The Earth is heated from the core and transfers heat in to the crust causing it to move. This movement depends on temperature. The higher the temperature the greater it's volume will become and the crust will begin to expand. It also depends on the density of the rocks. More dense rocks rise more rapidly than less dense rocks.

Thermal Convection

Scientists tried to come up with more ideas on how the Earth flowed. One theory was thermal convection. This occurs when material is heated and rises upward. Then the material spreads out, cools and sinks back into the core. Aurthur Holmes came up with this idea in 1929 when he was studying about the Earth. Although this is a widely accepted theory scientists are not sure if it is completely accurate as they cannot observe it directly.