Lava is magma, hot molten rock that flows through holes in the Earth's crust and onto the surface. Like magma, lava can be either viscous (~thick) or fluid (~thin). Blocky lava is so thick and slow that it barely moves along the ground. Other types of lava, like "pahoehoe", "aa", and "pillow lava", are thinner and flow faster. Lava sometimes sets things on fire. Sometimes it even destroys towns.