Evidence of Heat Flow: Volcanoes
The partial melting of rock deep in the Earth creates lava. Magma accumulates in an underground chamber and rises through the crust to the surface to form a volcano. Erupting "pyroclastic" material can include anything from lava (molten fluid rock) to ash (tiny, dust-sized fragments of frozen melt from an exploding magmatic foam).
Most volcanoes occur at plate boundaries. The Pacific Ring of Fire is a well-known series of volcanoes occurring around the Pacific plate.
Volcanoes that do not occur at plate boundaries are called hot spots. The Hawaiian Islands are examples of this type of volcano.