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While the Great Lakes formed only about 14,000 years ago, their foundations were laid over a billion years earlier. Today they contain approximately 84 percent of North America’s total supply of surface water.
Holding more than a fifth of the world’s freshwater, the five Great Lakes—Superior, Michigan, Huron, Ontario, and Erie—are more like inland seas than lakes. From numerous vantage points on the ground, these broad expanses of water are too vast to see land on the other side. Many commercial flights offer excellent views of the five iconic, interconnected lakes along the border between the United States and Canada.
The Great Lakes began forming around 14,000 years ago, as the last ice age was winding down, after the Laurentide Ice Sheet retreated north of the Great Lakes region, filling it with glacial meltwater. However, the foundation for these enormous lakes was set over a billion years ago, when the core of the ancient continent Laurentia began splitting in half. This breach, the Midcontinent Rift, is a 1200-mile-long geological cleft now located in the center of North America. The root of the rift is in what is now Lake Superior, with the two arms of the split reaching down into Alabama and Oklahoma.
Much of the Great Lakes area is locked in a deep freeze through winter. The scene brings to mind the period when the lakes started forming, as the last ice age was ending.
Around 10 million years after it began separating, the Midcontinent Rift stopped spreading, eventually filling with flood basalts from volcanic eruptions fueled by a plume of molten rock rising from deep in Earth’s mantle. In some places, basalt filled the rift to depths twelve miles beneath the surface. Then around 570 million years ago, a second split, the Saint Lawrence rift system, began opening, laying the foundation for Lake Ontario, Lake Erie, and the Saint Lawrence River.
The name Pictured Rocks was inspired by the streaks of mineral stains that paint the rocks on Lake Superior. Red and orange come from iron, blue and green from copper, and brown and black from manganese.
The first Great Lakes to form were ancestral lakes known as Early Lake Chicago and Early Lake Maumee. Early Lake Chicago formed in the basin where Lake Michigan is located today, while Early Lake Maumee formed in what is now the Lake Erie Basin. At that time, a large sheet of glacial ice still persisted north of the Great Lakes region and the ancestral lakes drained to the south, through what is now the Mississippi River Basin. As the ice sheet melted, the weight of the ice lifted, allowing the land to rise at a rate of about a foot per century—a phenomenon known as post-glacial isostatic rebound. As the land rose, the drainage patterns of the rapidly filling Great Lakes region shifted to the northeast, to its modern-day path through the Saint Lawrence River and Seaway to the North Atlantic Ocean.
You could fly over the Great Lakes en route to Chicago, Illinois; Detroit, Michigan; Toronto, Ontario; or a number of other North American destinations. The well-known shape of the five-lake system is hard to miss.
