The inner core of the Earth, which is made of Iron and Nickel, is still in the melted form which retains plenty of heat enough to keep an active geology at the Earth’s crust.
To understand the radioactivity concept as a cause of heat production inside the Earth, lets refer to Rogers et al. who affirm that since the radioactive elements are incompatible, the radiogenic heat that is produced “per unit mass” of the crust is “one hundred times greater” than that produced in the interior, but since the interior is more enormous than the crust, the overall heat is distributed between the mantle and the crust and the radioactive decay of elements provide enough heat to keep the earth’s crust in constant motion (54-55). Heat emitted as a result of radioactive decay or breaking apart of nuclei “not recognized in Lord Kelvin’s time” (Garrison 67) flows inside out through conduction and convection causing the occurrence of volcanoes and earthquakes. This radioactive decay caused the melting of the mass of the Earth at the first place when most of the molten Iron sank down in the interior emitting copious