PASADENA—While the gravity field of Earth is commonly thought of as constant, in reality there are small variations in the gravitational field as one moves around the surface of the planet.
These variations have typical magnitudes of about one–ten thousandth of the average gravitational attraction, which is approximately 9.8 meters per second per second. A global map of these variations shows large undulations at a variety of length scales. These undulations are known as gravity anomalies.
There are many such anomalies in Earth's gravity field, but one of the largest negative gravity anomalies (implying the attractions of gravity being a little less than average, or in other words, a mass deficit) 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.
With this new information, Simons and Hager were able to place new constraints on the variations in strength of the materials that constitute the outer layers of Earth's interior (the crust and mantle). Their work appears in the December 4 issue of the journal Nature.
About 18,000 years ago, Hudson Bay was at the center of a continental–sized glacier. Known as the Laurentide ice sheet, this glacier had a thickness of several kilometers. The weight of the ice bowed the surface of Earth down. The vast majority of the ice eventually melted at the end the Ice Age, leaving a depression in its wake.
While this depression has endured for thousands of years, it has been gradually recovering or "flattening itself out." The term "glacial rebound" refers to this exact behavior, whereby the land in formerly glaciated areas rises after the ice load has disappeared.
Evidence of this is seen in coastlines located near the center of the former ice sheet. These coastlines have already risen several hundred meters and will continue to rebound.
"The rate at which the area rebounds is a function of the viscosity of Earth," says Simons. "By looking at the rate of rebound going on, it's possible to learn about the planet's viscosity."
Simons says that geophysicists have known for some time about the Hudson Bay gravity anomaly, but have hitherto been uncertain how much of the gravity anomaly is a result of glacial rebound and how much is due to mantle convection or other processes.
The gravity anomaly is measured from both the ground and from space. Simons and Hager use a gravity data set developed by researchers at the Goddard Space Flight Center.
However, knowing how much of an anomaly exists at a certain site on Earth is not sufficient to determine the pliability of the materials beneath it. For this, Simons and his former M.I.T. colleague Hager have developed a new mathematical tool that looks at the spatial variations of the spectrum of the gravity field.
In many instances, this approach allows one to separate the signatures of geologic processes that occur at different locations on Earth. In particular, Simons and Hager were able to isolate the glacial rebound signature from signatures of other processes, such as manifestations of plate tectonics, that dominate that gravity field but are concentrated at other geographic locations.
Having an estimate of incomplete postglacial rebound allowed Simons and Hager to derive a model of how the viscosity of the mantle changes with depth. Simons and Hager propose one such model that explains both the gravity anomaly as well as the uplift rates estimated from the coastlines.
Their favored model suggests that underneath the oldest parts of continents (some of which are over 4 billion years old) 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.