Lava is molten rock that flows out of a volcano or breaks through cracks in the Earth's surface. Deep underground, rock gets so hot from the Earth's internal heat that it melts into a thick, glowing liquid called magma. When that magma reaches the surface, we call it lava.

Lava can be incredibly hot, reaching temperatures of about 1,300 to 2,200 degrees Fahrenheit. It flows like a river of fire, sometimes moving slowly enough that you could walk away from it, other times rushing forward at speeds faster than you can run. As lava cools, it hardens back into solid rock, which is why volcanic islands like Hawaii are made largely of layer upon layer of hardened lava.

When lava cools quickly, it can form shiny black glass called obsidian. When it cools while full of gas bubbles, it creates pumice, a rock so light it can float on water.