The mechanism by which tectonic plates move is still a subject of much debate among Earth scientists. The Earth is dynamic thanks to its internal heat, which comes from deep within the mantle from the breakdown of radioactive isotopes. This causes convection in the mantle – hot rocks rise and cold rocks descend. This very slow motion in the solid state transfers stresses to the lithosphere, just as convection in a boiling pan of thick soup will cause the skin to buckle where the convection cells meet.
As the theory of plate tectonics developed, mantle convection was long thought to be responsible for the movement of tectonic plates across the Earth’s surface. This theory is now largely out of favour, with modern imaging techniques unable to identify convection cells in the mantle sufficiently large to drive plate movement. Instead, it is thought to be caused by 'slab pull'. Newly formed oceanic lithosphere at mid ocean ridges is less dense than the asthenosphere, but becomes denser with age as it cools and thickens. This causes it to sink into the mantle at subduction zones, pulling slabs of lithosphere apart at divergent boundaries and resulting in sea floor spreading or rifting. How plate movement operates in detail, however, is highly controversial.