What causes the mantle to flow?

The mantle present in the Earth's crust flows due to a process known as mantle convection; convection currents that transport heat to the surface of the Earth are responsible for this activity. These currents permit molecules to travel through liquids or gases, an activity known as convection.

Heat needs to flow to a cooler area, and the heat found in the interior of the Earth wants to move to the planet's surface because it is colder. When convection occurs, warm air moves to the surface of the Earth, while cool air flows to the inside of the earth. Convection currents are also a contributing factor to the movement of tectonic plates.