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

Cools, moves to the side, and sinks, making a circular pattern.

Explanation:

It cools because it dissipates some of this heat to the lower crust. As it moves to the side, it drags along the crust hence powering the tectonic movements. As it cools, the liquid rock becomes denser and begins to sink to the bottom of the mantle as more hot and less dense liquid rock from the bottom rises. This forms a circular convection loop similar to that of water in a container being heated from below.  

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