Great Lakes Started Forming Before North America Existed

The Great Lakes were filled roughly 20,000 years ago as glaciers retreated, pouring their melt into the depressions left behind. But the world's largest group of freshwater lakes can be traced back much further—hundreds of millions of years, in fact, according to a new study, which finds they started not with water, but with heat. Researchers say a hot spot now residing under the Atlantic island nation of Cape Verde sat beneath what's now the Great Lakes some 300 million years ago when the continents were joined as the supercontinent Pangaea, and it had a major role in creating the lakes that today hold more than 20% of the world's fresh water, reports Phys.org. As University of Houston seismologist Aibing Li tells Live Science, "It was the hot spot [that] made the first imprint."

Hot spots occur where hot plumes rise up from the Earth's mantle, forming volcanoes. But they can be difficult to track because old volcanoes disappear over time. For this study, researchers tracked the movement of land over the Great Meteor (New England) hot spot, now found in the North Atlantic Ocean, using the presence of kimberlites—igneous rocks that form when diamond-rich magma cools. They found the hot spot tracked along the border of what's now Ontario and Quebec, across to Vermont and New Hampshire and out into the ocean between 150 million and 115 million years ago. But researchers also modeled the movement of land over the little-studied Cape Verde hot spot after realizing its prior presence under the Great Lakes would explain odd movements of seismic waves (radial anisotropy anomalies) in the crust below the lakes.

Based on the movement of tectonic plates, researchers knew the Cape Verde hot spot was once under North America. But their analysis showed it was under what's now Lake Superior between about 300 million and 225 million years ago. The hot spot then passed through Lake Huron and northern Lake Erie, followed by west-central New York and central Maryland, and then northern Virginia and finally the ocean about 170 million years ago. The hot spot would've stretched the crust and "probably caused lithosphere thinning and low topography needed for forming the Lakes during the glacial era," reads the study published Dec. 25 in Geophysical Research Letters. Now, researchers wonder "whether it's a general rule that large, inland lakes occur in places where hot spots once sat," per Live Science. (More Great Lakes stories.)