Geology of Minnesota

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 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.

Precambrian bedrock has been mined for metallic minerals, including iron ore, on which the economy of Northeast Minnesota was built.

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.

[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.

[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.

Eventually, the ice sheet melted, and the Red River gave Lake Agassiz a northern outlet toward Hudson Bay.

In the absence of glacial scouring and drift, this region presents a widespread and highly dissected aspect absent from other parts of the state.

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.

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.

There are surface exposures of rocks first formed in volcanic activity some 2,700 mya during construction of the Archaen-Superior province,[18] including Ely greenstone, metamorphosed and highly folded volcanics once thought to be the oldest exposed rock on earth;[19] 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.

Topsoils are thin and poor and their parent soils derived from the rock beneath or nearby rather than from glacial till, which is sparse.

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.

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.

[27] Bedrock in this region is mainly Archean, with small areas of Lower Paleozoic and Upper Mesozoic sedimentary rocks along the western border.

[28] By late Wisconsinan times this bedrock had been covered by clayey glacial drift scoured and transported south from sedimentary rocks in Manitoba.

[29] In the western part of the region in the Red River Valley, fine-grained glacial lake deposits and decayed organic materials up to 50 meters in depth form rich, well-textured, and moisture-retentive, yet well-drained soils (mollisols), which are ideal for agriculture.

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.

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.

[12] Ojakangas and Matsch extend the region west past the moraine to a line running north from the Iowa border between Mankato and New Ulm to the latitude of the Twin Cities, then encompassing the latter metropolis with a broad arc east to the St. Croix River.

[39] Under Sansome's classification (followed here), Southeastern Minnesota is generally coterminous with the Paleozoic Plateau Section of the Eastern Broadleaf Forest Province.

It is an area of karst topography, with thin topsoils lying atop porous limestones, leading to formation of caverns and sinkholes.

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.

[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.

At St. Anthony Falls, the Mississippi dropped 50 feet (15 m) over a limestone ledge; these waterfalls were used to drive the flour mills that were the foundation for the city's 19th century growth.

[47] Along the eastern edge of the region are the Dalles of the St. Croix River, a deep gorge cut by runoff from Glacial Lake Duluth into ancient bedrock.

[48] Interstate Park here contains the southernmost surface exposure of the Precambrian lava flows of the Midcontinent Rift, providing a glimpse of Minnesota's volcanic past.

Bedrock geologic map illustrating the main geologic regions of Minnesota. Igneous bodies lie across the northern half of the state, with intrusions throughout the northwest and basalts and other igneous rocks of the Duluth Complex and Beaver Bay Complex from the Midcontinent Rift System bordering Lake Superior in the northeast. In the southeast along the Mississippi River, up through the Twin Cities , canyons carve into Cambrian and Ordovician bedrock that were formed in shallow tropical seas that covered southern and south-central Minnesota during that time. The geology of the southwest includes old rocks, the 3.6 billion year old Morton Gneiss and the Paleoproterozoic Sioux Quartzite , overlain by the Cretaceous sedimentary sequence that includes the Dakota Sandstone , Niobrara Formation , and Pierre Shale , and is continuous across much of the western Midwest to the eastern slopes of the Rocky Mountains . Central Minnesota contains an ancient fault system from the Penokean orogeny .
Map of Minnesota bedrock by age.
Shaded relief image: Superior Upland in the northeast, the flat Red River Valley in the northwest, Central Minnesota's irregular landscape, the Coteau des Prairies and Minnesota River in the southwest, and the southeast's dissected Driftless Area along the Mississippi River below its confluences with the Minnesota and St. Croix in East Central 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.
Geologic regions
The Mesabi and Gunflint Ranges , once part of a single mountain range, were split when the center was engulfed by magma blistering up during the Midcontinent Rift, creating the Duluth Complex . Lava from the rift formed dense volcanic rocks which sank, creating the Lake Superior basin, edged by the North Shore Volcanic Group along the lakeshore.
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.
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.
Map of soils and sediments constituting Quaternary geology of Minnesota.
Relief map of the eastern part of Central Minnesota: Mississippi River enters from northwest and crosses diagonally to the southeast; Minnesota River in the valley of River Warren enters from the south; 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