BLOCKS, as PROPOUNDED by the Plate Tectonic Theory.The convection currents in the mantle GENERATED by the interior heating, courtesy of RADIOACTIVE material, is believed to be the cause of the movement of these plates.The mantle extends to a depth of 2,890 km.It is the thickest layer of the Earth consisting of the upper mantle and LOWER mantle.While the inner core (5,150 to 6,276 km) is solid, the outer core (2,897 to 5,150 km) is liquid.