It is widely known by lovers of the Great Lakes that their unique shape was caused by glaciers melting and receding northward. That was approximately 20,000 years ago. However, new research published in Geophysical Research Letters suggests these treasured lakes started forming hundreds of millions of years ago, long before, by the theory of plate tectonics, Pangea likely separated into the continents we recognize today.

This study out of the University of Houston points to a new theory that as North America began to split away, the Great Lakes basin moved over what is known as the Cape Verde hotspot. This process caused lithosphere thinning and subsequent surface subsidence, creating a pre-existing depression for the lakes to form during the glacial age. Today, the Cape Verde hotspot is located under the island nation of the same name in the Atlantic Ocean.

“I wouldn’t say the lake fully formed [in Pangea], it’s just that the hotspot created this low topography,” said Aibing Li, professor of geophysics and seismology at the University of Houston, and one of the researchers involved with this study.

Hotspots are also known as “mantle plumes,” a large column of hot rock rising from deep in the Earth’s mantle — which separates the planet’s crust from its core. The hotspots can produce volcanoes when tectonic plates move over them and form a volcano chain, like the Hawaii island chain. In the Great Lakes region, the lithosphere was too strong for the Cape Verde hotspot to break through. Instead of forming volcanoes, the hotspot produced lowlands.

When Li started reading literature about how the Great Lakes formed, she said scientists agreed that the lakes were formed by the glaciers during the Ice Age, 2.5 million years ago. That is pretty recent compared to the new theory based on this research, which suggests that the initial lake-forming process started about 300 to 200 million years ago.

Li said they investigated this after creating a seismic velocity anisotropy model of the region, mostly covering the east coast and eastern parts of the Great Lakes. She and the other researchers noticed seismic waves moving faster horizontally than vertically under the Great Lakes. This anomaly made them reevaluate past plate activity, which led them to the Cape Verde hotspot.

“We saw some very strong anisotropy signatures in the Great Lakes, and that was very puzzling because we didn’t expect to see any anomalies here,” said Li. “This anomaly reflects that the lithosphere here was strongly stretched horizontally.”

This study potentially challenges long-held assumptions about lake formation, and certainly opens the door for more questions than answers previously held, and for more research to be done on other large inland bodies of water around the world.

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