A brief prehistory of Lake Michigan
July 30, 2010
By Patricia Coorough Burke, Karen Morgan, Joshua Pierce, and Michael Timm
The story of Lake Michigan starts over a billion years ago—before the first dinosaurs were even a twinkle in their parents’ eyes. During this primordial period in our planet’s history, the precursor to the North American continent was literally being pulled apart.
A rift had developed in the middle of the North American plate, with magma pushing up from Earth’s mantle and threatening to split the plate in two.
Called the Midcontinent or Keweenawan Rift, it was similar to the modern-day East African Rift where the African Plate is being split into two new plates by upwelling magma—the plates are stretched and broken by faults that allow the magma to reach the surface, forming volcanoes like Mt. Kilimanjaro.
Some 1.1 billion years ago, lava seeped out of the Keweenawan Rift and forced the continent itself apart like a forceps. Although the spreading ultimately halted and the plate remained intact, the rift remained. A weak spot, a sort of planetary hernia, had developed. The sheer weight of the volcanic layers that had oozed out and solidified into rock above began to depress the crust, creating a preliminary basin. This natural low spot started to collect water.
The Shallow Seas
Over the next several hundred million years North America was repeatedly flooded as the prevailing climate fluctuated. The shallow, salty seas over what is today the Great Lakes region hosted a variety of life including coral and mollusks but also forms alien to us today—like the squidlike cephalopod and what’s become Wisconsin’s state fossil, the trilobite.
When these hard-shelled creatures died and settled to the then-seafloor, the calcium in their hard parts formed rocks—mainly limestone, composed of calcium carbonate (calcite), and dolostone, composed of calcium/magnesium carbonate (dolomite). Over time, layers and layers of dead creatures and sediment were compressed into sedimentary rocks. The base of Lake Michigan today is actually a layer of dolomite formed between 440 and 417 million years ago, during the Silurian Period.
Prehistorically, water drained from the preliminary basin at the continent’s interior along much the same routes as it does today—north toward present-day Hudson Bay and east toward the present-day Gulf of St. Lawrence. Millions of years before the glaciers, river systems excavated valleys and canyons through layers of sandstone, limestone, and shale on their way. Like varicose veins soon to become swollen beyond their fractal channels, the course of these prehistoric rivers would determine the future footprint of the Great Lakes.
Compared to the time it took the sagging basin of rock beneath them to develop, the Great Lakes themselves formed in just the blink of an eye of geologic time.
Just more than one million years ago, during a planetary ice age when ice covered an estimated 30 percent of the Earth’s land area, a glacial ice sheet descended on the region from the north. The glacier’s scouring power deepened and enlarged river valleys in the basin, scraping the softer surface rocks from the harder bedrock below.
Glaciers “advanced” when their leading edges accumulated more ice and “retreated” when existing ice melted. When enough ice accumulated, the massive bodies actually flowed like slow rivers of molasses, exerting tremendous amounts of force onto and across the land like a giant plow—powered by the weight of sometimes more than a mile-high mountain of ice. Like rivers, the glaciers slogged through lowlands, scouring them lower, depositing huge rock debris piles, or moraines, that marked their farthest advances.
This melt-and-freeze cycle of glaciation repeated for thousands of years. Interestingly, the size of glaciers in the pre-Great Lakes region may have been enhanced from prehistoric “lake effect” snows, which piled up dozens of feet of new snow directly in the area, too fast for it to melt.
Not only did glaciers sculpt the actual shapes of the Great Lakes’ basins, but their massive presence also altered where water collected and how it drained throughout the region by damming previous drainage ways.
The glaciers melted and retreated from the area during a natural global warming phase, and by approximately 10,000 years ago they had retreated permanently to the north. Once the ice was gone, all that weight was no longer pressing down on the earth—the underlying bedrock began to rebound like a trampoline bouncing back after you jump on it. This “isostatic rebounding” continues today, with areas of Lake Michigan rebounding between 30 and 50 centimeters per century, as estimated using Global Positioning System satellite data.
For the last few thousand years, the shape and depth of the newly carved lake basins changed, even as melted glacial ice slowly filled them.
It’s thought that what’s now Lake Michigan, for example, went through many iterations—as Lake Chicago (a blobby lake smaller in surface area near Chicago and cut off to the north by glaciers), Lake Algonquin (when Lakes Michigan and Huron were deeper and connected due to glacial damming), Lake Chippewa (an emaciated-seeming narrower version of Michigan), Lake Nipissing (a superlake connecting Michigan even more to Superior and Huron), and finally the familiar drooping shape we instantly recognize as Lake Michigan today.
And even that’s not a constant. From a geological perspective, Lake Michigan is but a transitory form—a two-bit actor in the ongoing drama of the planet’s much larger rock and water cycles.
Patricia Coorough Burke, Karen Morgan, Joshua Pierce, and Michael Timm contributed to this report.
— about the Great Lakes’ formation: www.on.ec.gc.ca/greatlakeskids/greatlakesmovie5.html.
— about Wisconsin geology: uwex.edu/wgnhs
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