North America

North America's little Archean continents slammed together in a series of mountain-building collisions. The core of the modern continent was formed 1,850 million years ago when five of these collisions occurred at once around northeastern Canada. This unified piece of ancient continental crust, called a craton, lies exposed at the surface in the Canadian Shield, and forms a solid foundation under much of the rest of the continent.

In the two billion years of the Proterozoic eon (2,500-570 million years ago), North America's geologic setting became more like the world as we know it. The cores of the modern continents were assembled, and the first collections of continents, or supercontinents, appeared. Life, however, was limited to bacteria and algae, and unbreatheable gases filled the atmosphere. Rampant erosion filled the rivers with mud and sand, because no land plants protected Earth's barren surface from the action of rain, wind, heat, and cold.

Sometimes tectonic stresses pulled the forming continents apart, creating cracks hundreds of miles or kilometers long in the crust. These cracks quickly filled with upwellingmagma to form dikes of solid rock. There are so many of these dikes of black rock that they are collectively called dike swarms.

Rich accumulations of both rare and common metallic elements make Proterozoic rocks a significant source of mineral wealth for North America, as on other continents. Chromium, nickel, copper, tin, titanium, vanadium, and platinum ores are found together in the onion-like layers of crystallized igneous rocks called layered intrusions. Greenstone belts are mined for copper, lead, and zinc, each of which is mixed with sulfur to form a North America. Illustration by Hans & Cassidy. Courtesy of Gale Group. sulfide mineral. Sulfide minerals of lead and zinc are found in limestones formed in shallow seas, while mines in the ancient continental river and delta sediments uncover buried vanadium, copper, and uranium ores.

Most of the steel framework for buildings and machines and tools comes from the processing of a rich and peculiar legacy of the Precambrian environment. Volcanos under the seas of Archean and Proterozoic time erupted huge amounts of ferric iron (Fe²⁺) into water filled with dissolved oxygen. The iron minerals that formed from the reaction of ferric iron and oxygen, hematite and iron hydroxides and sulfides, settled gently on the floors of lakes and quiet seas, season after season, for more than two billion years. The layers of ironminerals and chert formed amazing evenly striped rocks which have provided the world with its iron for more than a century. These banded iron formations are found in Greenland, Canada, and the Mesabi Range of Minnesota.

The banded iron formations disappear from the rock record at around 1.7 billion years ago, about the same time that oxides (minerals formed by reaction with oxygen) appeared abundantly in stream deposits. Some geologists theorize that previous to 1.7 billion years ago, oxygen was busy oxidizing iron in the sea to enter the atmosphere, and when the iron supply ran out, then the oxygen-rich atmosphere bubbled up out of the sea.

Somewhat similar to a continent-sized zipper, a huge rift opened from Kansas to Michigan's upper peninsula around 1,150 million years ago. Its tectonic activity shut down before tearing the continent in half, but left a trough 93 mi (150 km) wide filled with up to 10 mi (15 km) of stacked basalt lava flows and stream sediments. The rift is exposed today in the Keewenaw peninsula in upper Michigan. It once contained giant boulders of pure copper, some weighing several tons.

During the middle to late Proterozoic eon, continental collisions attached new pieces of continental crust to North America's southern, eastern, and western borders. Between 30% and 40% of North America joined the continent in the Proterozoic. The crust underlying the continental United States east of Nevada joined the craton, as well as the crust underlying the Sierra Madre Occidental of Sonora, Chihuahua, and Durango in Mexico. The Mazatzal Mountains, whose rootoutcrop in the Grand Canyon's inner gorge, rose in these mountain-building times in southern and central North America.