Lake Superior Geology

Gravity anomaly map highlighting the Midcontinent Rift. Areas in red indicate denser rock. Stein, C. A., S. Stein, M. Merino, G. Randy Keller, L. M. Flesch, and D. M. Jurdy (2014), Was the Mid-Continent Rift part of a successful seafloor-spreading episode?, Geophys. Res. Lett., 41(5), 1465-1470, doi:10.1002/2013GL059176, 2014.

The copper of the Great Lakes formed during a spectacular period in Earth's history—at a time when the North American continent was splitting apart. This separation began about 1.1 billion years ago and at its peak had a length of over 3,000 kilometers. During this time, huge amounts of lava poured out of the earth's crust, forming sequences thousands of feet thick. In the present day, these relatively dense, massive flood basalts are only exposed in the Lake Superior region.

Midcontinent Rift Formation

Detailed Geologic Timeline

Isle Royale Geology and Soils

Lake Superior Cross-sectional Timeline

NPS image

The following slides feature a cross-sectional diagram along this line.

Long before Lake Superior formed (1.06-1.04 billion years ago), deep underground in the hot and highly pressurized basalt, water stripped minerals as it moved about.

As the superheated water made its way toward the surface, both pressure and temperature decreased.

This decrease in pressure and temperature allowed for the precipitation of minerals in the cracks and voids of the volcanic layers resulting in the largest native copper deposit ever discovered.

Melting ice filled the basin that would become Lake Superior.

Part of a series of articles titled
Copper Connections.

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Why Copper?

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History Guides Archaeology

Where this belongs

• Isle Royale National Park
• Keweenaw National Historical Park

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