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Extrusive refers to the mode of igneous volcanic rock formation in which hot magma from inside the Earth flows out (extrudes) onto the surface as lava or explodes violently into the atmosphere to fall back as pyroclastics or tuff. This is as opposed to intrusive rock formation, in which magma does not reach the surface. The main effect of extrusion is that the magma can cool much more quickly in the open air or under seawater, and there is little time for the growth of crystals.
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.