Geology of Minnesota
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The geology of Minnesota is the natural history and study of the physical features of the U.S. state of Minnesota. This history can be divided into three periods: (1) formation of the Precambrian bedrock by volcanism and the deposition of sedimentary rock, heavily modified by processes such as faulting, folding and erosion, during a lengthy period of geologic instability from the origin of the planet until 1,100 million years ago; (2) deposition of successive layers of sedimentary rock by repeated inundations during which the land was below the level of the sea; and (3) a period of glaciation which eroded previous rock formations, deposited deep layers of glacial till, and carved the beds and valleys of modern lakes and rivers.
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[edit] Formation of Precambrian bedrock
Minnesota contains some of the oldest rocks on earth, granitic gneisses formed some 3600 mya (million years ago), or 80% as old as the planet.[1] About 2700 mya the first pieces of volcanic rock that would later form the U.S. state of Minnesota began to rise up out of an ancient ocean, forming the Canadian Shield.[2] Much of the underlying gneiss rock of today's state had already been formed nearly a billion years earlier, but still 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. About 2000 mya, much of the water had drained away. Heavy mineral deposits containing iron collected on the shores of a receding sea to form the Mesabi, Cuyuna, Vermilion, and Gunflint iron ranges from the center of the state up into what is now Canada. These regions also showed the first signs of life as algae grew in the shallow waters.
Over 1100 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, thence south through eastern Minnesota down into what is now Kansas.[4] The rifting stopped before the land could become two separate continents. About 100 million years later, the last volcano in the area went quiet, and the North American craton became stable.
[edit] Formation of sedimentary rock
Five hundred fifty million years ago, the area found itself repeatedly inundated with water of a shallow sea that grew and receded through several cycles.[1] The land mass of what is now North America ran along the equator, and Minnesota had a tropical climate. Small marine creatures such as trilobites, coral, and snails lived in the sea. The shells of the tiny animals sank to the bottom, eventually forming limestone and sandstone. The region remained coastline for a long period, with creatures resembling crocodiles and sharks sliding through the nearby seas. Other land animals followed as the dinosaurs disappeared, but much of the historical record of this time was chiseled away as glaciers expanded and retreated across the region through several cycles starting about 2 million years ago.
[edit] Recent history
Several ice ages came and went over the Minnesota Territory. The ice continued to retreat for the last time about 12,500 years ago. Melting glaciers filled the lakes and etched the river valleys of the state. Minnesota was on the southern edge of Lake Agassiz at this time, a massive lake with a volume rivaling that of all the Great Lakes combined. Dammed by the northern ice sheet, glacial River Warren was the outlet for the immense flow of this lake, which drained south through the valleys now used by the Minnesota and Mississippi Rivers. Falls on the river were precursors to the Saint Anthony Falls. Eventually the ice sheet melted, and the Red River gave Lake Agassiz a northern outlet toward Hudson Bay, leaving behind a fertile valley as the lake drained away. The peatlands and flat plains of northwestern Minnesota now occupy the former lakebed.
At this time, a number of giant animals roamed the area. Beavers were the size of bears, and mammoths were 14 feet (4.3 m) high at the shoulder and weighed 10 tons. Even buffalo were much larger than they are today. Glaciers continued to retreat and the climate became warmer in the next few millennia. The giant creatures eventually died out about 9,000 years ago.
- See also: Glacial history of Minnesota
[edit] Minnesota today
The resulting landscape is relatively flat; only 1699 feet of elevation separate the high and low points.[5] But the state's features, its people, and its economy all have roots in its geology. The Iron Range contains deposits of ore in northeast Minnesota. In the Red River Valley, the American Crystal Sugar Company sows 500,000 acres of sugar beets.[6] In southwest Minnesota, on top of the Coteau des Prairies, an area circumvented by the last glaciation, wind speeds average 16 mph, high enough for commercially viable wind power.[7] In Minneapolis, the 50 foot drop in the Mississippi River over a limestone ledge at Saint Anthony Falls provided water power to the first flour mills in the city, the basis for its tremendous growth in the last half of the 19th century.
[edit] References
- ^ a b c Ojakangas, Richard W.; Charles L. Matsch (1982). Minnesota's Geology, Illus. Dan Breedy, Minneapolis, Minnesota: University of Minnesota Press. ISBN 0-8166-0953-5.
- ^ Breining, Greg (December 2005). Compass American Guides: Minnesota, 3rd Edition, 3rd, Compass American Guides. ISBN 1-4000-1484-0.
- ^ USGS, America's Volcanic Past: Minnesota Retrieved 5 January 2007
- ^ Michigan State University Dept. of Geography, The Mid-Continent Rift Zone Retrieved 5 January 2007
- ^ Elevations and Distances in the United States. U.S Geological Survey (2005). Retrieved on 2006-11-06.
- ^ Cooperative Profile. American Crystal Sugar Company. Retrieved on 2006-12-05.
- ^ Wind Energy Projects Throughout the United States of America. The American Wind Energy Association. Retrieved on 2006-11-26.
