Researchers Find New Ways to Model Plate Tectonics, Soil Erosion
The evolution of wax-microplates is observed from above. A picture is taken every 15 seconds, each image size is 3.0mm across, and the pulling speed is 35 microns/sec. The dark line is the rift. An offsetted spreading center is formed in image (A). As time progresses (A to D), a microplate is formed and rotates counter clockwise and increases in size. In image E the microplate breaks off and is frozen into the newly formed crust (D). The spiral shape can be explained by a model that works both for wax and Earth. Image from milou.msc.cornell.edu/lay_wax.cfm.
Thanks to a tub of molten wax, Cornell University's Eberhard Bodenschatz is able to witness 100 million years of tectonic evolution on the ocean floor — the creation of transform faults, rift valleys and spiral structures called "microplates" — in the course of a single hour. Bodenschatz believes the dynamics of the Earth's crust tell a large portion of the history of this planet, and his wax-tectonics modeling experiments shave away millions of years of field research by simulating portions of Earth's drama in the laboratory. Meanwhile, Daniel Rothman, a geophysicist at MIT, is combining physical reasoning, the analysis of digital elevation maps and computer simulation to predict and describe the mechanics of water-driven landscape erosion. Both researchers reported the latest results of their respective experiments at the APS Centennial meeting in Atlanta.
"Except for the occasional earthquake or volcano eruption, we typically witness few large scale changes in the Earth's crust; the earth's time scale dwarfs the life span of a human being millions of times over," says Bodenschatz. "But undeniably, the earth moves, shaping the land masses upon which we live. There is no reason to expect that in another hundred million years, the North American plate won't be oriented in a direction opposite to its current configuration." Unfortunately, he adds, "Humans lack the time to witness the full extent of this fascinating creation/demolition derby."
Clearly, an experimental system that allows the study of the dynamics of rift formation processes and its dependence on the material parameters of the solidifying crust was badly needed. Following some initial work in the mid 1970's, Bodenschatz and his collaborators developed a technique in which molten wax is frozen at the surface by a flow of cold air. The experimental apparatus is heated from below so that the wax melts and cools from above to create a crust, which represents the Earth's cold, hard lithosphere, while the molten wax below represents its plastic upper mantle. The solid layer is then pulled apart with constant velocity and the formation of the rift between the two solid plates is studied. While peering into the wax tank, one can observe and characterize the many phenomenon occurring on the planet. These in turn can be measured and analyzed to determine the factors contributing to their behavior. Most remarkably, says Bodenschatz, "These phenomena seem to scale to the Earth. That wax spreading patterns should form and be similar to the Earth is fascinating, and leads to deeper questions of dynamics in general."
For example, Bodenschatz found that at very slow spreading rates, the rift remains straight with a deep valley. At fast rates, the pattern is dominated by transform faults and fracture zones. But medium rates were, where Bodenschatz observed the most interesting phenomenon. Microplates, tiny chunks of solid wax that appear at the rift and begin rolling upwards much like a rolling snowball gathering snow, grow with constant velocity in the direction of the rift resulting in a spiral shape. Bodenschatz theorizes that microplates form in a similar fashion on the ocean floor.
Rothman's goal at MIT is to explain why a landscape exhibits its particular features, and why so many - whether it be the underwater continental slope off the coast of Oregon or the Amazon river basin - exhibit similar features. "Based on simple assumptions concerning the tendency of running water to run in the same direction, our model challenges existing theories of landscape evolution that assumes landscapes evolve to a form that minimizes the rate of energy dissipation," he says. On a more practical level, Rothman's work may make it easier for analysts to find oil, predict floods, or determine whether the topological features on the surface of Mars were sculpted by running water. "From the air, riverbed erosion might look like limbs branching off a tree trunk, crystals forming on an icy windowpane, or a nerve cell's axions," says Rothman, and he hopes to make mathematical sense of the dendritic patterns of the river's branches.
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