The Earth's gravity is generally perceived as a constant, with one "g" indicating an acceleration of 9.8 meters per second per second. However, small variations in the gravitational field are discernable at different locations, typically with magnitudes of one-ten thousandth of the average gravitational attraction, implying a "mass deficit" in such places. These undulations, known as gravity anomalies, are spread throughout the Earth, but one of the largest is centered over Hudson Bay, Canada. Using a new approach to analyzing planetary gravity fields, two geophysicists, Mark Simons at the California Institute of Technology and Bradford Hager at M.I.T., have shown that incomplete glacial rebound can account for a substantial portion of the Hudson Bay gravity anomaly. About 18,000 years ago, Hudson Bay was at the center of a continental-sized glacier with a thickness of several kilometers. The weight of the ice bowed the surface of the Earth down and after it receded, it left depression in its wake. Having an estimate of incomplete post-glacial rebound allowed Simons and Hager to derive a model of how the viscosity of the mantle changes with depth. Their favored model suggests that underneath the oldest parts of continents the viscosity of the outer 400 kilometers of Earth is much stiffer than under the oceans. Therefore, these continental keels can resist the erosion by the convective flow that drives plate tectonics. For more information, Robert Tindol, "Caltech", at (626) 395-3631.
The Internet happened.” Those three words spoken yesterday by Marc Ostertag, North America president of B&R Automation at Pacific Design & Manufacturing, now taking place in Anaheim through Feb. 11, continues to bring ever-lasting changes to our ways of life and will undoubtedly transform manufacturing.
When you think of the DARPA Robotics Challenge, you may imagine complex humanoid contraptions made of metal and wires that move like a Terminator Series T-90. But what actually happened at the much-vaunted event was something just a bit different.
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