Archean 2.8 to 2.7 bya (WI only);
4.0-2.5 billion years ago
Earth’s oldest rocks form
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 often referred to as greenstone belts because it occurs as long narrow strips of rock. Note: the grey color band on the northern edge of the Wisconsin geological map is a greenstone belt. Archean greenstones are made up of several rock types that form a general sequence:
At the bottom of a typical Archean greenstone sequence is volcanic basalt called pillow basalt. This is lava that erupted underwater and is similar in composition to the modern ocean floor and Hawaii.
Above the basalt is another type of volcanic rock called rhyolite. Rhyolite comes from a more explosive type of volcano than basalt, such as the types found in the Cascades and Japan and the rock produced by this type of an eruption is often fragmented and brecciated.
The third major component of greenstone belts is greywacke. Greywacke is a sedimentary rock made up of sand, clay and volcanic rock fragments. Due to metamorphism the clay is altered to the mineral chlorite and the rock is altered to a metamorphic rock called schist.
Iron formations are another important component of Archean greenstone belts. Iron formations are also sedimentary rocks probably formed by volcanic hot springs and precipitated by bacteria.
Associated with these greenstone belts are granitic intrusions that intruded into, and deformed and altered the greenstone.
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 has been functioning in some form or another for over 2.5 billion years and also suggests that the Archean was a time of ocean floor and continental crust building.
Many greenstone and granite terranes formed during the Archean. After these terranes formed it is thought that many collided and formed an Archean supercontinent called Kenorland. This would have been similar to the supercontinent Pangea, but much older. The formation of this supercontinent was probably the first of several major mountain building events that metamorphosed, folded and partially melted these terranes. Please note that few rocks from this time survived. The rocks that have survived are highly altered having been subjected multiple times to high temperatures and pressures. Consequently it makes it very difficult to understand what the world was like this far back in time.
(2.8 - 2.9 billion years ago)
Wisconsin’s Oldest Rocks
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. In Minnesota some are over 3 billion years old. The shape, size and location of these terranes are not known. 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
(2.6 to 2.2 billion years ago)
This is a relatively quite time in Wisconsin geologically speaking an there is intense weathering of the rocks. Mountains that formed during earlier parts of the Archean are quickly eroded down to flat, rolling granitic plains called peneplains. At this time there are no plants or animals. The continents are bare rock. With no plants or ground cover, erosion is intense and quick. Much of the original Archean terranes are eroded into pebbles, sand and clay, destroying much of the Archean mountains and original crust. These sediments are washed into the ocean and deposited on the ocean floor.