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.