Tectonic plates shift as a result of the intense heat at the Earth's core, which causes molten rock in the mantle layer to rise, while cooler rock near the surface sinks back down. This is a process referred to as thermal convection.
This occurs in a current cycle that takes thousands of years to complete. When molten lava rises to the surface of the Earth's crust, it pushes through thin areas and creates new oceanic crust. As this happens, tectonic plates shift in various ways, moving toward and away from each other. The Earth is made up of a dozen major tectonic plates and several smaller plates, all of which are constantly moving.
As molten lava creates new oceanic crust and thermal convection causes cooler rock to sink, the coastal regions of the Earth are constantly losing land mass, while simultaneously gaining land mass in other areas. Mount Kilauea, an active volcano in Hawaii, is an example of this. Since 1983, approximately 500 acres of land have been added to the island by molten lava accumulation, according to the National Park Service.
Earth’s land masses move at an average rate of about 0.6 inch a year. The coast of California moves at a much faster speed of approximately 2 inches per year, causing the tectonic plates in this area to grind violently, resulting in frequent earthquakes.