The material separation of the earth according to their density has played a vital role in this process. It has a very dense metallic core, comparatively less dense rocky mantle and low density crust. It might have happened two ways; the heat raised by radio active combination that caused the interior melting, and the energy created by its formation process. The answer for how the earth’s external crust became less dense is clear. It happened as the densest materials submerged in to the melted interior of the planet.
As mentioned earlier, the heat created by the radioactive decay or the internal melting naturally have the tendency to flow out. Subsequently other natural phenomena such as convection currents, magnetic field, plate motion, faults, volcanoes, and many more occur. As the outer crusts are comparatively cooler, the outward flow of heat through them makes the earth always geologically active. The moon or other small planets are cool and inactive because they are small in size and have little heat flow from interior (Seeds, Backman,