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{{short description|Geological rift in the center of the North American continent}}
[[File:NA basement rocks.gif|right|thumb|300px|Geological map of North America showing the Midcontinent Rift in white, here labeled Keweenawan Rift. Lake Superior now occupies the apex of the rift; the section to its north marked "SUPERIOR" is the [[Superior Craton]]]]

The '''Midcontinent Rift System''' (MRS) or '''Keweenawan Rift''' is a {{convert|2000|km|mi|-2|abbr=on}} long geological [[rift]] in the center of the [[North America|North American continent]] and south-central part of the [[North American plate]]. 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. [[aulacogen|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. Those arms meet at [[Lake Superior]], which is contained within the [[rift valley]]. The lake's [[North Shore (Lake Superior)|north shore]] in [[Ontario]] and [[Minnesota]] defines the northern arc of the rift. From the lake, the rift's eastern arm trends south to central lower [[Michigan]], and possibly into [[Indiana]], [[Ohio]], [[Kentucky]], [[Tennessee]], and [[Alabama]].<ref name="GSA Grenville Front">{{cite journal|last1=Stein|first1=Carol|title=Is the "Grenville Front" in the central United States really the Midcontinent Rift?|journal=GSA Today|date=May 2018|volume=28|issue=5|pages=4–10|url=https://www.geosociety.org/gsatoday/science/G357A/article.htm|access-date=1 May 2018|doi=10.1130/GSATG357A.1}}</ref> The western arm runs from Lake Superior southwest through portions of [[Wisconsin]], [[Minnesota]], [[Iowa]], and [[Nebraska]] to northeastern [[Kansas]],<ref name = MRW/> and possibly into [[Oklahoma]].<ref name=Schmus />

==Formation and failure==
[[File:Superior upland.jpg|thumb|right|Rift rocks are exposed in the [[buff (colour)|buff-colored]] areas around [[Lake Superior]] (black). The buff area on the north margin is the [[Lake Nipigon]] area.]]
The rock formations created by the rift included [[gabbro]] and [[granite]] [[intrusive rock]]s and [[basalt]] [[lava]]s.<ref name = MNGeol/> In the Lake Superior region, the upwelling of this molten rock may have been the result of a [[hotspot (geology)|hotspot]] which produced a [[triple junction]].<ref name = MRW>{{Cite web| title = Middle Proterozoic and The Mid-continent Rift | work = Minnesota's Rocks and Waters | publisher = [[Winona State University]] | url = http://geogroup4.wikispaces.com/file/view/Mid-continentRift.ppt| access-date = 13 April 2008 }}(Powerpoint presentation)</ref> 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 the [[Afar Depression]] of the [[East African Rift]] system.

The southwest and southeast extensions represent two arms of the triple junction while a third [[Aulacogen|failed arm]] extends north into Ontario as the [[Nipigon Embayment]].<ref name=Schmus>{{Cite journal |last1=Van Schmus |first1 = W. R. |last2=Hinze |first2=W. J. |title=The Midcontinent Rift System |journal=Annual Review of Earth and Planetary Sciences |volume=13 |issue=1 |pages=345–383 |date=May 1985 |url= https://kuscholarworks.ku.edu/dspace/bitstream/1808/104/1/fac6cit13.pdf |doi=10.1146/annurev.ea.13.050185.002021 |access-date=10 June 2007 |bibcode=1985AREPS..13..345V |hdl = 1808/104 |hdl-access=free }}</ref><ref name=Kean>{{Cite web |last=Kean |first=William F. |title=Keweenawan Rift System |work=Field Trips, Northern Wisconsin and Upper Michigan |publisher=University of Wisconsin-Milwaukee |date=24 November 2000 |url=http://www.uwm.edu/People/wkean/fieldtrip/ArCraig/keewenaw.htm |access-date=8 June 2007 |url-status=bot: unknown |archive-url=https://web.archive.org/web/20080706161521/http://www.uwm.edu/People/wkean/fieldtrip/ArCraig/keewenaw.htm |archive-date=6 July 2008 }}</ref> This failed arm includes [[Lake Nipigon]], Ontario.

The rift system may have been 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.<ref name=Schmus /> Later compressive forces from the Grenville Orogeny likely played a major role in the rift's failure and closure.<ref name=Schmus/><ref name=Meso/> 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.<ref name = Reeves>{{Cite journal |last1= Reeves |first1= T. K. |last2=Carroll |first2=Herbert B. |title = Geologic Analysis of Priority Basins for Exploration and Drilling | publisher = U.S. Department of Energy, Office of Scientific and Technical Information | date=April 1999 | url = http://www.osti.gov/bridge/servlets/purl/6060-JKD5at/webviewable/6060.PDF |access-date = 10 June 2007 |doi= 10.2172/6060 }}</ref> But after about 15–22 million years the rift failed.<ref name = Meso>{{Cite journal |last1=Ojakangas |first1=R. W. |last2=Morey |first2=G. B. |last3=Green |first3=J. C. |title=The Mesoproterozoic Midcontinent Rift System, Lake Superior Region, USA |journal=[[Sedimentary Geology (journal)|Sedimentary Geology]] |volume=141–142 |pages=421–442 |date = 1 June 2001 |doi=10.1016/S0037-0738(01)00085-9 |bibcode=2001SedG..141..421O }} (Abstract)</ref><ref name = Schneider>{{cite conference |first1=D. A. |last1=Schneider |first2=D. K. |last2=Holm |first3=V. |last3=Chandler |title=A Superior Swath: Proterozoic Geology of the North American Midcontinent |publisher=American Geophysical Union |date=Fall 2006 |bibcode=2006AGUFM.T42A..06S |id = Abstract #T42A-06 }}</ref> The Midcontinent Rift is the deepest closed or healed rift yet discovered; no known deeper rift ever failed to become an ocean.<ref name = Reeves/>

==The rift today==
[[File:Keweenaw structure.jpg|thumb|360px|right|Volcanic strata protrude at [[Isle Royale]] and the [[Keweenaw Peninsula]]<ref name = Huber>{{Cite web
| last = Huber
| first = Norman King
| title = The story of Isle Royale National Park
| work = Bulletin 1309
| publisher = [[United States Geological Survey]]
| url = http://www.minsocam.org/MSA/collectors_corner/usgs/b1309.htm
| date = 1975 }}</ref>]]
Lake Superior occupies a basin created by the rift.<ref name=Schmus/> Near that lake, rocks produced by the rift can be found on the surface of [[Isle Royale]] and the [[Keweenaw Peninsula]] of the Upper Peninsula of Michigan,<ref name = Huber/> northwest Wisconsin,<ref name = Anderson>{{Cite web
| last = Anderson
| first = Raymond R.
| title = Introduction to the Midcontinental Rift
| publisher = Iowa Department of Natural Resources Geological Survey
| url = http://www.iihr.uiowa.edu/igs/introduction-to-the-midcontinental-rift/
| date = 25 November 2014 }}</ref> and on the [[North Shore (Lake Superior)|North Shore]] of Superior in Minnesota and Ontario.<ref name = MNGeol>{{Cite book| last = Ojakangas | first = Richard W. |author2=Charles L. Matsch | others = Illus. Dan Breedy | title = Minnesota's Geology | year = 1982 | publisher = University of Minnesota Press | location= Minneapolis, Minnesota | isbn = 978-0-8166-0953-6 }}</ref> Similar rocks are exposed as far south as [[Interstate Park]] near [[Saint Paul, Minnesota]],<ref name=Schmus/><ref name=Jol>{{Cite web
|last = Jol
|first = Harry M.
|title = Interstate State Park, A Brief Geologic History
|publisher = University of Wisconsin at Eau Claire
|date = 2006
|url = http://www.uwec.edu/jolhm/Interstate2006/history.htm
|url-status = dead
|archive-url = https://archive.today/20121215050555/http://www.uwec.edu/jolhm/Interstate2006/history.htm
|archive-date = 15 December 2012
|df = dmy-all
}}</ref><ref>[https://www.leg.state.mn.us/docs/2010/mandated/100444.pdf ''Interstate State Park Management Plan''], Minnesota Department of Natural Resources Division of Parks and Trails] (2009), p. 26.</ref> but elsewhere the rift is buried beneath more recent sedimentary rocks up to {{convert|9|km|mi|0|abbr=on}} thick.<ref name = Palacas/> Where buried, the rift has been mapped by [[Gravity anomaly|gravity anomalies]] (its dense [[basalt]]ic rock increases gravity locally),<ref name = Anderson1>{{Cite web
| last = Daniels
| first = David L.
|author2=Snyder, Stephen L.
| title = Bouguer gravity anomaly map of north-central United States
| publisher = Minnesota Geological Survey
| date = 1991
| hdl = 11299/60081
| url = http://conservancy.umn.edu/handle/11299/60081}}</ref>
[[aeromagnetic survey]]s, and [[seismic]] data.<ref name = Hinze2>{{Cite web
| last = Hinze
| first = W. |author2=Kellogg, D. |author3=Merritt, D.
| title = Gravity and Aeromagnetic Anomaly Maps of the Southern Peninsula of Michigan
| work = Report of Investigation 14
| publisher = Michigan State Geological Survey
| date = 1971
| url = http://www.michigan.gov/documents/deq/GIMDL-RI14_216260_7.pdf}}</ref>

A slightly older but possibly related geologic feature is the {{Convert|2700000|km2|mi2|abbr=on}} [[Mackenzie Large Igneous Province]] in Canada, which extends from the [[Arctic]] in [[Nunavut]] to near the [[Great Lakes]] in [[Northwestern Ontario]].<ref>{{cite book|author1=National Research Council|author2=Division on Engineering and Physical Sciences|author3=Commission on Physical Sciences, Mathematics, and Applications|author4=Geophysics Research Board, Assembly of Mathematical and Physical Sciences, Geophysics Study Committee|title=Continental Tectonics|url=https://books.google.com/books?id=s1F2eoTkQm0C&pg=PT191|year=1980|publisher=National Academies Press|isbn=978-0-309-02928-5|page=180}}</ref>

==Natural resources==
[[File:Iowa magnetic map.jpg|thumb|right|Iowa magnetic anomaly map showing the rift curving from the north center to the southwest of the state.<ref>{{Cite web|url=https://mrdata.usgs.gov/catalog/cite-view.php?cite=64|title=Iowa magnetic and gravity maps and data|last=Kucks|first=Robert P.|author2=Hill, Patricia L.|date=2005|publisher=U.S. Geological Survey|access-date=2009-09-03}}</ref>]]
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. Oil identified as Precambrian has been found seeping out of the Nonesuch Shale in the White Pine mine in Michigan.<ref>J.A. Seglund, "Midcontinent rift continues to show promise as petroleum prospect," ''Oil and Gas Journal'', 15 May 1989, p.55-58.</ref> 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.<ref name = Palacas>{{Cite web
| last = Palacas
| first = James G.
| title = Superior Province (051)
| work = National Oil and Gas Assessment
| publisher = United States Geological Survey
| date = 1995
| url = http://certmapper.cr.usgs.gov/data/noga95/prov51/text/prov51.pdf
| access-date = 10 June 2007 }}</ref> These include two "dry holes" drilled by [[Amoco]]: a {{convert|7238|ft|m|0|abbr=on}} well in [[Alger County, Michigan]] in 1987 and 1988, and one in [[Bayfield County, Wisconsin]] to a depth of {{convert|4966|ft|m|0|abbr=on}} in 1992.<ref>Albert B. Dickas, ''Results of the Middle Proterozoic Midcontinent rift frontier play along Lake Superior's south shore'', Oil & Gas Journal, 18 September 1995, pp. 80–82.</ref> In 1987 [[Amoco]] also drilled a {{convert|17,851|ft|m|0|abbr=on}} dry hole that penetrated rift sediments in Iowa.<ref>"[https://www.iihr.uiowa.edu/igs/publications/uploads/RIFS-2006-2.pdf Exploring the Midcontinent Rift]". ''Iowa Geological Survey Resource Information Fact Sheet 2006-2.</ref>

The Michigan [[Copper Country]] in the [[Upper Peninsula]] and [[Isle Royale]] contains major [[native copper]] deposits in Keweenawan-age rocks associated with the rift. A [[Copper mining in Michigan|copper mining industry]] was developed in [[Pre-Columbian era|Precolumbian]] 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 the [[Duluth Complex]] north of the lake.<ref>{{Cite web | last = Kraker | first = Dan | title = FAQ: Everything you need to know about PolyMet | url = http://www.mprnews.org/story/2013/12/02/environment/copper-mining-faq | date = 2 December 2013 | access-date = 2013-12-10 }}</ref> Once thought to be [[Economic geology|uneconomic to mine]],<ref name = Jira>{{Cite web |last=Jira |first=Mark |author2=David Southwick |title=Mineral potential and geology of the Duluth Complex |work=Mineral Potential and Geology of Minnesota |publisher=Minnesota Geological Survey, University of Minnesota |url=http://www.mngs.umn.edu/mnpot/dcmplx.html |access-date=2008-04-13 }}</ref> these deposits have attracted renewed interest from resource companies.<ref>{{Cite web | last = Kraker | first = Dan | title = PolyMet's copper-nickel mine rekindles decades-old environmental debate | publisher = [[Minnesota Public Radio]] | date = 5 December 2013 | url = http://www.mprnews.org/story/2013/12/05/environment/polymet-copper-nickel-mining-history | access-date = 10 December 2013}}</ref>

==Gallery==
<gallery perrow=3 widths=210px heights=210px>
File:Sleeping Giant PP.JPG|[[Sleeping Giant (Ontario)|Sleeping Giant]] in [[Ontario]]
File:Isle Royale aerial.jpg|Eroded volcanic strata at [[Isle Royale]] in [[Michigan]]
Image:Mount McKay Thunder Bay.jpg|A mafic [[sill (geology)|sill]] at [[Thunder Bay]], Ontario
File:Palisade, Shovel Point.jpg|Cliffs at [[Palisade Head]] and Shovel Point in [[Minnesota]]
File:JayCookeStatePark1.jpg|Saint Louis River in [[Jay Cooke State Park]] in Minnesota
File:InterstateStateParkMN arf3.JPG|Dalles of the [[St. Croix River (Wisconsin-Minnesota)|St. Croix River]] at [[Interstate State Park]], Minnesota-Wisconsin
</gallery>

==See also==
*{{annotated link|Coldwell Complex}}
*[[Geology of Ontario]]
*{{annotated link|Mamainse Point Formation}}
*{{annotated link|Volcanology of Canada}}
*{{annotated link|Volcanology of Eastern Canada}}

==References==
{{reflist|30em}}

==External links==
* [https://wim.usgs.gov/geonarrative/MRS_mineral_deposits/ "Mineral Deposits of the Midcontinent Rift System"]. United States Geological Survey Mineral Resources Program, Eastern Mineral and Environmental Resources Science Center. A detailed and well-illustrated resource focusing on the mineral resources of the Midcontinent Rift and its geologic history.
* Stein, Seth et al. (2016) [https://eos.org/features/new-insights-into-north-americas-midcontinent-rift "New Insights into North America’s Midcontinent Rift"]. ''[[Eos (magazine)|Eos]]'', 97, https://doi.org/10.1029/2016EO056659.

{{Large igneous provinces}}

{{Use dmy dates|date=August 2019}}

[[Category:Aulacogens]]
[[Category:Geologic provinces of the United States]]
[[Category:Geology of Alabama]]
[[Category:Geology of Indiana]]
[[Category:Geology of Iowa]]
[[Category:Geology of Kansas]]
[[Category:Geology of Kentucky]]
[[Category:Geology of Michigan]]
[[Category:Geology of Minnesota]]
[[Category:Geology of Nebraska]]
[[Category:Geology of Ohio]]
[[Category:Geology of Oklahoma]]
[[Category:Geology of Ontario]]
[[Category:Geology of Tennessee]]
[[Category:Geology of Wisconsin]]
[[Category:Mesoproterozoic rifts and grabens]]
[[Category:Large igneous provinces]]

Midcontinent Rift System - Revision history

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