Midcontinent Rift System

It formed when the continent's core, the North American craton, began to split apart during the Mesoproterozoic era of the Precambrian, about 1.1 billion years ago.

The rift failed, leaving behind thick layers of igneous rock that are exposed in its northern reaches, but buried beneath later sedimentary formations along most of its western and eastern arms.

From the lake, the rift's eastern arm trends south to central lower Michigan, and possibly into Indiana, Ohio, Kentucky, Tennessee, and Alabama.

[1] The western arm runs from Lake Superior southwest through portions of Wisconsin, Minnesota, Iowa, and Nebraska to northeastern Kansas,[2] and possibly into Oklahoma.

[3][6] Had the rifting process continued, the eventual result would have been sundering of the North American craton and creation of a sea.

[15] A slightly older but possibly related geologic feature is the 2,700,000 km2 (1,000,000 sq mi) Mackenzie Large Igneous Province in Canada, which extends from the Arctic in Nunavut to near the Great Lakes in Northwestern Ontario.

[16] The Proterozoic Nonesuch Shale formation in the Keweenawan Rift contains enough organic carbon (greater than 0.5%) to be considered a potential source rock for petroleum.

Midcontinent Rift System
Rift rocks are exposed in the buff-colored areas around Lake Superior (black). The buff area on the north margin is the Lake Nipigon area.
Volcanic strata protrude at Isle Royale and the Keweenaw Peninsula [ 9 ]
Iowa magnetic anomaly map showing the rift curving from the north center to the southwest of the state. [ 17 ]