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.