Images are copyright © 1997 by Walter S. Kiefer. All rights reserved.
Although the surface of the Earth is solid, its interior is soft and flows like a very viscous fluid. This flow, called mantle convection, is an important method of heat transport within the Earth. Mantle convection is the driving mechanism for plate tectonics, which is the process ultimately responsible for producing earthquakes, mountain ranges, and volcanos on Earth. Mantle convection is also important on Venus and Mars.
The four images shown at the top of the page illustrate the process of mantle convection. They are based on a computer simulation by Dr. Walter Kiefer (LPI) and Dr. Louise Kellogg (University of California, Davis). The calculations were performed in spherical axisymmetric geometry, with the rotational symmetry axis being vertical in each image. This gives each image the appearance of left-right symmetry about the center line of the image. The black region in the center of each image is the core. The Rayleigh number, which determines the vigor of the convective flow, is 107 in this calculation, which is close to but somewhat less than the degree of convective vigor occurring on Earth today. Most of the heat (about 75%) transported in this model originates from radioactive elements in the mantle, with the rest of the heat coming out of the core.
The different colors represent variations in temperature within the mantle. Red and orange represent hot temperatures, implying material that is rising through the mantle. The hot, pipe-like structure in the center of each image is an example of a mantle plume. On Earth, mantle plumes are thought to produce volcanic features such as the Hawaiian islands. Some volcanic features on Venus and Mars are believed to form in a similar manner. Models that include calculation of the magmatism that occurs in mantle plumes are described on my Mars research page. Blue and green represent cold temperatures, implying material that is sinking into the mantle. These cold regions represent the subduction of lithospheric slabs on Earth. The temperature difference between the hottest and coldest material is about 1500 degrees Kelvin.
The rising and sinking motions deform the outer surface of a planet, contributing to surface topography. Hot, rising material pushes the surface of the planet upward, producing high topography. Similarly, cold sinking material pulls the surface of the planet downward, producing low topography. The gray regions show this topography with considerable vertical exaggeration. The actual topography caused by this process is a few kilometers in amplitude.
These images show the evolution of the mantle's thermal structure over a several hundred million year period. A hot mass of material develops near the base of the mantle and rises toward the surface in the plume at the center of each image. As this material approaches the surface, it pushes the topography up and may also cause a period of enhanced volcanic activity. Thermal structures of this type are believed to underlie volcanic islands such as Hawaii, where there is geologic evidence for fluctuations with time in both the surface topography and the volcanism rate. In addition, these images also show lateral motions of cold sinking regions, analogous to subduction zones on Earth, and the formation of a new rising structure near the base of the mantle.
Abstract of Kiefer and Kellogg time-dependent mantle convection manuscript (includes additional color figures).
Walter S. Kiefer, firstname.lastname@example.org