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Rocks resulting from cooled-off lava include obsidian, lava rock, basalt or pumice. These are known as extrusive igneous rocks because they are formed outside of the crust. There are about 700 different types of igneous rock. More examples of igneous rocks include tuff, diorite and andesite.Here are some uses of igneous rocks: Granite is used for building and construction because of its strong properties. It is also used for head stones and kitchen counter tops.here are about 700 different types of igneous rock. More examples of igneous rocks include tuff, diorite and andesite. Here are some uses of igneous rocks: Granite is used for building and construction because of its strong properties. It is also used for head stones and kitchen counter tops.
Some extrusive igneous rocks cool so rapidly that crystals do not develop at all. These form a glass, such as obsidian (Figure 4.14). Others, such as pumice, contain holes where gas bubbles were trapped when the material was still hot and molten.The holes make pumice so light that it actually floats in water. The most common extrusive igneous rock is basalt, a rock that is especially common below the oceans (Figure 4.15).Figure 4.13: Extrusive igneous rocks form after lava cools above the surface. The lave spills out from the Earth at a volcano.he holes make pumice so light that it actually floats in water. The most common extrusive igneous rock is basalt, a rock that is especially common below the oceans (Figure 4.15). Figure 4.13: Extrusive igneous rocks form after lava cools above the surface.