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.