New research using GPS satellites shows that much of Canada continues to rise by up to half an inch a year after being covered during the last ice age that ended 10,000 years ago, while sites in much of the US south of the Great Lakes, are sinking.
The reason, says Seth Stein, a professor of geological sciences at Northwestern University in Evanston, Ill., is that material from Earth’s sticky but still fluid mantle is slowly moving north from under the Midwest and Northeast, where it was pushed from the weight of heavy ice sheets tens of thousands of years ago.
“The mantle material was squeezed out and now has to flow back north,” Stein said. “It is amazing that we can actually see this going on now. The glaciers continue to make their presence felt.”
The most recent advance in ice cover across the continent, called the Wisconsin glaciation because of scars it left on that state, lasted from 115,000 years ago until 10,000 years ago.
Ice that was thousands of feet thick over Hudson Bay lowered Earth’s crust in that area by as much as several hundred feet. When the glaciers receded, the crust began to rebound and shifting ice masses redistributed lakes and rivers, including filling the Great Lakes that had been gouged out by the ice.
Stein, postdoctoral researcher Giovanni Sella, and colleagues at several other institutions used readings from GPS satellites and ground-level transponders at more than 200 sites across the continent to “see” the land moving as it continues to adjust from the glaciers. Sella is to report on the findings next week during a joint meeting of the American and Canadian geophysical unions in Montreal.
“If you take the load off of road tar, it won’t pop back immediately,” Stein said. ‘The earth is similar – the ground continues to rebound as the viscous mantle flows back in.”
The GPS system can detect motions as small as 1/25th of an inch per year.
As the mantle, an 1,800-mile-thick layer that lies between Earth’s crust and molten core, moves back in, the changes are registered at the surface.
Post-glacial rebound also affects the water levels of the Great Lakes. “You can see it on the tide gauges. With the northern shores tipping up, water levels are dropping, while on the southern side, the shores are sinking and water levels are rising,” Stein said.
This impacts not only industries and residents along the lakes’ shores, but also the international management of water levels and navigation through them.
The small motions may also be one cause of the mysterious earthquakes that periodically rattle the center of North America, including the St. Lawrence Valley, northern New England into Newfoundland and perhaps even in the New Madrid earthquake zone in the Mississippi Valley.
“This idea has been around for a while, but until now, no one knew how large the ground movements were across the area,” said Stein. “We believe they may have significant effects.”
He said scientists aren’t sure how long it takes the mantle to level out after an ice age. Ice ages seem, at least for the past few million years, to happen on 100,000-year cycles, with warm intervals lasting 20,000 or 30,000 years.
“It depends on just how gooey the mantle is as to how fast it flows back in,” Stein said. “We’re learning a lot about the mantle from these readings and studying several different models that people have come up with for how the mantle behaves over time to see what best fits our flow data.”