Why there is a hotspot inside earth?

There are two main contributors to the heat within the earth.

The first, and the classic answer to the question, is that the suns gravity causes the earths insides to rub against each other, breaking up the rock and generating heat through friction.

The second, and more important, factor generating about twice as much heat as all human activity combined is a huge ongoing nuclear reaction at the Earth's core. This is fed by nuclear elements, such as uranium and plutonium, 'falling' through the earth's liquid core towards the center of the earth, where it is compacted by the combined weight of the entire planet.

http://blogs.scientificamerican.com/observations/2011/07/18/nuclear-fission-confirmed-as-source-of-more-than-half-of-earths-heat/

SCIENCE!