Earth’s oldest rocks form
4.0-2.5 billion years ago
The oldest known rocks on the Earth are 4 billion years old and were collected from the Northwest Territories, Canada. These rocks mark the beginning of a new eon in the Earth’s history, the Archean. Three basic types of rock are known from the Archean: granite, gneiss and greenstone. All have undergone some degree of metamorphism in which the original rock was altered by high temperatures and pressures.
Gneiss is the oldest type of rock in Wisconsin. It is a metamorphic rock that has been so altered that it is difficult to determine the nature of the original rock. Gneiss can be derived from volcanic rock, sedimentary rock, or granite.
Greenstone belts and granitic intrusions are similar to the types of rocks that form in island arc subduction zones today, such as modern day Japan. These ancient rocks provide evidence that plate tectonics have been functioning in some form or another for over 2.5 billion years and suggest that the Archean was a time of ocean floor and continental crust building.
Keewatin - Wisconsin’s Oldest Rocks
2.8 - 2.9 billion years ago- Late Archean
During this time Wisconsin and North America did not exist. Instead, a series of islands or small plates, called terranes, were scattered over the globe. Remnants of these terranes have been dated in Wisconsin to about 2.8-2.9 billion years old and in Minnesota some are over 3 billion years old. The shape, size, and location of these terranes are unknown. Much of the original Archean terranes have been destroyed by weathering or are buried below younger rocks. The small amount of Archean rock that survives today has been severely altered and metamorphosed to gneiss and schist making the original rock unidentifiable.
At least two different Archean terranes have been identified in Wisconsin. In the northern part of the state the remains of one Archean terrane is exposed in Iron, Ashland, Bayfield and Sawyer counties. In the central part of the state the remains of a second, different, Archean terrane is exposed in Wood, Jackson and Clark counties. Whether or not any Archean terranes occur south of this is unknown due to the thick layer of Paleozoic sedimentary rock covering the southern part of the state.
The End of the Archean and beginning of the Proterozoic
(2.2 to 2.6 billion years ago)
This was a relatively quiet time in Wisconsin, geologically speaking, and there was intense weathering of the rocks. Mountains that formed during earlier parts of the Archean were quickly eroded down to flat, rolling granitic plains called peneplains. At this time there are no plants or animals and the continents were bare rock. With no plants or ground cover, erosion was intense and quick. Much of the original Archean terranes were eroded into pebbles, sand and clay; destroying much of the Archean mountains and original crust. These sediments were washed into the ocean and deposited on the ocean floor.