There are three types of rocks: igneous rocks, sedimentary rocks and metamorphic rocks. Igneous rocks are formed when molten rocks (known as magma) from underneath the Earth's surface rise up, cool down and solidify. This process can take place both above the surface of Earth or below it. When the magma solidifies over the surface of Earth, the resulting igneous rocks are known as extrusive rocks. When the magma cools down and solidifies under the surface of Earth, the resulting rocks are known as the intrusive rocks.
Since these rocks cool slowly underneath the Earth, they have a long time available to them and hence develop larger crystals. In comparison, extrusive rocks have small grained crystals. Granite is an example of an intrusive rock, while basalt is an extrusive rock.
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