It's the temperature differences in the mantle. Warmer magma near the core rises because it becomes less dense. It reaches the surface, pushes against the tectonic plates, then cools and falls back down to be replaced by new warm magma.
Explanation: All the rocky planets in our Solar System, in-
cluding the Earth, initially formed much hot-
ter than their surroundings and have since been
cooling to space for billions of years. The result-
ing heat released from planetary interiors pow-
ers convective flow in the mantle. The man-
tle is often the most voluminous and/or stiffest
part of a planet, and therefore acts as the bottle-
neck for heat transport, thus dictating the rate at
which a planet cools. Mantle flow drives geo-
logical activity that modifies planetary surfaces
through processes such as volcanism, orogene-
sis, and rifting. On Earth, the major convective
currents in the mantle are identified as hot up-
wellings like mantle plumes, cold sinking slabs
and the motion of tectonic plates at the surface.
On other terrestrial planets in our Solar System,
mantle flow is mostly concealed beneath a rocky
surface that remains stagnant for relatively long
periods of time. Even though such planetary
surfaces do not participate in convective circu-
lation, they deform in response to the underly-
ing mantle currents, forming geological features
such as coronae, volcanic lava flows and wrin-
kle ridges. Moreover, the exchange of mate-
rial between the interior and surface, for exam-
ple through melting and volcanism, is a conse-
quence of mantle circulation, and continuously
modifies the composition of the mantle and the
overlying crust. Mantle convection governs the
geological activity and the thermal and chemical
evolution of terrestrial planets, and understand-
ing the physical processes of convection helps
us reconstruct histories of planets over billions
of years after their formation.
