Geology of Minnesota

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.

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. 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. 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. The rifting stopped before the land could become two separate continents. About 100 million years later, the last volcano went quiet.

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. 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.

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. 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. 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.