Stacks of lava that are very thick can take years to cool down completely. Lava forms a thin crust at the top that cools in 10 to 15 minutes, but the flow underneath can take much longer to cool.
Lava is molten rock that erupts from volcanoes. Lava floats to the surface of the Earth because it is less dense than the rock around it. When it escapes from the surface, it is called magma. Lava that is not very thick can flow very far from a volcano, but thick lava stays close, creating the base of the volcano. The temperature of lava depends on the minerals in it and their composition.