Plate tectonics provides a general framework for understanding the world-
wide distribution of seismicity, volcanism, and mountain building. These
phenomena are largely associated with plate interactions at plate margins.
However, an explanation must also be given for the relative motions of the
plates. The basic mechanism responsible for plate tectonics must provide
the energy for the earthquakes, volcanism, and mountain building. The only
source of energy of sufficient magnitude is heat from the interior of the
Earth. This heat is the result of the radioactive decay of the uranium iso-
topes 238U and 235U, the thorium isotope 232Th, and the potassium isotope 40K as well as the cooling of the Earth. An accurate estimate of the heat
lost from the interior of the Earth can be obtained from measurements of
the surface heat flow. The energy associated with seismicity, volcanism, and
mountain building is about 1% of the heat flow to the surface.
Heat can be converted to directed motion by thermal convection. Consider
a horizontal fluid layer in a gravitational field that is heated from within and
cooled from above. The cool fluid near the upper boundary is heavier than
the hotter fluid in the layer. Buoyancy forces cause the cool fluid to sink,
and it is replaced by hot rising fluid. Laboratory experiments show that
under appropriate conditions two-dimensional convection cells develop. A thin thermal boundary layer of cool fluid forms
adjacent to the upper boundary of the layer. Thermal boundary layers from
two adjacent cells merge and separate from the upper boundary to form a
cool descending plume. The negative buoyancy of the cool descending plume
drives the flow. The thin thermal boundary layer is directly analogous to the
lithosphere. The separation of the thermal boundary layers to form the cool
descending plume is analogous to subduction.