Minnesota at a Glance Precambrian Geology

Abstract

What do the cliffs along the North Shore of Lake Superior, the smooth outcrops in the Boundary Waters Canoe Area Wilderness, the immense iron mines on the Mesabi Iron Range, and the knobby outcrops within the Minnesota River valley have in common? They are part of the very old bedrock that underlies all of Minnesota. Minnesota is situated at the southern edge of the Canadian Shield (Fig. 1)—the nucleus of the continent of North America that formed during Precambrian time. This period of time encompasses about 85% of Earth's history. Geologists consider Precambrian time to have begun with the formation of planet Earth about 4,550 million years (Ma) ago and to have ended about 541 Ma, when organisms with hard parts, such as shells, rapidly diversified. The rocks formed in Minnesota during this enormous span of time record a complicated geologic history that involved volcanoes, ocean islands, mountain chains, earthquakes, and unstable geologic conditions that were very different from the Minnesota of today. Precambrian Minnesota resembled modern-day Indonesia for a while; later, it resembled modern-day California; and still later it resembled parts of the Middle East and eastern Africa.