Some extrusive igneous rocks cool so rapidly that crystals do not develop at all.
These form a glass, such as obsidian. 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.