock is intrusive.
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Rocks containing vesicles, like this type pictured above, called scoria, are called vesicular.
Igneous rocks are classified into two categories: intrusive and extrusive rocks. Extrusive rocks form from lava that crystallizes on the earth's surface. Two good indicators to help you decide whether you are looking at an extrusive rock are small crystal size and the presence of vesicles. While most extrusive rocks have small crystal size, occasionally you may see porphyritic rocks, which have large crystals in a matrix of smaller crystals. Another unique extrusive rock is obsidian, which cools so quickly that it has no crystals.
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