Geologic History of the Midwestern United States

Hadean to Proterozoic (4.6 billion to 541 million years ago)

Canadian Shield

The shape and position of North America has changed dramatically over the last few billion years, and geologic processes continue these changes today. The Earth is estimated to be approximately 4.6 billion years old. The oldest rocks known are located in northern Quebec and date to 4.3 billion years ago. Rocks dating to 4 billion years old are found on almost every continent. In North America they are found exposed at the surface in parts of Canada, composing the Canadian Shield, the stable core of the North American continental landmass.

The Canadian Shield is the original core, or craton, of the North American continent. It includes much of Greenland, more than half of Canada, and it extends into the Adirondack Mountains of New York and the Superior Upland region of the Midwest. It is an accumulation of smaller plates and terranes that formed over a period of hundreds of millions of years, between 2.5 and 1.25 billion years ago. The shield was the first section of the North American continent to emerge above sea level, and it remains the largest exposure of Precambrian-aged rock in the world.

Banded iron formations

The late Archean to Proterozoic of the Midwest was characterized by the deposition and presence of banded iron formations. With the evolution of photosynthetic organisms (still single-celled at this time), their waste product, oxygen, was released into the oceans and ultimately the atmosphere. The oxygen reacted and combined with dissolved iron particles to produce bands of iron oxides such as hematite and magnetite.

Penokean Mountains

Between 1.8 and 1.9 billion years ago, an orogenic event produced the Penokean Mountains, which extended from Minnesota into northern Wisconsin and Michigan. The Penokean Orogeny occurred as the Pembine-Wausau island arc and Marshfield terrane collided from the south, increasing the size of the Superior landmass. Erosion of the Penokean Mountains led to deposits of sandstone along the margin of a shallow sea. Around 1.7 billion years ago, after millions of years of heat and pressure, this sandstone produced the metamorphic Sioux Quartzite of Minnesota and Iowa. The Baraboo Quartzite of the Devil’s Lake area in central Wisconsin also dates to this time.

Midcontinent Rift System (Keweenawan Rift)

Around 1.1 billion years ago, a rift valley called the Midcontinent Rift System (Keweenawan Rift) began to split North America apart. The Midcontinent Rift System extended from the eastern edge of modern-day Lake Superior to Kansas. Intense volcanism occurred along the rift, producing igneous deposits with a thickness of some 7.6 kilometers (25,000 feet). The spreading occurred for 20 million years, after which the rift zone began to sink and was gradually filled with sediment.

Paleozoic (541 to 252 million years ago)

Overlying the older rock of the Canadian Shield, much of the remaining geologic history of the Midwest records the presence of shallow seas, the formation of the supercontinent Pangaea, and—most recently—the modern ice age.

Cambrian and Ordovician (541 to 444 million years ago)

Cambrian deposits are recorded in Wisconsin, Illinois, Minnesota, and Iowa. During this time, shallow seas covered much of the Midwest, with several transgression and regression episodes recorded as well. Nearly all of what would become North America (a continent called Laurentia) was located just south of the equator at the dawn of the Cambrian, and it drifted within the tropics for most of its existence.

During the middle of the Ordovician period, about 470 million years ago, the Iapetus Ocean began to close as Baltica (proto-Europe) approached the North American plate from the southeast. The intense pressure of the colliding plates and islands smashing into the side of North America caused its edge to crumple, crushing and folding it into mountains. This mountain-building event is called the Taconic Orogeny, and the resulting Taconic Mountains stretched from Newfoundland to Georgia, sharing roughly the same location and orientation of the Appalachian Mountains today, but with towering peaks in eastern Canada and New England.

This folding propagated far to the west, forming waves parallel to the Taconic Mountains themselves that ran roughly southwest to northeast. Nearest the mountains, the crust warped downwards from central New York to central North Carolina, creating the Appalachian Basin. From western Ohio to Alabama, it was warped up into the Cincinnati Arch, and then down again further inland, creating the Michigan and Illinois basins. These formations are prominent features of Midwestern geology. The inland basins were flooded by the ocean for nearly all of their existence.

Silurian to Mississippi (444 to 323 million years ago)

The mountain building ceased around the beginning of the Silurian, but the Taconic Mountains still played a major role in the formation of the Midwest. As sediment was eroded from the western side of the Taconic Mountains, deposits spread away, through the Northeast and Midwest, as far west as Wisconsin. Thousands of feet of this sediment built up on the floors of the seas that filled the Midwestern basins, and this sediment, along with the millions of generations of marine organisms that lived there, formed the Silurian and Devonian bedrock of much of the area.

For about 60 million years, the eastern margin of North America was relatively quiet. The subduction of the oceanic Iapetus plate caused volcanic eruptions that occasionally spread ash over the Midwest, but for the most part, the Taconics were slowly eroding. Finally, Baltica collided with North America near the end of the Devonian period, around 380 million years ago, and mountain-building began again, creating the Acadian Mountains. The Acadian Mountains effectively replaced the Taconics, creating a massive range that was similar in location and extent.

Just as in the Taconic mountain-building period, compression from the Acadian continental collision warped the crust downward, reinforcing the inland seas. Sediments eroding from the mountains formed the Catskill Delta, creating a new wedge of sediments stretching into a shallow inland sea; the Devonian and Mississippian rocks of Ohio are evidence of this event.

At the time of the Acadian mountain building and subsequent erosion during the Devonian, the Midwest was located at the Equator and experienced a tropical climate. Baltica and North America were united as one larger landmass. Africa, South America, India, Australia, Antarctica, and Florida formed a second continent—Gondwana—in the southern hemisphere.

Pennsylvanian to Permian (323 to 252 million years ago)

During the Carboniferous, the collision of North America with Gondwana was the genesis of Pangaea. This event also resulted in the formation of both the Appalachian Mountains and, in Arkansas and Oklahoma, the Ouachita Mountains.

During the Pennsylvanian to Permian, sea levels began to fluctuate and ultimately fall. Regression of the sea was caused by a combination of glaciation at the South Pole and the forces of mountain building. Along the margins of the retreating sea, enormous coastal swamps formed. These swamps extended from modern Pennsylvania to Alabama and inland as far as Ohio, Indiana, Illinois, and Kansas. They were dominated by strange trees related to modern clubmosses and horsetails. When the vegetation in the swamps died, it fell into stagnant, oxygen-poor water. The lack of oxygen in the water slowed decomposition, forming huge deposits of peat. Sediment covered these deposits, compressing and ultimately metamorphosing them into some of the largest coal beds in the world. The extensive coal beds in North America and western Europe give the Carboniferous period its name.

The burial of so much carbon lowered the amount of carbon dioxide (CO₂) in the Earth's atmosphere, and the abundant plant life raised global oxygen levels. Global temperatures continued to fall, accelerating glaciation in the southern hemisphere and lowering sea level worldwide. Ultimately, the interior seaways regressed from the Midwest, which is why there is no marine sediment younger than Pennsylvanian in age across this area of North America until the development of the Western Interior Seaway in the Cretaceous.