Earth's tectonic plates move due to the movement of magma in the mantle underneath the crust. Extreme temperatures inside the planet's core cause a convection cycle in which hot magma rises to the surface and eventually sinks back toward the core as it cools.
There are many massive convection cells in the mantle, and each cell has an effect on the movement of the plate or plates above it. The convection process pushes the plates, causing them to collide in some places and become separated in others.
The separation of two tectonic plates, known as a divergent plate boundary, causes magma to rise to the surface in the gap between the plates. This magma then cools to form new crust, which further pushes the two plates apart. This action also results in a collision at the opposite side of the plate, as the divergent boundary pushes it into the adjacent plate. The collision occurs at a convergent boundary, which can lead to the formation of either mountains or volcanoes.
Scientists also believe that tectonic plates may have a convection cycle that aids in their movement. Most plates have thinner, warmer parts that tend to rise, while the denser, colder crust sinks deeper into the mantle.