Lava is molten rock that pours out of volcanoes or from cracks in the earth. It comes from deep in the earth where the heat is great. There, it is called magma. When lava first comes to the surface it is red-hot, reaching temperatures from 7 to 12 times hotter than boiling water.
Lands that were once covered with lava often become quite fertile after weathering has broken the lava into fine soil. Some lavas, such as a glassy lava called perlite, are heated in furnaces. They expand into a frothy material used to manufacture lightweight concrete.
There are two kinds of lava. One kind, called aa, is viscous (sticky) and moves slowly. The other kind, called pahoehoe, is so fluid that when it first erupts it flows down the side of a volcano faster than a person can run. Both kinds cool at the surface first, forming a solid crust over a liquid core.
Many regions of the earth consist of piled-up sheets of lava. In the Northwestern United States, the Columbia lava plateau is made up of a great lava pile more than 5,000 feet (1,500 meters) thick in places. The islands of Hawaii are a chain of volcanoes built mostly of lava. The mountain belt of southern Mexico also has a great lava pile.