MARK SIMONS / CALIFORNIA INSTITUTE OF TECHNOLOGY, 1997 / SCIENCE PHOTO LIBRARY MARK SIMONS / CALIFORNIA INSTITUTE OF TECHNOLOGY, 1997 / SCIENCE PHOTO LIBRARY
Gravity anomalies. Free-air gravity anomaly map, showing the low-gravity anomaly in Hudson Bay, Canada, which is partly due to the last ice age. These anomalies are the differences between the theoretical value of gravity at the surface and the measured value. The colours range from purple (low gravity), through blue, green (normal) and yellow to red (high). The Hudson Bay anomaly makes a small but significant 0.005% difference. It occurs partly because the area's rocks are still recovering from being compressed by the Laurentide ice sheet 10,000 years ago. The high density of molten rock under the area also contributes.
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