Understanding the New View of Tectonic Plates
September 7, 2010 1 Comment
Tectonic plates of Earth has ever been a mystery as to how it works. But now this mystery seems to be resolved. Scientists at Caltech have developed new computer algorithms that for the first time allow for the simultaneous modeling of the earth’s mantle flow, large-scale tectonic plate motions, and the behavior of individual fault zones, to produce an unprecedented view of plate tectonics and the forces that drive it. A paper describing the whole earth model and its underlying algorithms was published in the August 27 issue of the journal Science and also featured on the cover.The work illustrates the interplay between making important advances in science and pushing the envelope of computational science. To create the new model, computational scientists at Texas’s Institute for Computational Engineering and Sciences (ICES) pushed the envelope of a computational technique known as Adaptive Mesh Refinement (AMR).
Partial differential equations such as those describing mantle flow are solved by subdividing the region of interest (such as the mantle) into a computational grid. Ordinarily, the resolution is kept the same throughout the grid. However, many problems feature small-scale dynamics that are found only in limited regions. AMR methods adaptively create finer resolution only where it’s needed. This leads to huge reductions in the number of grid points, making possible simulations that were previously out of reach. The complexity of managing adaptivity among thousands of processors, however, has meant that current AMR algorithms have not scaled well on modern petascale supercomputers. Petascale computers are capable of one million billion operations per second. To overcome this long-standing problem, the group developed new algorithms that allows for adaptivity in a way that scales to the hundreds of thousands of processor cores of the largest supercomputers available today.
[Image Details:Tectonic plate motion (arrows) and viscosity arising from global mantle flow simulation. Plate boundaries, which can be seen as narrow red lines are resolved using an adaptively refined mesh with 1km local resolution. Shown are the Pacific and the Australian tectonic plates and the New Hebrides and Tonga microplates.]
With the new algorithms, the scientists were able to simulate global mantle flow and how it manifests as plate tectonics and the motion of individual faults. The AMR algorithms reduced the size of the simulations by a factor of 5,000, permitting them to fit on fewer than 10,000 processors and run overnight on the Ranger supercomputer at the National Science Foundation. A key to the model was the incorporation of data on a multitude of scales. Many natural processes display a multitude of phenomena on a wide range of scales, from small to large. For example, at the largest scale—that of the whole earth—the movement of the surface tectonic plates is a manifestation of a giant heat engine, driven by the convection of the mantle below. The boundaries between the plates, however, are composed of many hundreds to thousands of individual faults, which together constitute active fault zones. Gurnish said:
The individual fault zones play a critical role in how the whole planet works and if you can’t simulate the fault zones, you can’t simulate plate movement—and, in turn, you can’t simulate the dynamics of the whole planet.
In the new model, the researchers were able to resolve the largest fault zones, creating a mesh with a resolution of about one kilometer near the plate boundaries. Included in the simulation were seismological data as well as data pertaining to the temperature of the rocks, their density, and their viscosity—or how strong or weak the rocks are, which affects how easily they deform. That deformation is nonlinear—with simple changes producing unexpected and complex effects.
Normally, when you hit a baseball with a bat, the properties of the bat don’t change—it won’t turn to Silly Putty. In the earth, the properties do change, which creates an exciting computational problem. If the system is too nonlinear, the earth becomes too mushy; if it’s not nonlinear enough, plates won’t move. We need to hit the ‘sweet spot.
[Image Details:Cross section showing the adaptively refined mesh with a finest resolution of about 1km in the region from the New Hebrides to Tonga in the SW Pacific. The refinement occurs both around plate plate boundaries and dynamically in response to the nonlinear rheology.]
One surprising result from the model relates to the energy released from plates in earthquake zones. Much of the energy dissipation occurs in the earth’s deep interior. We never saw this when we looked on smaller scales.