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 isotopes 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, andmountain 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, as illustrated. 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. The buoyancy body force on the cool descending plume is analogous to the body force on the descending lithosphere.
Ascending mantle plumes can also be associated with thermal convection. There is expected to be a hot thermal boundary layer at the base of the convecting mantle. Buoyancy forces on the low-density mantle rock would be expected to form hot ascending plumes either at the base of the mantle (the D′′-layer of seismology) or at an interface in the lower mantle between an upper convecting layer and an isolated lower layer. The fluidlike behavior of the Earth’s crystalline mantle can be explained by solid-state creep processes. At low stress levels and temperatures approaching the rock solidus, the dominant creep process is the diffusion of ions and vacancies through the crystal lattice.
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