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.
Wearables are changing the way we see ourselves. With onboard sensors that have access to our bodies, we are starting to know our physical selves like never before, quantifying our activity, our heart rate, breathing, and even our muscle effort.
This year, Design News is getting a head start on the Fourth of July celebration. In honor of our country and its legacy of engineering innovation -- in all of its forms -- we are taking you on an alphabetical tour through all 50 states to showcase interesting engineering breakthroughs and historically significant events.
Focus on Fundamentals consists of 45-minute on-line classes that cover a host of technologies. You learn without leaving the comfort of your desk. All classes are taught by subject-matter experts and all are archived. So if you can't attend live, attend at your convenience.