The great span of Precambrian time is divided for convenience into three major parts—Hadean (4,600-4,000 million years ago), Archean (4,000-2,500 million years ago) and Proterozoic (2,500-541 million years ago). Each of these is divided still further (see Fig. 1). In Minnesota, only deposits of Archean and Proterozoic age are represented in the rock record. The ages of rocks are determined by measuring the tiny amounts of certain naturally occurring radioactive isotopes they contain. Before isotopic dating methods were developed in the 1950s, the ages of rocks assigned to the Precambrian were not known. The crystalline rocks beneath the fossil-bearing strata were assigned arbitrarily to the Precambrian, without knowledge of the vast length of time they represented. The fossil-bearing rocks represent the most recent part of earth history, called the Phanerozoic (541 million years ago to present, see Fig. 1).
Archean rocks make up the oldest of the Precambrian rocks, and are subdivided into two terranes on the basis of their age and metamorphic history.
The oldest group of Archean rocks crops out in the valley of the Minnesota River between New Ulm and Ortonville, and similar rocks underlie the southwestern quarter of the state. Collectively, these rocks are called the gneiss (pronounced "nice") terrane, a family of coarse-grained streaky or banded rocks that formed between about 3,500 and 2,600 million years ago, miles below the surface of an Archean continent (Fig. 2). This continent or continental fragment is about 900 million years older than the Archean volcanic rocks of northern Minnesota. It may have been joined to the northern volcanic rocks during a period of continental growth about 2,700-2,600 million years ago.
The younger Archean rocks occur mainly in northern Minnesota, and may be seen in Voyageurs National Park, in the western part of the Boundary Waters Canoe Area Wilderness, and in scattered areas elsewhere between Lake of the Woods and Ely. These originally were parts of volcanic islands similar to those which make up Indonesia or Japan in the western Pacific. Greenstone (a dark greenish-gray, fine-grained, weakly metamorphosed basalt), metadacite (a grayish-white, fine-grained metamorphosed volcanic rock), and graywacke (a layered gray rock made up of sand and mud eroded from volcanic sources) were the main materials in the upper parts of the Archean volcanic islands (Fig. 3). Lower parts, revealed by deep erosion, contain large amounts of granite and related coarse-grained rocks that crystallized at depth from the molten state. Much of the granite formed when the volcanic islands were squeezed together by tectonic forces about 2,700 million years ago to form an Archean continent. The Late Archean rocks, collectively, are called the greenstone-granite terrane.
Rocks of the gneiss terrane are quarried in the Minnesota River valley for use as building stone and crushed rock aggregate used for road and railroad construction. Rocks of the greenstone-granite terrane have been explored extensively for various metals including gold, copper, zinc, lead, and iron, but the only deposits found so far that have been large enough and rich enough to mine are the now-closed iron mines near Ely and Soudan. The underground Soudan mine is open for public tours.
Proterozoic rocks form two belts of very different age and geologic history in the eastern part of Minnesota. The older belt (Early Proterozoic) crops out from St. Cloud to the neighborhood of Moose Lake and Carlton (south of Duluth), north to the Mesabi Iron Range near Eveleth and Hibbing. The predominant rock types in this older belt consist mainly of metamorphosed slate and graywacke with minor amounts of volcanic rocks and iron formation, as well as a large proportion of later granitic rocks (Fig. 4). These rocks and those like them beneath the ground surface are part of a former mountain belt—the Penokean orogen—that extended from Lake Huron to South Dakota, and perhaps farther, about 2,000 to 1,800 million years ago. The eroded remnants of this belt have geologic similarities to modern mountain belts along the west coast of North America, and geologists infer that mountains comparable to those in western California existed long ago in Minnesota.
In southwestern Minnesota, the Early Proterozoic Sioux Quartzite accumulated as sand deposited by braided streams that flowed within several fault-bounded basins, on an erosional surface developed on older Archean rocks. Later metamorphism has recrystallized the sandstone into the hard quartzite seen today in the bluffs at Blue Mounds State Park (Fig. 5). Thin beds of reddish-brown mudstone (catlinite) in the quartzite are still being quarried—and carved—at Pipestone National Monument.
The younger belt (Middle Proterozoic) runs along the North Shore of Lake Superior and continues south along the Minnesota-Wisconsin border all the way to Kansas, beneath younger rocks. This younger belt of rocks consists mainly of gabbro and anorthosite, as well as volcanic rocks such as basalt and rhyolite (Fig. 6). These rocks formed around 1,100 million years ago along the Midcontinent rift system, a major feature that formed by the spreading apart of older continental crust. Fractures in the thinned crust allowed magmas to work their way to the surface, where they erupted as volcanoes. The lava flows produced by these volcanoes are exposed along Lake Superior, and their well-preserved flow features are the same as those in modern-day volcanic rocks in Hawaii. The Lake Superior agate, for which Minnesota is famous, formed when vesicles (frozen gas bubbles) in these basalts were filled in by thin bands of red and white quartz (Fig. 7). The base of the volcanic pile was intruded by coarse-grained gabbro, anorthosite, and granite, some of which may have supplied magmas that were feeding lava flows higher up in the sequence. When volcanism ceased, blankets of sand—now sandstone—were deposited in a basin on top of the volcanic rocks.
The iron mines on the Mesabi range, near Hibbing and Virginia, are developed in Early Proterozoic rocks. The Mesabi Iron Range (Fig. 8) is one of the largest mining districts in the world, and in 1992 accounted for 74% of the total iron ore production in the United States. Several quarries developed in Early Proterozoic granite are in operation near St. Cloud. These quarries provide building stone used not only in Minnesota but around the world, as well as crushed rock aggregate used for railroad and highway construction. The Sioux Quartzite also is quarried for aggregate.
Rocks similar to the Middle Proterozoic North Shore volcanic rocks are mined extensively for copper in Michigan. In Minnesota, a large reserve of copper, nickel, and associated platinum and gold exists at the base of the Duluth Complex, along the northwest edge of the Middle Proterozoic system. The sandstones that overlie the volcanic rocks have been quarried in the past for building and paving stone.
Written by T.J. Boerboom, 1994, Minnesota at a Glance: Precambrian Geology: Minnesota Geological Survey; modified for web by A.J. Retzler, June 2020, using additional source(s) cited below.
(1) Morey, G.B., and Dahlberg, H., 1995, Geology of Minnesota: A Guide for Teachers: State of Minnesota, Department of Natural Resources, Fifth Printing, 34 p.