Geology of Minnesota

Add links
Source: Wikipedia, the free encyclopedia.
Midwest to the eastern slopes of the Rocky Mountains. Central Minnesota contains an ancient fault system from the Penokean orogeny
.

The geology of Minnesota comprises the

soils of the U.S. state of Minnesota
, including their formation, development, distribution, and condition.

The state's geologic history can be divided into three periods. The first period was a lengthy period of geologic instability from the origin of the planet until roughly 1,100 million years ago. During this time, the state's

glacial till
over most of the state, and created the beds and valleys of modern lakes and rivers.

Minnesota's geologic resources have been the historical foundation of the state's economy. Precambrian bedrock has been mined for metallic minerals, including iron ore, on which the economy of Northeast Minnesota was built. Archaen granites and gneisses, and later limestones and sandstones, are quarried for structural stone and monuments. Glacial deposits are mined for aggregates, glacial till and lacustrine deposits formed the parent soil for the state's farmlands, and glacial lakes are the backbone of Minnesota's tourist industry. These economic assets have in turn dictated the state's history and settlement patterns, and the trade and supply routes along the waterways, valleys and plains have become the state's transportation corridors.

Geological history

Precambrian bedrock

Map of Minnesota bedrock by age.
St. Croix
in East Central Minnesota

Minnesota contains some of the oldest rocks on Earth, granitic

North American craton. Much of the underlying gneiss rock of today's state had already formed nearly a billion years earlier, but lay underneath the sea.[3]
Except for an area where islands appeared in what is now the northern part of the state, most of the region remained underwater.

In Middle Precambrian time, about 2,000 mya, the land rose above the water. Heavy mineral deposits containing iron had collected on the shores of the receding sea to form the Mesabi, Cuyuna, Vermilion, and Gunflint iron ranges from the center of the state north into Northwestern Ontario, Canada.[4] These regions also showed the first signs of life as algae grew in the shallow waters.

Over 1,100 mya, a rift formed and lava emerged from cracks along the edges of the rift valley. This Midcontinent Rift System extended from the lower peninsula of Michigan north to the current Lake Superior, southwest through the lake to the Duluth area, and south through eastern Minnesota down into what is now Kansas.[5] The rifting stopped before the land could become two separate continents. About 100 million years later, the last volcano went quiet.

Late Precambrian and Paleozoic sedimentary rock

The mountain-building and rifting events left areas of high relief above the low basin of the Midcontinent rift. Over the next 1,100 million years, the uplands were worn down and the rift filled with sediments, forming rock ranging in thickness from several hundred meters near Lake Superior to thousands of meters further south.[6] While the crustal tectonic plates continued their slow drift over the surface of the planet, meeting and separating in the successive collision and sundering of continents, the North American craton remained stable.[7] Although now free of folding and faulting caused by plate tectonics, the region continued to experience gradual subsidence and uplift.[8]

Five hundred fifty million years ago, the state was repeatedly inundated with water of a shallow sea that grew and receded through several cycles. The land mass of what is now North America ran along the equator, and Minnesota had a tropical climate.[9] Small marine creatures such as trilobites, coral, and snails lived in the sea. The shells of the tiny animals sank to the bottom, and are preserved in limestones, sandstones, and shales from this era.[4] Later, creatures resembling crocodiles and sharks slid through the water, and fossil shark teeth have been found on the uplands of the Mesabi Range.[10] During the Mesozoic and Cenozoic eras other land animals followed as the dinosaurs disappeared, but much of the physical evidence from this era has been scraped away or buried by recent glaciation. The rock units that remain in Minnesota from this time period are of Cambrian and Ordovician age, from the Mount Simon Sandstone at the bottom of the sequence of sedimentary rocks to the Maquoketa Group at the top.

Ice ages

See also: Glacial history of Minnesota
Northern hemisphere glaciation during the last ice ages. The creation of 3 to 4 km thick ice sheets caused a global sea level drop of about 120 m.

In the

Saint Lawrence River.[12]

Giant animals roamed the area.

buffalo were much larger than today. The glaciers continued to retreat and the climate became warmer over the next few millennia; the giant creatures died out about 9,000 years ago.[13]

This glaciation has drastically remodeled most of Minnesota, and all but two of the state's regions are covered with deep layers of glacial till.

Archaen and Proterozoic rocks were more resistant to glacial erosion
than the sedimentary bedrock first encountered in many other regions, and glacial till is relatively sparse. While the effects of glacial erosion are clearly present and there are some areas of glacial till, older rocks and landforms remain unburied and exposed across much of the region.

Contemporary features

Geologic regions

Contemporary Minnesota is much quieter geologically than in the past. Outcroppings of lava flows and magma intrusions are the only remaining traces of the volcanism that ended over 1,100 mya. Landlocked within the continent, the state is far from the seas that once covered it, and the continental glacier has receded entirely from North America. Minnesota's landscape is a relatively flat peneplain; its highest and lowest points are separated by only 518 metres (1,699 ft) of elevation.[15]

While the state no longer has true mountain ranges or oceans, there is a fair amount of regional diversity in

landforms and geological history, which in turn has affected Minnesota's settlement patterns, human history, and economic development. These diverse geological regions can be classified several ways. The classification used here principally derives from Sansome's Minnesota Underfoot: A Field Guide to Minnesota's Geology, but is also influenced by Minnesota's Geology by Ojakangas and Matsch. These authorities generally agree on areal borders, but the regions as defined by Ojakangas and Matsch are more geographical in their approximations of areas of similar geology, while Sansome's divisions are more irregular in shape in order to include within a region all areas of similar geology, with particular emphasis on the effects of recent glaciation. As glaciation and its residue has largely dictated regional surface geology and topography, Sansome's divisions are often coextensive with ecological provinces, sections, and subsections.[16]

Northeastern Minnesota: ancient bedrock

Northeastern Minnesota is an irregularly-shaped region composed of the northeasternmost part of the state north of Lake Superior, the area around

International Falls. Excluded are parts of the beds of glacial lakes Agassiz and Upham, the latter now occupied by the upper valley of the Saint Louis River and its tributary the Cloquet. This area is coextensive with the Northern Superior Uplands Section of the Laurentian Mixed Forest.[17]

North Shore Volcanic Group
along the lakeshore.

Known as the

Midcontinent Rift.[20] The Precambrian bedrock
formed by this activity has been eroded but remains at or close to the surface over much of the area.

The entire area is the raw southern edge of the Canadian Shield. Topsoils are thin and poor and their parent soils derived from the rock beneath or nearby rather than from glacial till, which is sparse.[21] Many of this region's lakes are located in depressions formed by the differential erosion of tilted layers of bedded rock of the Canadian Shield; the crevasses thereby formed have filled with water to create many of the thousands of lakes and swamps of the Superior National Forest.[22]

In post-glacial times Northeastern Minnesota was covered by forest broken only by these interconnected lakes and wetlands. Much of the area has been little changed by human activity, as there are substantial forest and wilderness preserves, most notably the Boundary Waters Canoe Area Wilderness and Voyageurs National Park. In the remainder of the region, lakes provide recreation, forests are managed for pulpwood, and the underlying bedrock is mined for valuable ores deposited in Precambrian times. While copper and nickel ores have been mined, the principal metallic mineral is iron. Three of Minnesota's four iron ranges are in the region, including the Mesabi Range, which has supplied over 90% of the state's historic output, including most of the natural ores pure enough to be fed directly into furnaces. The state's iron mines have produced over three and a half billion metric tons of ore.[23] While high-grade ores have now been exhausted, lower-grade taconite continues to supply a large proportion of the nation's needs.

Northwestern Minnesota: glacial lakebed

Northwestern Minnesota is a vast plain in the bed of Glacial Lake Agassiz. This plain extends north and northwest from the

Rainy River watershed within the state, at approximately the level of Lake Agassiz’ Herman Beach.[24] In ecological terms, it includes the Northern Minnesota Peatlands of the Laurentian Mixed Forest,[25] the Tallgrass Aspen Parklands,[26] and the Red River Valley Section of the Prairie Parklands.[27]

Bedrock in this region is mainly

mollisols), which are ideal for agriculture.[30] To the north and east, much of the land is poorly drained peat, often organized in rare and distinctive patterns known as patterned peatland.[31]
[32] At marginally higher elevations within these wetlands are areas of black spruce, tamarack, and other water-tolerant species.

Southwestern Minnesota: glacial river and glacial till

Relief map of Southwestern Minnesota, transected by the Minnesota River in the valley of the glacial River Warren. To the northwest, the faint U-shaped Big Stone Moraine; the triangular wedge in the southwest is the Coteau des Prairies. The rough-appearing areas to the north and east are part of Central Minnesota.

Southwestern Minnesota is in the watersheds of the Minnesota River, the Missouri River, and the Des Moines River.[33] The Minnesota River lies in the bed of the glacial River Warren, a much larger torrent that drained Lake Agassiz while outlets to the north were blocked by glaciers. The Coteau des Prairies divides the Minnesota and Missouri River valleys, and is a striking landform created by the bifurcation of different lobes of glacial advance. On the Minnesota side of the coteau is a feature known as Buffalo Ridge, where wind speeds average 16 mph (26 km/h). This windy plateau has been developed for commercial wind power.[34]

Between the river and the plateau are flat prairies atop varying depths of glacial till. In the extreme southwest portion of the state, bedrock outcroppings of Sioux Quartzite are common, with less common interbedded outcrops of an associated metamorphosed mudstone named catlinite. Pipestone, Minnesota is the site of historic Native American quarries of catlinite, which is more commonly known as "pipestone". Another notable outcrop in the region is the Jeffers Petroglyphs, a Sioux Quartzite outcropping with numerous petroglyphs which may be up to 7000–9000 years old.[35]

Drier than most of the rest of the state, the region is a transition zone between the prairies and the Great Plains. Once rich in wetlands known as prairie potholes,[36] 90%, or some three million acres (12,000 km²), have been drained for agriculture in the Minnesota River basin.[37] Most of the prairies are now farm fields.[37] Due to the quaternary and bedrock geology of the region, as well as the reduced precipitation in the region, groundwater resources are neither plentiful, nor widely distributed, unlike most other areas of the state. Given these constraints, this rural area hosts a vast network of water pipelines which transports groundwater from the few localized areas with productive groundwater wells to much of the region's population.

Southeastern Minnesota: bluffs, caves and sinkholes

Limestone bluffs cut by the Root River in Olmsted County
Relief map of Southeastern Minnesota: Owatonna Moraine on extreme west; junction of the state's principal rivers in Central Minnesota in the north; the Wisconsin border along the St. Croix and Mississippi Rivers to the east.

Southeastern Minnesota is separated from Southwestern Minnesota by the Owatonna Moraine, the eastern branch of the

Twin Cities, then encompassing the latter metropolis with a broad arc east to the St. Croix River.[38] This moraine runs south from the Twin Cities in the general area of Minnesota State Highway 13 and Interstate 35. Sansome attaches this moraine to her description of West-Central Minnesota, given its similarity in glacial features to that region.[39] Under Sansome's classification (followed here), Southeastern Minnesota is generally coterminous with the Paleozoic Plateau Section of the Eastern Broadleaf Forest Province.[40]

The bedrock here is lower

Driftless area. As the topsoils are shallower and poorer than those to the west, dairy farming
rather than cash crops is the principal agricultural activity.

Central Minnesota: knob and kettle country

Map of soils and sediments constituting Quaternary geology of Minnesota.

Central Minnesota is composed of (1) the

gerrymander
-like region.

The bedrock ranges in age from Archean granites to Upper Mesozoic Cretaceous sediments,[43] and underlying the eastern part of the region (and the southerly extension to Iowa) are the Late Precambrian Keweenawan volcanics of the Midcontinent Rift, overlaid by thousands of meters of sedimentary rocks.[44]

At the surface, the entire region is "Moraine terrain", with the

drumlins, eskers, kames, outwash plains and till plains
, all relics from recent glaciation. In the multitude of glacier-formed depressions are wetlands and many of the state's "10,000 lakes", which make the area prime vacation territory. The glacial deposits are a source of aggregate, and underneath the glacial till are high-quality granites which are quarried for buildings and monuments.

East Central Minnesota: bedrock valleys and outwash plain

St. Croix
enters from northeast. East Central Minnesota is composed of the eastern half and all territory on the image north of the Mississippi.
Limestone over sandstone in the gorge of Minnehaha Creek

The subregion of East Central Minnesota is that part of Central Minnesota near the junction of three of the state's great rivers. Included are

Twin Cities metropolitan area.[45]
The region has the same types of glacial landforms as the remainder of Central Minnesota, but is distinguished by its bedrock valleys, both active and buried.

The valleys now hold three of Minnesota's largest rivers, which join here. The St. Croix joins the Mississippi at

flour mills
that were the foundation for the city's 19th century growth.

Other bedrock

Chain of Lakes in Minneapolis and lakes and dry valleys in St. Paul.[46]

North of the metropolitan area is the Anoka Sandplain, a flat area of sandy

Midcontinent Rift, providing a glimpse of Minnesota's volcanic past.[49]

References

Notes

  1. ^ Ojakangas and Matsch, Minnesota's Geology, p. 23.
  2. ^ Ojakangas and Matsch, Minnesota's Geology, pp. 25–32.
  3. ^ "Minnesota". America’s Volcanic Past: Places. U.S. Geological Survey, Cascades Volcanic Observatory. Archived from the original on 2009-01-10. Retrieved 2007-06-30.
  4. ^ a b "Natural History - Minnesota's Geology". Minnesota Dept. of Natural Resources. 2007. Archived from the original on October 10, 2006. Retrieved 2007-06-30.
  5. ^ Van Schmus et al., The Midcontinent Rift System, pp. 345, 349.
  6. ^ Ojakangas and Matsch, Minnesota’s Geology, pp. 57–59.
  7. ^ Townsend, Catherine L.; John T. Figge (2002). "Dance of the Giant Continents". Northwest Origins. Burke Museum of History and Culture, University of Washington. Retrieved 2007-04-02.
  8. ^ Ojakangas and Matsch, Minnesota’s Geology, p. 65.
  9. ^ Ojakangas and Matsch, Minnesota’s Geology, pp. 65–95.
  10. ^ Ojakangas and Matsch, Minnesota's Geology, pp. 91–93.
  11. ^ Ojakangas and Matsch, Minnesota’s Geology, pp. 97, 108.
  12. ^ a b Lusardi, B.A. (1997). "Quaternary Glacial Geology" (PDF). Minnesota at a Glance. Minnesota Geological Survey, University of Minnesota. Archived from the original (PDF) on 2007-02-03. Retrieved 2007-06-30.
  13. ^ Ojakangas and Matsch, Minnesota’s Geology, pp. 116–18.
  14. ^ Jirsa and Southwick, Mineral Potential and Geology of Minnesota, Glacial cover in Minnesota.
  15. ^ "Elevations and Distances in the United States". U.S. Geological Survey. 2005. Archived from the original on 2008-06-01. Retrieved 2007-06-30.
  16. ^ Ecological Classification System, Ecological Land Classification Hierarchy.
  17. ^ Ecological Classification System, Northern Superior Uplands Section.
  18. ^ Van Schmus et al., The Midcontinent Rift System, p. 348.
  19. ^ Chandler, A Geophysical Investigation of the Ely Greenstone Belt in the Soudan Area Archived November 28, 2007, at the Wayback Machine, pp. 4–5; Nute, Rainy River Country, p. 3. Older rocks have since been found in Minnesota (the Morton gneiss of Southeast Minnesota) and Greenland and Labrador. Ojakangas and Matsch, Minnesota's Geology, p. 24.
  20. ^ Sansome, Minnesota Underfoot: A Field Guide to Minnesota's Geology, pp. 19, 30, 36; Ojakagas and Matsch, Minnesota's Geology, pp. 37–40, 50–55, 191.
  21. ^ Heinselman, The Boundary Waters Wilderness Ecosystem, pp. 13–14.
  22. ^ Ojakangas and Matsch, Minnesota’s Geology, pp. 185, 190–91.
  23. doi:10.2113/gsecongeo.94.1.133
    .
  24. ^ Compare regional map at Sansome, Minnesota Underfoot at p. 173, with image showing Herman Beach at Ojakanas and Matsch, Minnesota’s Geology, at p. 201. The upland portions of the Red River watershed atop the moraine are different in character, with gravelly soil, lakes, and other glacial landforms, and therefore are assigned to the Central Minnesota region.
  25. ^ Ecological Classification System, Northern Minnesota and Ontario Peatlands Section
  26. ^ Ecological Classification System, Tallgrass Aspen Parklands Province.
  27. ^ Ecological Classification System, Red River Valley Section.
  28. ^ Sansome, Minnesota Underfoot, p. 9 ("Bedrock Geology Map").
  29. ^ Heinselman, Forest Sites, Bog Processes, and Peatland Types in the Glacial Lake Agassiz Region, Minnesota Archived 2007-07-23 at the Wayback Machine, p. 331.
  30. ^ Sansome, Minnesota Underfoot, p. 174, 181.
  31. ^ Heinselman, Forest Sites, Bog Processes, and Peatland Types in the Glacial Lake Agassiz Region, Minnesota Archived 2007-07-23 at the Wayback Machine, pp. 354–61; Sansome, Minnesota Underfoot, pp. 191–93.
  32. USGS via Microsoft Research Maps
    . Retrieved 2007-06-30.
  33. ^ Ojakangas and Matsch, Minnesota’s Geology, p. 223; Sansome, Minnesota Underfoot, pp. 110–11.
  34. ^ "U.S. Wind Energy Projects — Minnesota". The American Wind Energy Association. Archived from the original on April 5, 2008. Retrieved 2008-04-07.
  35. ^ "History". Jeffers Petroglyphs. Minnesota Historical Society. Retrieved 15 January 2011.
  36. ^ "Map". Wetlands Component Assessment Regions. National Resources Conservation Service, United States Department of Agriculture. Retrieved 2007-06-30.
  37. ^ a b "Wetlands". Fact Sheets. Minnesota River Basin Data Center, Minnesota State University, Mankato. Retrieved 2007-06-30.
  38. ^ Minnesota’s Geology, p. 222 (map).
  39. ^ Sansome, Minnesota Underfoot, p. 110.
  40. ^ Ecological Classification System, Paleozoic Plateau Section.
  41. ^ Sansome, Minnesota Underfoot, p. 129-31.
  42. ^ While Sansome places the north bank of the Minnesota River in this region, this article follows Ojakangas and Matsch in assigning the lowlands along the north bank to Southwest Minnesota, and the uplands to Central Minnesota. Compare regional map at Sansome, Minnesota Underfoot at p. 109, with Ojakanas and Matsch, Minnesota’s Geology, at p. 222.
  43. ^ Sansome, Minnesota Underfoot, p. 9 ("Bedrock Geologic Map").
  44. ^ Ojakangas and Matsch, Minnesota’s Geology, p. 59.
  45. ^ Ojakangas and Matsch attach the metropolitan area to Southeastern Minnesota. Minnesota’s Geology, p. 232 (map).
  46. ^ Ojakangas and Matsch, Minnesota’s Geology, p. 236
  47. ^ Anoka Conservation District. "Geologic History of the Anoka Sandplain". Guide to Anoka County's Natural Resources. Anoka County. Retrieved 2007-04-03.
  48. ^ Black, Robert F. (2005). "Chapter 10: St. Croix Dalles Interstate Park". Geology of Ice Age National Scientific Reserve of Wisconsin, NPS Scientific Monograph No. 2. National Park Service. Retrieved 2007-06-30.
  49. ^ Jol, Harry M. (2006). "Interstate State Park, A Brief Geologic History". University of Wisconsin at Eau Claire. Archived from the original on 2012-12-15. Retrieved 2007-06-30.

Sources

Retrieved from "https://en.wikipedia.org/w/index.php?title=Geology_of_Minnesota&oldid=1219807558"