Question

What is the source for geothermal energy?

Answer 1

Lets start with the basics first. Do you know what geo means?

Answer 2

volcano ...

Answer 3

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Answer 4

Geo means earth, and thermal means heat.
Earthheat energy, so it's not necessarily a volcano, but perhaps the lava underneath it.

Answer 5

The geothermal energy of the Earth's crust originates from the original formation of the planet (20%) and from radioactive decay of minerals (80%). The geothermal gradient, which is the difference in temperature between the core of the planet and its surface, drives a continuous conduction of thermal energy in the form of heat from the core to the surface.
for more reading https://en.wikipedia.org/wiki/Geothermal_energy