TheMidcontinent Rift System (MRS) orKeweenawan Rift is a 2,000 km (1,200 mi) long geologicalrift in the center of theNorth American continent and south-central part of theNorth American plate. It formed when the continent's core, theNorth American craton, began to split apart during theMesoproterozoic Era of theProterozoic Eon, about 1.1 billion years ago.The rift failed, leaving behind thick layers ofigneous rock that are exposed in its northern reaches, but buried beneath later sedimentary formations along most of its western and eastern arms. Those arms meet atLake Superior, which is contained within therift valley. The lake'snorth shore inOntario andMinnesota defines the northern arc of the rift. From the lake, the rift's eastern arm trends south to central lowerMichigan, and possibly intoIndiana,Ohio,Kentucky,Tennessee, andAlabama.[1] The western arm runs from Lake Superior southwest through portions ofWisconsin,Minnesota,Iowa, andNebraska to northeasternKansas,[2] and possibly intoOklahoma.[3]
The rock formations created by the rift includedgabbro andgraniteintrusive rocks andbasaltlavas.[4] In the Lake Superior region, the upwelling of this molten rock may have been the result of ahotspot which produced atriple junction.[2] The hotspot domed the rocks of the Lake Superior area. Voluminous basaltic lava flows erupted from the central axis of the rift, similar to the present-day rifting underway in theAfar Depression of theEast African Rift system.
The southwest and southeast extensions represent two arms of the triple junction while a thirdfailed arm extends north into Ontario as theNipigon Embayment.[3][5] This failed arm includesLake Nipigon, Ontario.
The rift system may have been the result of extensional forces behind the continental collision of theGrenville Orogeny to the east which in part overlaps the timing of the rift development.[3] Later compressive forces from the Grenville Orogeny likely played a major role in the rift's failure and closure.[3][6] Had the rifting process continued, the eventual result would have been sundering of the North Americancraton and creation of a sea. The Midcontinent Rift appears to have progressed almost to the point where the ocean intruded.[7] But after about 15–22 million years the rift failed.[6][8] The Midcontinent Rift is the deepest closed or healed rift yet discovered; no known deeper rift ever failed to become an ocean.[7]
Lake Superior occupies a basin created by the rift.[3] Near the present lake, rocks produced by the rift can be seen on the surface ofIsle Royale and theKeweenaw Peninsula of the Upper Peninsula of Michigan,[9] northwest Wisconsin,[10] and on theNorth Shore of Superior in Minnesota and Ontario.[4] Similar rocks are exposed as far south asInterstate Park nearSaint Paul, Minnesota,[3][11][12] but elsewhere the rift is buried beneath more recent sedimentary rocks up to 9 km (6 mi) thick.[13] Where buried, the rift has been mapped bygravity anomalies (its densebasaltic rock increases gravity locally),[14]aeromagnetic surveys, andseismic data.[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 theArctic inNunavut to near theGreat Lakes inNorthwestern Ontario.[16]
TheProterozoicNonesuch Shale formation in the Keweenawan Rift contains enough organic carbon (greater than 0.5%) to be considered a potentialsource rock for petroleum. Oil identified as Precambrian has been found seeping out of the Nonesuch Shale in the White Pine mine in Michigan.[18] A few deep wells were drilled to explore for oil and gas in rift rocks as far southwest as Kansas. No oil and gas were found, but the explorations did make some deep rock samples available.[13] These include two "dry holes" drilled byAmoco: a 7,238 ft (2,206 m) well inAlger County, Michigan in 1987 and 1988, and one inBayfield County, Wisconsin to a depth of 4,966 ft (1,514 m) in 1992.[19] In 1987Amoco also drilled a 17,851 ft (5,441 m) dry hole that penetrated rift sediments in Iowa.[20]
The MichiganCopper Country in theUpper Peninsula andIsle Royale contains majornative copper deposits in Keweenawan-age rocks associated with the rift. Acopper mining industry was developed inPrecolumbian times, reactivated in the 1840s and continued for more than a century. Some low-grade copper and nickel deposits, among the largest in the world, also exist in theDuluth Complex north of the lake.[21] Once thought to beuneconomic to mine,[22] these deposits have attracted renewed interest from resource companies.[23]
White hydrogen could be found or produced in the Mid-continental Rift System at scale for a renewablehydrogen economy. Water could be pumped down to hot iron-rich rock toproduce hydrogen and the hydrogen could be extracted.[24]
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