Mantle convection - is when the plates move around the asthenosphere and the magma moves in circular patterns due to heat.
Movement of tectonic plates is related to convection currents in the earth's mantle. Tremendous heat and pressure within the earth cause the hot magma to flow in convection currents. These currents cause the movement of the tectonic plates that make up the earth's crust.
Superposition is the geologic principle which explains why the bottom stratum in a section of rock is the oldest. The word literally means placing one thing on top of another where the lower strata are the older.