The Canadian Shield: Foundation of a Continent

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, a stable core of the North American continental landmass.

The oldest rocks found on Earth are 4.3-billion-year-old green- stone beds found along the eastern shore of Hudson Bay in northern Quebec. The oldest known materials are 4.4-billion-year-old zircons from Western Australia.

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.

Seven distinct provinces compose the nucleus that is the Canadian Shield. The Superior Province is found in south central Canada, and it extends into northeastern Minnesota, northern Wisconsin, and the Upper Peninsula of Michigan. Metamorphic gneisses exposed within the Minnesota River Valley represent the oldest rocks found in the Midwest, dating back 3.5 billion years. The oldest rocks from Wisconsin are represented by 2.8-billion-year-old gneiss, while Precambrian outcrops in a small area of northwestern Iowa—the Sioux Quartzite in Gitchie Manitou State Preserve—date to 1.7 billion years ago. No Precambrian rocks are exposed in Indiana, Illinois, or Ohio (Figure 1.3).