The geology of Minnesota comprises the rock, minerals, and 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 Precambrian bedrock was formed by volcanism and the deposition of sedimentary rock and then modified by processes such as faulting, folding and erosion. In the second period, many layers of sedimentary rock were formed by deposition and lithification of successive layers of sediment from runoff and repeated incursions of the sea. In the third and most recent period starting about 1.8 million years ago, glaciation eroded previous rock formations and deposited deep layers of 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
Minnesota contains some of the oldest rocks on Earth, granitic gneisses that formed some 3,600 mya (million years ago) — roughly 80% the age of the planet.Ojakangas and Matsch, Minnesota's Geology, p. 23. About 2,700 mya, the first volcanic rocks that would later underlie Minnesota began to rise up out of an ancient ocean, forming the Superior Craton.Ojakangas and Matsch, Minnesota's Geology, pp. 25–32. This craton later assembled into the Canadian Shield, which became part of the 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. 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. 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.Van Schmus et al., The Midcontinent Rift System, pp. 345, 349. 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.Ojakangas and Matsch, Minnesota’s Geology, pp. 57–59. 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. Although now free of folding and faulting caused by plate tectonics, the region continued to experience gradual subsidence and uplift.Ojakangas and Matsch, Minnesota’s Geology, p. 65.
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.Ojakangas and Matsch, Minnesota’s Geology, pp. 65–95. 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. Later, creatures resembling crocodiles and sharks slid through the water, and fossil shark teeth have been found on the uplands of the Mesabi Range.Ojakangas and Matsch, Minnesota's Geology, pp. 91–93. 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, Ordovician, and Devonian age, from the Mount Simon Sandstone at the bottom of the sequence of sedimentary rocks to the Cedar Valley Group at the top.
Ice ages
In the Quaternary Period starting about two million years ago, glaciers expanded and retreated across the region. The ice retreated for the last time about 12,500 years ago.Ojakangas and Matsch, Minnesota’s Geology, pp. 97, 108. Melting glaciers formed many of the state's lakes and etched its river valleys. They also formed a number of proglacial lakes, which contributed to the state's topography and soils. Principal among these lakes was Lake Agassiz, a massive lake with a volume rivaling that of all the present Great Lakes combined. Dammed by the northern ice sheet, this lake's immense flow found an outlet in glacial River Warren, which drained south across the Traverse Gap through the valleys now occupied by the Minnesota and Mississippi Rivers. Eventually, the ice sheet melted, and the Red River gave Lake Agassiz a northern outlet toward Hudson Bay. As the lake drained away its bed became peatlands and a fertile plain. Similarly, Glacial Lake Duluth, in the basin of Lake Superior, was dammed by a glacier; it drained down the ancient course of the Midcontinental Rift to the St. Croix River and the Mississippi. When the glaciers receded, the lake was able to drain through the Great Lakes to the Saint Lawrence River.
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 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.Ojakangas and Matsch, Minnesota’s Geology, pp. 116–18.
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.Jirsa and Southwick, Mineral Potential and Geology of Minnesota, Glacial cover in Minnesota. The driftless area of Southeastern Minnesota was untouched by the most recent glaciation. In the absence of glacial scouring and drift, this region presents a widespread and highly dissected aspect absent from other parts of the state. Northeastern Minnesota was subject to glaciation and its effects, but its hard 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
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 of elevation.
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.Ecological Classification System, Ecological Land Classification Hierarchy.
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 Jay Cooke State Park and the Nemadji River basin southwest of Duluth, and much of the area east of U.S. Highway 53 that runs between Duluth and 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.Ecological Classification System, Northern Superior Uplands Section.
Known as the Arrowhead for its shape, this region shows the most visible evidence of the state's violent past. There are surface exposures of rocks first formed in volcanic activity some 2,700 mya during construction of the Archaen-Superior province,Van Schmus et al., The Midcontinent Rift System, p. 348. including Ely greenstone, metamorphosed and highly folded volcanics once thought to be the oldest exposed rock on earth;Chandler, A Geophysical Investigation of the Ely Greenstone Belt in the Soudan Area , 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. Proterozoic formations created about 1,900 mya that gave the area most of its mineral riches; and more recent intrusive gabbro and extrusive basalts and rhyolites of the Duluth Complex and North Shore Volcanic Group, created by magma and lava which upwelled and hardened about 1,100 mya during the Midcontinent Rift.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. The Precambrian bedrock formed by this activity has been eroded but remains at or close to the surface over much of the area.