Lake Superior Geology

The Greenstone Flow: One of Earth's largest lava flows creates a prominent bluff on the Keweenaw.

NPS photo

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.

Float copper on display in Calumet, MI

NPS photo

Glaciers played an important role in shaping the Lake Superior basin. Their slow, yet powerful, movement sheared copper from the host rock and transported it far away, sometimes hundreds of miles. These transported pieces are called float copper.

Melting ice filled the basin that would become Lake Superior.