The Great Lakes formed where they did 20,000 years ago thanks to a hotspot that sat under the supercontinent Pangaea 300 million years ago, before North America even existed.
New research finds that the Cape Verde hotspot, which still exists under the island nation in the Central Atlantic Ocean, heated and stretched the crust under the spot that would eventually become the Great Lakes. This process, which happened over tens of millions of years, led to a low spot in the topography of the region, which glaciers later scraped out during the ice age. After the glaciers retreated, their melt filled the lakes, which now hold 21% of the world’s fresh water.
“It was the hotspot which made the first imprint,” said Aibing Li, a seismologist at the University of Houston and a co-author of the new paper, published Dec. 25 in the journal Geophysical Research Letters.
Hotspots are plumes of hot material that rise from the mantle, Earth’s middle layer. When hotspots interact with the crust, they can create volcanoes, such as the Hawaiian Islands. Yellowstone National Park also formed because of a hotspot, which left a trail of volcanism through Oregon, Idaho, Montana, Nevada and Wyoming as the North American continent crept over it.
The traces of ancient hotspots are harder to detect, as old volcanoes erode. However, there are two hotspots in the Atlantic today — the Great Meteor hotspot and the Cape Verde hotspot — that geologists know, based on how the tectonic plates have moved over hundreds of millions of years, must have once been under North America. The Great Meteor hotspot traced a line under what is now the border of Ontario and Quebec and then cut across modern-day Vermont and New Hampshire and out into the Atlantic between 150 million and 115 million years ago. This process is confirmed by the presence of kimberlites, rocks from rapid volcanic eruptions that can carry diamonds to the surface.
The Cape Verde hotspot, on the other hand, had been little studied. Li and her team were working on understanding the formation and evolution of the North American continent when they discovered something odd in the Great Lakes region: In the crust under the lakes, earthquake waves moved oddly — they traveled at different velocities going horizontally versus vertically. This phenomenon is called “radial anisotropy.”
“Usually you see this anisotropy when the…
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