CHICAGO — As an ice sheet thousands of feet thick began its final crawling retreat from North America to the Arctic toward the end of the last glacial period some 10,000 years ago, it left behind the planet’s largest freshwater system. At least that’s what scientists have long believed about the formation of the Great Lakes.

But a recent study suggests that the timeline actually stretches further back — beyond the evolution of early humans and past the age of dinosaurs — to 200 to 300 million years ago when a hot spot, or plume of hot material, from underneath Earth’s crust created a low point that the glaciers would finish carving out and filling with water much later.

“Most of upper North America was covered by glaciation. But why, in this particular area, (are there) these Great Lakes?” said Aibing Li, a seismologist at the University of Houston who co-authored the study, which was published in the scientific journal Geophysical Research Letters in December. “We’ll see this surface feature, and we usually just consider that’s just some very shallow process. It could have, actually, some deeper source, a deeper origin. It’s not just like, randomly, on the surface, you have something special. Anything that happened in that area must have some reason.”

The continents, as they are today, didn’t exist all those hundreds of millions of years ago until the supercontinent Pangaea broke. As North America shifted away, the site of the modern-day Great Lakes passed over the Cape Verde hot spot, Li found.

When ocean hot spots interact with the planet’s surface, it usually results in volcano chains like in Hawaii. But when these hot spots are located within a continent, the impact is more difficult for scientists to detect through the rigid, thick outermost layer of Earth.

So, Li wasn’t really expecting to find what she did. As they used a model to study seismic movements in the country’s northeast, her team noticed that earthquake waves under the crust there traveled at different velocities, horizontally and vertically.

“We actually were puzzled, at the beginning, with what we saw,” she said.

Upon closer inspection, they realized the plate movements indicated that the hot spot — now under an African island nation of the same name located in the central Atlantic Ocean — had sat for a long time right below what would later become the eastern part of the lakes, specifically lakes Huron, Erie and Ontario.

Li explained that as hot, buoyant material from a hot spot plume rises under a continent, it finds resistance from the thick outer layer.

“It’ll lift, push upward,” Li said. “It tries to escape.”

Since the last major ice age started 2.7 million years ago and until it ended about 12,000 years ago, the Laurentide Ice Sheet in North America expanded and melted between several millennia of cold and warm periods. This allowed the glacier to finish shaping the massive bodies of water.

Maureen Long, a seismologist and professor of Earth and planetary sciences at Yale University, said the study contributes to scientific discussion about how natural geographical features can be affected by processes deep below.

“It is a hypothesis that still needs to be tested, but it really does illuminate the connection between what happens in the deep Earth and what we see at the surface,” Long said. “And whether or not this particular hypothesis turns out to be right, that connection … is very real, so this is a really important line of research.”

Long, an expert in mantle dynamics in the northeast United States, also believes like Li that there has to be a reason for the Great Lakes to be located where they are.

“(It’s) very simple physics. If you fill something with water, the water is going to go to the low point,” Long said. “The more recent glaciation formed the actual lakes and filled them — that part is relatively well understood. The part that we’re still puzzling over is, why was there sort of this low point there in the first place that then got carved out by the glaciers?”

While Li plans to soon find out if lakes Michigan and Superior could’ve also been created in the same way as their eastern counterparts, she hopes the study encourages more scientific research into the formation of inland water bodies besides the Great Lakes.

“When you have a new idea, it’s not easy (for it) to be accepted,” Li said. “But I think this can stimulate some more discussions. And science always evolves.”

Long said the study illustrates the concept of geologic time, which humans don’t often think of as they go about their day-to-day lives on timescales of minutes and hours.

“The features that we see, mountains or lakes or rivers, those features get shaped over millions to tens of millions to hundreds of millions to billions of years. It’s amazing to think about,” Long said. “The Earth is 4.6 billion years old — that’s a lot of time. And we see a snapshot of what the Earth is today, but over that long history and that long timescale, a lot has changed.”

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