Geology

The start of the Archean Eon is defined by the isotopic ages of the oldest rocks. Prior to the Archean, the Earth was in the Astronomical stage of its development. The oldest materials we have are not rocks, but minerals. To be precise, they are Australian zircons grains that are about 4.2-4.4 billion years old. They probably got into the Archean rock formation through a river.

During the Archean, volcanic activity produced rare rocks like Komalite, a rock low in silicon, potassium, and aluminum, with a high magnesium content. Granite rocks dominated in the remnants of the surviving Archean crust. Rocks from the Archean Eon are often highly deformed metaporphized deep water sediments, including gray wackes, mudstone, volcanic sediments, and Banded Iron formations. Carbonate rocks were also present in the Archean, which indicates a more acidic ocean than now. Greenstone belts are also typical of the eon. They consist of alternating layers of sedimentary and igneous rock. Rock from this age only makes up for 7% of the world's cratons, and Archean rocks can mainly be found in Greenland, Canada, the Baltic Shield, Scotland, India, Brazil, Australia, and Western Africa.

Banded Iron Formations (or BIFs), are an important type of rock formation from this eon. The oldest ones date back to the Mesoarchean. Basically, a Banded Iron Formation is a rock formation consisting of layers of iron-poor layers and iron-rich layers. This indicates oxygen "cycles" or "pulses", the causes of which have never been discovered. BIFs formed until about 1.8 billion years ago, and they contain 20 times the amount of oxygen we have in our atmosphere today.

When the Archean began, the Earth's heat flow was nearly three times higher than it is today, and was still twice that level when the eon ended. The heat mostly came from planetary accretion, heat from the formation of the Earth's core, and heat produced by radioactive elements. The early crust was very unstable and melted and re-melted a lot. Any surviving Hadean rocks were sucked back into the mantle and melted down, or "erased" them and all their isotopic records. All rocks could be sucked back into the mantle if they were heavy enough.

There are lots of theories surrounding Archean geology. One area where opinions are divided is the formation of the Archean and how it grew. Scientists who doubt plate tectonics in the Archean existed argue that protocontinents formed at hotspots (areas with abnormal heat), rather than subduction zones (areas where plates slide into the mantle). Partial melting in downward directed diapirs produced intermediate and felsic rocks. Others accept that continent formation is part of the plate tectonic process, which is thought to have operated since the start of the Archean.

Plate tectonics in the Archean would have been different from today. Because the Earth is thought to have been hotter, there would be more vigorous tectonic activity than today, resulting in faster recycling of the Earth's crust. This may have prevented cratonisation and continent formation until the mantle cooled and convection slowed down. Another theory is that the oceanic lithosphere was too buoyant to subduct, and the tectonic events caused the loss of most Archean rocks.

Understanding what cratons are is a big part of understanding Archean geology. They appear when the (mostly volcanic) rock is buried deep, but not deep enough to melt. Instead, the heat and the pressure make it metamorphic rock. These areas have fresh igneous rock on top and metamorphic rock on the bottom. For some reason, there was a lot of cratonisation towards the end of the Archean, which has not been replicated since. Cratons mainly sit on the continental crusts.