Lava flow types, illustration. Lava is molten rock that is produced by volcanoes as an upwelling of the magma found beneath the Earth's crust. The lava can range in temperature from 500 to over 1000 degrees Celsius. It has a chemical composition specific to its volcano. Lava flow speeds vary, but they are generally viscous and slow-moving. They will slowly solidify to form black volcanic rock. At top is aa lava (with a rough, rubbly surface). At centre is pahoehoe lava (with an undulating surface). At bottom is a block lava flow (with a blocky, angular surface).