The Midcontinent Rift System (MRS) or Keweenawan Rift is a long geological rift in the center of the North American continent and south-central part of the

North American plate The North American Plate is a tectonic plate covering most of North America, Cuba, the Bahamas, extreme northeastern Asia, and parts of Iceland and the Azores. With an area of , it is the Earth's second largest tectonic plate, behind the Paci ...

. It formed when the continent's core, the North American craton, began to split apart during the Mesoproterozoic era of the

Precambrian The Precambrian (or Pre-Cambrian, sometimes abbreviated pꞒ, or Cryptozoic) is the earliest part of Earth's history, set before the current Phanerozoic Eon. The Precambrian is so named because it preceded the Cambrian, the first period of th ...

, 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. Those arms meet at

Lake Superior Lake Superior in central North America is the largest freshwater lake in the world by surface areaThe Caspian Sea is the largest lake, but is saline, not freshwater. and the third-largest by volume, holding 10% of the world's surface fresh wa ...

, which is contained within the

rift valley A rift valley is a linear shaped lowland between several highlands or mountain ranges created by the action of a geologic rift. Rifts are formed as a result of the pulling apart of the lithosphere due to extensional tectonics. The linear d ...

. The lake's north shore in

Ontario Ontario ( ; ) is one of the thirteen provinces and territories of Canada.Ontario is located in the geographic eastern half of Canada, but it has historically and politically been considered to be part of Central Canada. Located in Central Ca ...

and

Minnesota Minnesota () is a state in the upper midwestern region of the United States. It is the 12th largest U.S. state in area and the 22nd most populous, with over 5.75 million residents. Minnesota is home to western prairies, now given over t ...

defines the northern arc of the rift. From the lake, the rift's eastern arm trends south to central lower

Michigan Michigan () is a state in the Great Lakes region of the upper Midwestern United States. With a population of nearly 10.12 million and an area of nearly , Michigan is the 10th-largest state by population, the 11th-largest by area, and t ...

, and possibly into

Indiana Indiana () is a U.S. state in the Midwestern United States. It is the 38th-largest by area and the 17th-most populous of the 50 States. Its capital and largest city is Indianapolis. Indiana was admitted to the United States as the 19th ...

Ohio Ohio () is a U.S. state, state in the Midwestern United States, Midwestern region of the United States. Of the List of states and territories of the United States, fifty U.S. states, it is the List of U.S. states and territories by area, 34th-l ...

Kentucky Kentucky ( , ), officially the Commonwealth of Kentucky, is a state in the Southeastern region of the United States and one of the states of the Upper South. It borders Illinois, Indiana, and Ohio to the north; West Virginia and Virginia ...

Tennessee Tennessee ( , ), officially the State of Tennessee, is a landlocked U.S. state, state in the Southeastern United States, Southeastern region of the United States. Tennessee is the List of U.S. states and territories by area, 36th-largest by ...

, and

Alabama (We dare defend our rights) , anthem = " Alabama" , image_map = Alabama in United States.svg , seat = Montgomery , LargestCity = Huntsville , LargestCounty = Baldwin County , LargestMetro = Greater Birmingham , area_total_km2 = 135,7 ...

. The western arm runs from Lake Superior southwest through portions of

Wisconsin Wisconsin () is a state in the upper Midwestern United States. Wisconsin is the 25th-largest state by total area and the 20th-most populous. It is bordered by Minnesota to the west, Iowa to the southwest, Illinois to the south, Lake M ...

Iowa Iowa () is a U.S. state, state in the Midwestern United States, Midwestern region of the United States, bordered by the Mississippi River to the east and the Missouri River and Big Sioux River to the west. It is bordered by six states: Wiscon ...

, and

Nebraska Nebraska () is a state in the Midwestern region of the United States. It is bordered by South Dakota to the north; Iowa to the east and Missouri to the southeast, both across the Missouri River; Kansas to the south; Colorado to the sout ...

to northeastern

Kansas Kansas () is a U.S. state, state in the Midwestern United States, Midwestern United States. Its Capital city, capital is Topeka, Kansas, Topeka, and its largest city is Wichita, Kansas, Wichita. Kansas is a landlocked state bordered by Nebras ...

, and possibly into

Oklahoma Oklahoma (; Choctaw: ; chr, ᎣᎧᎳᎰᎹ, ''Okalahoma'' ) is a state in the South Central region of the United States, bordered by Texas on the south and west, Kansas on the north, Missouri on the northeast, Arkansas on the east, New ...

Formation and failure

The rock formations created by the rift included

gabbro Gabbro () is a phaneritic (coarse-grained), mafic intrusive igneous rock formed from the slow cooling of magnesium-rich and iron-rich magma into a holocrystalline mass deep beneath the Earth's surface. Slow-cooling, coarse-grained gabbro is ...

and

granite Granite () is a coarse-grained ( phaneritic) intrusive igneous rock composed mostly of quartz, alkali feldspar, and plagioclase. It forms from magma with a high content of silica and alkali metal oxides that slowly cools and solidifies un ...

intrusive rock Intrusive rock is formed when magma penetrates existing rock, crystallizes, and solidifies underground to form '' intrusions'', such as batholiths, dikes, sills, laccoliths, and volcanic necks.Intrusive RocksIntrusive rocks accessdate: March ...

s and

basalt Basalt (; ) is an aphanitic (fine-grained) extrusive igneous rock formed from the rapid cooling of low-viscosity lava rich in magnesium and iron (mafic lava) exposed at or very near the surface of a rocky planet or moon. More than 90 ...

lava Lava is molten or partially molten rock (magma) that has been expelled from the interior of a terrestrial planet (such as Earth) or a moon onto its surface. Lava may be erupted at a volcano or through a fracture in the crust, on land or ...

s. In the Lake Superior region, the upwelling of this molten rock may have been the result of a hotspot which produced a

triple junction A triple junction is the point where the boundaries of three tectonic plates meet. At the triple junction each of the three boundaries will be one of three types – a ridge (R), trench (T) or transform fault (F) – and triple junctions can b ...

.(Powerpoint presentation) 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 The Afar Triangle (also called the Afar Depression) is a geological depression caused by the Afar Triple Junction, which is part of the Great Rift Valley in East Africa. The region has disclosed fossil specimens of the very earliest hominins; ...

of the

East African Rift The East African Rift (EAR) or East African Rift System (EARS) is an active continental rift zone in East Africa. The EAR began developing around the onset of the Miocene, 22–25 million years ago. In the past it was considered to be part of ...

system. The southwest and southeast extensions represent two arms of the triple junction while a third failed arm extends north into Ontario as the Nipigon Embayment. This failed arm includes

Lake Nipigon Lake Nipigon (; french: lac Nipigon; oj, Animbiigoo-zaaga'igan) is part of the Great Lakes drainage basin. It is the largest lake entirely within the boundaries of the Canadian province of Ontario. Etymology In the Jesuit Relations the lake is ...

, Ontario. The rift system may have been the result of extensional forces behind the continental collision of the

Grenville Orogeny The Grenville orogeny was a long-lived Mesoproterozoic mountain-building event associated with the assembly of the supercontinent Rodinia. Its record is a prominent orogenic belt which spans a significant portion of the North American continent, f ...

to the east which in part overlaps the timing of the rift development. Later compressive forces from the Grenville Orogeny likely played a major role in the rift's failure and closure. Had the rifting process continued, the eventual result would have been sundering of the North American

craton A craton (, , or ; from grc-gre, κράτος "strength") is an old and stable part of the continental lithosphere, which consists of Earth's two topmost layers, the crust and the uppermost mantle. Having often survived cycles of merging and ...

and creation of a sea. The Midcontinent Rift appears to have progressed almost to the point where the ocean intruded. But after about 15–22 million years the rift failed. (Abstract) The Midcontinent Rift is the deepest closed or healed rift yet discovered; no known deeper rift ever failed to become an ocean.

The rift today

Lake Superior occupies a basin created by the rift. Near that lake, rocks produced by the rift can be found on the surface of Isle Royale and the

Keweenaw Peninsula The Keweenaw Peninsula ( , sometimes locally ) is the northernmost part of Michigan's Upper Peninsula. It projects into Lake Superior and was the site of the first copper boom in the United States, leading to its moniker of " Copper Country." A ...

of the Upper Peninsula of Michigan, northwest Wisconsin, and on the North Shore of Superior in Minnesota and Ontario. Similar rocks are exposed as far south as Interstate Park near

Saint Paul, Minnesota Saint Paul (abbreviated St. Paul) is the capital of the U.S. state of Minnesota and the county seat of Ramsey County. Situated on high bluffs overlooking a bend in the Mississippi River, Saint Paul is a regional business hub and the center ...

, but elsewhere the rift is buried beneath more recent sedimentary rocks up to thick. Where buried, the rift has been mapped by

gravity anomalies The gravity anomaly at a location on the Earth's surface is the difference between the observed value of gravity and the value predicted by a theoretical model. If the Earth were an ideal oblate spheroid of uniform density, then the gravity meas ...

(its dense

ic rock increases gravity locally), aeromagnetic surveys, and

seismic Seismology (; from Ancient Greek σεισμός (''seismós'') meaning "earthquake" and -λογία (''-logía'') meaning "study of") is the scientific study of earthquakes and the propagation of elastic waves through the Earth or through other ...

data. A slightly older but possibly related geologic feature is the Mackenzie Large Igneous Province in Canada, which extends from the

Arctic The Arctic ( or ) is a polar regions of Earth, polar region located at the northernmost part of Earth. The Arctic consists of the Arctic Ocean, adjacent seas, and parts of Canada (Yukon, Northwest Territories, Nunavut), Danish Realm (Greenla ...

Nunavut Nunavut ( , ; iu, ᓄᓇᕗᑦ , ; ) is the largest and northernmost territory of Canada. It was separated officially from the Northwest Territories on April 1, 1999, via the '' Nunavut Act'' and the '' Nunavut Land Claims Agreement Act'' ...

to near the

Great Lakes The Great Lakes, also called the Great Lakes of North America, are a series of large interconnected freshwater lakes in the mid-east region of North America that connect to the Atlantic Ocean via the Saint Lawrence River. There are five lakes ...

Northwestern Ontario Northwestern Ontario is a secondary region of Northern Ontario in the Canadian province of Ontario which lies north and west of Lake Superior and west of Hudson Bay and James Bay. It includes most of subarctic Ontario. Its western boundary is the ...

Natural resources

The

Proterozoic The Proterozoic () is a geological eon spanning the time interval from 2500 to 538.8million years ago. It is the most recent part of the Precambrian "supereon". It is also the longest eon of the Earth's geologic time scale, and it is subdivided i ...

Nonesuch Shale The Nonesuch Shale is a Proterozoic geologic formation that outcrops in Michigan and Wisconsin, United States, but has been found by drill holes to extend in the subsurface as far southwest as Iowa. The Nonesuch is a lacustrine sequence of shale ...

formation in the Keweenawan Rift contains enough organic carbon (greater than 0.5%) to be considered a potential

source rock In petroleum geology, source rock is rock which has generated hydrocarbons or which could generate hydrocarbons. Source rocks are one of the necessary elements of a working petroleum system. They are organic-rich sediments that may have been depo ...

for petroleum. Oil identified as Precambrian has been found seeping out of the Nonesuch Shale in the White Pine mine in Michigan. 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. These include two "dry holes" drilled by

Amoco Amoco () is a brand of fuel stations operating in the United States, and owned by BP since 1998. The Amoco Corporation was an American chemical and oil company, founded by Standard Oil Company in 1889 around a refinery in Whiting, India ...

: a well in Alger County, Michigan in 1987 and 1988, and one in

Bayfield County, Wisconsin Bayfield County is the northernmost county in the U.S. state of Wisconsin. As of the 2020 census, its population is 16,220. Its county seat is Washburn. The county was created in 1845 and organized in 1850. The Red Cliff Band of Lake Superio ...

to a depth of in 1992. In 1987

also drilled a dry hole that penetrated rift sediments in Iowa. The Michigan Copper Country in the

Upper Peninsula The Upper Peninsula of Michigan – also known as Upper Michigan or colloquially the U.P. – is the northern and more elevated of the two major landmasses that make up the U.S. state of Michigan; it is separated from the Lower Peninsula by ...

and Isle Royale contains major native copper deposits in Keweenawan-age rocks associated with the rift. A copper mining industry was developed in 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. Once thought to be uneconomic to mine, these deposits have attracted renewed interest from resource companies.

Gallery

File:Sleeping Giant PP.JPG, Sleeping Giant in

File:Isle Royale aerial.jpg, Eroded volcanic strata at Isle Royale in

Image:Mount McKay Thunder Bay.jpg, A mafic

sill Sill may refer to: * Sill (dock), a weir at the low water mark retaining water within a dock * Sill (geology), a subhorizontal sheet intrusion of molten or solidified magma * Sill (geostatistics) * Sill (river), a river in Austria * Sill plate, ...

Thunder Bay Thunder Bay is a city in and the seat of Thunder Bay District, Ontario, Canada. It is the most populous municipality in Northwestern Ontario and the second most populous (after Greater Sudbury) municipality in Northern Ontario; its populati ...

, Ontario File:Palisade, Shovel Point.jpg, Cliffs at

Palisade Head Palisade Head is a headland on the North Shore of Lake Superior in the U.S. state of Minnesota. It is within Tettegouche State Park but not contiguous with the rest of that park. Palisade Head is located at milepost 57 on scenic Minnesota Sta ...

and Shovel Point in

File:JayCookeStatePark1.jpg, Saint Louis River in

Jay Cooke State Park Jay Cooke State Park is a state park of Minnesota, United States, protecting the lower reaches of the Saint Louis River. The park is located about southwest of Duluth and is one of the ten most visited state parks in Minnesota. The western ...

in Minnesota File:InterstateStateParkMN arf3.JPG, Dalles of the St. Croix River at Interstate State Park, Minnesota-Wisconsin

References

External links