Gary A. Glatzmaier of the Institute of Geophysics & Planetary Physics at Los Alamos National Laboratory explains the computer modeling of field reversals. The first dynamically-consistent, three-dimensional computer simulation of the geodynamo (the mechanism in the Earth's fluid outer core that generates and maintains the geomagnetic field) was accomplished and published by Paul H. Roberts of the University of California at Los Angeles and myself in 1995. We programmed supercomputers to solve the large set of nonlinear equations that describe the physics of the fluid motions and magnetic field generation in the Earth's core. Image: Gary A. Glatzmaier, Paul H. Roberts COMPUTER SIMULATION shows a magnetic pole reversal taking place over a period of about 1,000 years. Magnetic field lines are blue where the field is directed inward and yellow where it is directed outward. The simulated geomagnetic field, which now spans the equivalent of over 300,000 years, has an intensity, a dipole-dominated structure and a westward drift at the surface that are all similar to the Earth's real field. Our model predicted that the solid inner core, being magnetically coupled to the eastward fluid flow above it, should rotate slightly faster than the surface of the Earth. This prediction was recently supported by studies of seismic waves passing through the core. In addition, the computer model has produced three spontaneous reversals of the geomagnetic field during the 300,000-year simulation. So now, for the first time, we have three-dimensional, time-dependent simulated information about how magnetic reversals can occur. The process is not simple, even in our computer model. Fluid motions try to reverse the field on a few thousand-year timescale, but the solid, inner core tries to prevent reversals because the field cannot change (diffuse) within the inner core nearly as quickly as in the fluid, outer core. Only on rare occasions do the thermodynamics, the fluid motions and the magnetic field all evolve in a compatible manner that allows for the original field to diffuse completely out of the inner core so the new dipole polarity can diffuse in and establish a reversed magnetic field. The stochastic (random) nature of the process probably explains why the time between reversals on the Earth varies so much. Answer originally posted on April 6,1998. « previous
Ali Oncel [email_address] Department of Earth Sciences KFUPM Final Lecture: Heat Flow Solid Earth Geophysics
Temperature contours and heat flow profile for a subduction zone Depressing in temperature as the cold slab subducts, leading to low heat flow in the forearc region . Migrating hot fluids upward from the top of the subducting plate produce magma and high heat flow at the volcanic arc.