Hydrology, Cryosphere & Earth Surface News

Long Live the Laurentian Great Lakes

Living in Geologic Time: Billion-year-old rifting events set the stage for Earth’s greatest lakes.

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From some vantage points, the Great Lakes feel more like vast inland seas than freshwater lakes. But the 6 quadrillion gallons (~23 quadrillion liters) sloshing in Superior, Michigan, Huron, Ontario, and Erie represent one fifth of the planet’s fresh water.

How long will the Great Lakes retain their honor of being the largest collection of freshwater lakes on the planet? Thanks to some ancient rifting events, possibly for a very long time to come.

Ancient Rifts and Ice Age Lakes

The story of the Great Lakes began over 1 billion years ago, when the ancient supercontinent Laurentia began splitting in half. Over the course of about 10 million years, the Midcontinent Rift System opened a massive fissure on its way to becoming a new ocean basin. But for reasons geologists don’t entirely understand, the rift failed, and no ocean was formed, leaving a 3,000-kilometer-long scar across what is now North America.

Eons of erosion have hidden this scar, which runs from Lake Superior in two forks down to Alabama and Oklahoma. “A billion years is a very long time, even by geologic standards,” said Seth Stein, a geophysicist at Northwestern University in Evanston, Ill.

Evidence of the rift is visible at Lake Superior, which is ringed by cliffs of billion-year-old basalt that erupted into the active rift, giving the largest lake a uniquely rugged shoreline. “Lake Superior looks very different from the rest of the Midwest,” where volcanic rocks are rare, Stein said.

Dark volcanic cliffs rim the shore of Lake Superior.
Lake Superior’s towering basalt cliffs stand in contrast to the sandy beaches that ring the other Great Lakes. Credit: Seth and Carol Stein

Lake Superior sits within the Midcontinent Rift scar, but all five Great Lake basins were carved out by glaciers during the last glacial period, when the region was buried under the Laurentide Ice Sheet. As the ice age came to a close, the glaciers retreated, and the lake basins filled with meltwater to form the five Great Lakes as we know them by around 10,000 years ago. Today, some scientists refer to the Great Lakes by their more formal name, the Laurentian Great Lakes.

Rounded stones are stacked into a fireplace on a rocky beach on the south shore of Lake Superior.
A smoky fire did not deter the hordes of summertime mosquitoes that plagued our backpacking trip on the south shore of Lake Superior. Credit: Mary Caperton Morton

A few summers ago, I drove hundreds of kilometers out of my way to visit Superior’s billion-year-old rocks on a backpacking trip through Porcupine Mountains Wilderness State Park, Mich., on the south shore of Lake Superior. Unfortunately, the pilgrimage was rushed. The moment we parked at the trailhead, clouds of mosquitoes swarmed the car. It was mid-June, and the bloodsuckers were so relentless—biting through bug spray, layers of clothes, a smoky fire, and a tent—that we cut our 4-day loop in half.

Next time I visit, I’ll avoid peak mosquito season. The Great Lakes offer a lifetime’s worth of hiking opportunities: I have my eye on the Superior Hiking Trail, a 499-kilometer-long, 2- to 4-week backpacking trip along the northwest shore of Lake Superior in Minnesota. Isle Royale, a large island in Lake Superior, “offers arguably the best backpacking in the Midwest,” Stein said.

More Water, More Problems

The Great Lakes have been a nexus of migration, transportation, fishing, and trade for thousands of years, with a series of lakes, rivers, and waterways connecting the upper Midwest to the Atlantic Ocean. Records started in 1918 show lake levels fluctuating seasonally, with the annual water budget controlled by inputs from rivers and outputs to the Atlantic Ocean and evaporation.

In 2014, after years of drought and higher temperatures triggered increased evaporation, water levels in the Great Lakes reached all-time lows. “For a long time, the dominant climate change narrative was that lake levels were going to drop drastically,” said Richard Rood, a climate scientist at the University of Michigan in Ann Arbor. “That narrative ended abruptly in 2015,” when lake levels started rising because of heavy rainfall and flooding across the Midwest.

By 2019, lake levels had drastically reversed course, exceeding their highest recorded levels. High water in the Great Lakes leads to shoreline flooding and erosion, which in turn threaten homes, industrial buildings, and port infrastructure.

“Now the narrative is that we will need to be able to cope with both highs and lows [in lake levels],” with those extremes persisting for longer time periods, perhaps several seasons in a row, Rood said.

Higher influxes of water into the Great Lakes also bring more pollutants, including nitrogen and phosphorus from agricultural runoff and Escherichia coli bacteria from overworked water treatment systems. Algae blooms and toxic water quality issues have increased.

The sheer size of the individual Great Lakes means that pollutants can stay in the system for a long time: A water droplet or molecule of pollutant will reside in Lake Superior for as long as 191 years, Lake Michigan for 99 years, and Lake Huron for 22 years, whereas the smaller Lakes Ontario and Erie have residence times of 6 and 2.6 years, respectively.

“Smaller lakes with shorter residence times can respond more quickly to changes than bigger lakes,” said Dale Robertson, a research hydrologist at the U.S. Geological Survey in Middleton, Wis. That means there’s hope for Lake Erie, where toxic algal blooms feeding off nutrients delivered by the Maumee River create water quality issues each summer. But larger lakes, like larger boats, cannot change course as quickly. It may take decades before we know how the larger lakes will respond to climatic changes, Robertson said.

Currently, around 35 million people rely on the Great Lakes for drinking water, and an additional 56 billion gallons are extracted each day for municipal, agricultural, and industrial use.

Those staggering numbers are likely to increase, Rood said, as people displaced from other regions of the country by climate-related impacts seek refuge in the Great Lakes Megalopolis.

“Compared to some places, like south Florida, the Great Lakes are going to look like a climate winner,” Rood said. “We need to be prepared for how those migrations will change our communities and water usage in the coming decades.”

Dozens of beachgoers enjoy a sandy beach and calm swimming area on Lake Michigan.
Holland Beach, Mich., on Lake Michigan is one of many sandy swimming beaches around the Great Lakes. Credit: Mary Caperton Morton

Long Live the Great Lakes

The long-term trajectory of large freshwater lakes often depends on their outlet: Endorheic or terminal lakes such as the Great Salt Lake have no exit to the ocean, and the majority of their water balance is lost through evaporation, making them saltier over time.

The Great Lakes are not an endorheic basin; the outlet that connects them to the Atlantic Ocean has a 535-million-year-old history that’s unlikely to peter out anytime soon. The Saint Lawrence River, along with the basins that hold Lakes Erie and Ontario, sits inside the Saint Lawrence Rift, a 1,000-kilometer-long scar that dates back to the opening of the Iapetus Ocean between the paleocontinents of Laurentia, Baltica, and Avalonia.

Unlike the long dead and buried Midcontinent Rift System, the Saint Lawrence Rift System is still seismically active, capable of generating earthquakes over magnitude 5. As long as water flows into the Great Lakes Basin, the Great Lakes are likely to retain their status as the world’s largest—and hopefully freshest—lakes for eons to come.

—Mary Caperton Morton (@theblondecoyote), Science Writer

Living in Geologic Time is a series of personal accounts that highlight the past, present, and future of famous landmarks on geologic timescales.

Citation: Morton, M. C. (2020), Long live the Laurentian Great Lakes, Eos, 101, https://doi.org/10.1029/2020EO144218. Published on 18 May 2020.
Text © 2020. The authors. CC BY-NC-ND 3.0
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