Mantle Convection
Mantle convection is a slow process when heat is transferred from the Earth’s core upwards to the surface. The heating of the mantle by the core creates convection cells and then hot mantle rises. It cools as it goes toward the crust until it reaches a less dense material and then spreads out and comes back down.
The Earth’s crust is broken into plates that float on top of the mantle. These plates are always in motion. Scientists believe that the rising mantle spreads and moves apart the plates in the crust. Intense heat causes the rocks to rise and then they cool and sink back down to the core. This convection, which looks like a lava lamp, is believed to be what causes the tectonic plates to move. Earthquakes occur when these plates grind against each other. Mountains form when the plates collide and deep trenches form when one plate slides under another plate. When the mantle pushes through the crust, volcanoes erupt.
Typical mantle convection speed is 20 mm/yr near the crust but can vary quite a bit. The convection in the upper mantle is much faster than the convection near the core. A single shallow convection cycle can take 50 million years and a deeper convection can be closer to 200 million years.