The Earth's outermost layer is made up of several large pieces, called tectonic plates. These plates are constantly moving around on the planet's surface. Sometimes, they move apart from each other (divergent boundaries), and sometimes they move towards each other (convergent boundaries). At convergent boundaries, one plate usually slides underneath the other (subduction).
Convection currents play a big role in moving the Earth's tectonic plates around. Convection occurs when hot material rises to the surface, while cooler material sinks down. This movement creates a circulating current. The hot mantle material beneath the Earth's crust is less dense than the colder crust material above it. So, it rises up towards the surface. As it rises, it pushes the crustal plates aside. The mantle material then cools off and sinks back down again. This cyclic motion of convection currents slowly moves the Earth's tectonic plates around over time.