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Geological map of North America showing (in white) the Midcontinent Rift, here labeled Keweenawan Rift.

The Midcontinent Rift System (MRS) or Keweenawan Rift is a 2,000 km. long geological rift in the center of the North American continent and south-central part of the North American plate. It trends north from central lower Michigan, turns west through Lake Superior with its outer arc following the line of the north shore of that lake in Minnesota and Ontario, then turns southwest through portions of Wisconsin, Minnesota, Iowa, Nebraska and Kansas.^[1] It is approximately 1,100 million years old, from the Precambrian Mesoproterozoic era.

Cliffs at Palisade Head on Lake Superior in Minnesota, view northeast to Shovel Point; both are surficial relics of the Midcontinent Rift.

The lava flows created by the rift were formed from basaltic magma. The upwelling of this magma may have been the result of a hotspot which produced a triple junction in the vicinity of Lake Superior. The hotspot made a dome that covered the Lake Superior area. Voluminous basaltic lava flows erupted from the central axis of the rift, similar to the rifting of the Afar Depression of the East African Rift system. The southwest and southeast extensions represent two arms of the triple junction while a third failed arm extends north into Ontario.^[2][3] This failed arm now forms Lake Nipigon. It is also possible that the rift is the result of extensional forces behind the continental collision of the Grenville Orogeny to the east which in part overlaps the timing of the rift development.^[2]

It is likely that later compressive forces from the Grenville Orogeny also played a major role in the rift's eventual failure and closure.^[4][2] Had the rifting process continued, the eventual result would have been sundering of the North American craton and creation of a sea. The Midcontinent Rift appears to have progressed almost to the point where the ocean intruded.^[5] But after about 10-20 million years the rift failed.^[4] The Midcontinent Rift is the deepest closed or healed rift yet discovered; no deeper rift ever failed to become an ocean.^[5]

Rift rocks are exposed in the lighter area around Lake Superior in this geological map.

Near Lake Superior, rocks produced by this rift can be found on the surface along the shores of the Keweenaw Peninsula of Upper Michigan,^[6] and on the North Shore of the lake in Minnesota.^[7] The lake itself lies in the rift valley formed by the rifting. Similar rocks are exposed as far south as Interstate Park near Saint Paul, Minnesota,^[8] but otherwise the rift is buried thousands of feet below the surface. Where buried, it has been mapped by gravity anomalies (its dense basaltic rock increases gravity locally),^[9] aeromagnetic surveys,^[10] and seismic data.^[11]

The Proterozoic Nonesuch Shale formation in the Keweenaw Rift contains enough organic carbon to be considered a potential source rock for petroleum. A few deep wells have been drilled to explore for oil and gas (so far unsuccessfully), in the rift system as far southwest as Kansas, making some deep rock samples available.^[12] The most recent oil exploration wells into rift rocks were a dry hole drilled by Amoco to 7,238 feet in Alger County, Michigan in 1987 and 1988, and a dry hole also drilled by Amoco to 4,966 feet in Bayfield County, Wisconsin in 1992.^[13]

The Michigan Copper Country contains major native copper deposits in Keweenawan-age rocks associated with the rift. (See Copper mining in Michigan)