Convection in the mantle is the same as convection in a pot of water on a stove. Convection currents within Earth’s mantle form as material near the core heats up. As the core heats the bottom layer of mantle material, particles move more rapidly, decreasing its density and causing it to rise. The rising material begins the convection current. When the warm material reaches the surface, it spreads horizontally. The material cools because it is no longer near the core. It eventually becomes cool and dense enough to sink back down into the mantle. At the bottom of the mantle, the material travels horizontally and is heated by the core. It reaches the location where warm mantle material rises, and the mantle convection cell is complete.

Plate tectonics is the unifying theory of geology. Plate tectonics theory explains why:

• Earth’s geography has changed through time and continues to change today.
• some places are prone to earthquakes while others are not.
• certain regions may have deadly, mild, or no volcanic eruptions.
• mountain ranges are located where they are.

Plate tectonic motions affect Earth’s rock cycle, climate, and the evolution of life.