The main source of heat of the earth that keeps plasticity the mantle is earth’s radioactive core. The part of the mantle close to the core is hotter than that closer to the crust. Therefore, the mantle has convectional currents. These currents drag with them the earth crust above and hence tectonic plate movements. At convergent boundaries, the earth's crust is pressed onto one another to form fold mountains such as the Himalayas. At divergent boundaries, faulting occurs and the subsequent crust depression is filled with water to form oceans.