I. Heat in the Earth. A. Earth is hot inside. Frying pan handle. Convection - heat transfer with deformation of the body. C. Convection occurs in Gases, Liquids and Solids.
Higher temperatures yield weaker materials - lower viscosity.
The temperature of the earth is hot enough to cause convection in both the molten iron core and in the overlying mantle. Lithospheric pieces are strong enough to behave as rigid plates, playing bumper cars. Divergent - plates moving apart. Mid-Ocean Ridges and Continental Rift zones. Few large quakes, volcanoes common.