During mantle convection, heat from the earth's core causes material in the mantle to expand and move upward. What happens when it reaches the top?

One common example is the mid ocean ridges where the magma comes to the top of the crust and spreads out creating new crust and driving the tectonic plates apart.

A more extreme example is Iceland, where the magma comes to the surface creating explosive volcanos that have form the Islands of Iceland.

A third example are the rift valleys where the rising magma cools before it reaches the surface for the most part. The crust is split apart as the magma pushes out in two directions as it rises. This is causing the rift valleys to become wider. It is possible that the rift Valley will become an ocean as the two parts of the crust are pushed further apart. Also there are volcanos associated with rift valleys such as Kilimanjaro in Tanzania and Mount Kenya, created by the rising magma.

When material in the mantle reaches the top during mantle convection, it spreads horizontally, cools down, and becomes denser. As it cools, it begins to sink back down towards the core, completing the convective cycle. This continuous movement of material within the mantle helps drive the process of plate tectonics on Earth's surface.