A new study from the University of Houston could change the way we understand how lakes were formed.
Aibing Li, a UH seismologist and co-author of the study, found that the Cape Verde hotspot, which still exists under the island nation in the Central Atlantic Ocean, may have been the main culprit in forming North America's Great Lakes.
"We saw something strange in this area," Li said. "Before, people thought the Great Lakes were formed during the ice age. Our study shows it started from the deep mantle."
The discovery suggests the process began hundreds of millions of years ago, before the supercontinent Pangaea began to separate. Over several millennia, as North America broke away and shifted above the Cape Verde hotspot, it traveled through where the Great Lakes are now located.
"When the hotspot was in the continent, it behaved differently from when they were in the ocean."
— Aibing Li, a seismologist and co-author of the study, University of Houston
Hotspots are plumes of hot material that rise from the mantle, which is Earth's middle layer. When hotspots interact with the Earth's surface, they can create volcanoes. But detecting the effects of an ancient hotspot that was once inside a continent is more difficult, Li said.
"When the hotspot was in the continent, it behaved differently from when they were in the ocean," Li said. "For example, a Hawaiian hotspot produced volcano chains. You can see that on the surface.
"But if the hotspot was in the continent, the lithosphere (the rigid, outermost layer of Earth) is very thick – we don't see the impact," Li continued.
Li said the team used a seismic velocity anisotropy model in northeastern America, including the eastern part of the Great Lakes region, when they noticed an anomaly: under the crust, earthquake waves traveled at different velocities going horizontally and vertically.
At first, the team didn't think a hotspot was involved. But as they looked closer, Li said plate movement made it clear that the Cape Verde hotspot had once sat under where the modern-day Great Lakes were for 300 million to 200 million years ago – back when North America was still part of Pangaea.
"My colleague showed me a video of hotspots and plate movement," Li said. "We saw the Cape Verde hotspot was right where the Great Lakes were for a pretty long time."
The discovery offers new insight into what shaped the Great Lakes, and, potentially, other lakes or land masses around North America, Li said. "I hope this will stimulate more research on this topic," she said. "This is a new idea, a change of the notion that lakes are formed more recently due to processes near the surface."
Li's next goal is to expand the research further west to cover the rest of the Great Lakes region to see whether her the theory remains true. She also hopes to learn whether her discovery applies to other inland lakes.
Additionally, Li wants to investigate the dynamics of hotspot-continent interaction in general.
"When the lithosphere is very different, for instance, a thin and weak lithosphere in the western U.S. versus a thick and strong one in the eastern U.S.," she said. "We want to know if a hotspot will produce different or similar features at depths and on the surface."
