The center of the Earth is hot because of radioactivity. It is also hot partly because of the energy that was released from the stuff that made up the Earth as that stuff fell and hit the rest of what was becoming Earth. (Its kinetic energy turned to heat.)
Because the Earth's core is hot, magma in the mantle is constantly moving. The magma pushes up in the boundaries between tectonic plates. The pressure from this makes them move.
So, the idea of plate tectonics is that the crust is made up of these plates that are pushed around by the magma of the mantle.
Plate tectonics refers to the description of the structure of the top layer of the earth, and the movements that take occur in this layer. that are responsible for the origin of most of the physical features of of the earth's surface. Among others, plate tectonics also explain why earthquakes occur and why earthquakes and volcanoes occur where they do.
As per plate tectonic theory the outer layer of earth is composed of about 30 rigid pieces called tectonic plates, that move about on the layer of molten rocks. The movement is very slow, about 10 centimeters per year, but it is there. This phenomenon has been going on for millions of years. Therefore, these plates have moved vast distances in spite of their slow speed. The movement of the plates take place because they are resting on a mass of molten core of the earth, with no firm bonding. The forces to move the plates is generated by the rotation of earth and the reaction occurring within the core of earth due to its intense heat.
Tectonic plates are made up of earth's crust and outermost part on its mantle. The thickness of the plates is not uniform. At most of the places these are about 100 kilometres thick, while at some places they are less than 8 kilometers thick. The continents and all other surface of the earth including ocean floors are embedded on the top of these pates.
The earths center is hot because it was formed out of the very hot matter separated from sun. AS the earth cooled, the outer surface was the fist to cool down and solidify. As we go down towards the center of the earth the degree of cooling is progressively less with the result that the temperature at the core of the earth is about 7000 degrees C. But the Earth's core is also cooling down surely but slowly.