What process provides the source or generates the thermal heat that leads to hotspots and melts the earth's crust?

Convection in the asthenosphere.

Explanation:

Convection in the asthenosphere

Heat which is present in the Earth maintain the malleability of the asthenosphere , and hence provides lubrication to the Earth's tectonic plates for movement .

The convection current that is generated in the asthenosphere , tries to push the magma upward via the vents that are present in the volcanoes and thereby spreads and create a new crust .

And this generates a hotspots on the Earth's crust .