The Archeozoic Era stretches from about 3.8 billion to 2.5 billion years ago. Traditionally, the beginning of the Archean is defined to coincide with the oldest rocks discovered. As recent discoveries have pushed back the earliest dated rocks to about 4.0 billion years old, the beginning of the Archean has also been pushed back correspondingly. However, most texts still continue to date the beginning to 3.8 billion years ago. As the Late Heavy Bombardment (LHB) ended with the Hadean, the newly forming crust continued to stabilize, and eventually led to the creation of the continents. When the continents first appeared is still under debate. The Earth in this period was moderately warm. Although the sun was about 30% cooler than it is today, the geological activity of the earth was much higher, leading to a somewhat temperate climate. Most of the earth was covered with oceans. The atmosphere contained mostly methane and little to no oxygen; therefore it is considered a reducing atmosphere. Although recent discoveries may change this view, it is generally believed that life first evolved in the Archean. Some of the oldest fossils of life on Earth include the Apex Chert (3.465 billion years old) and stromatolites (3.45 billion years old) from Australia, and the Swaziland microfossils from Africa (also about 3.45 billion years old). Dating the oldest life forms is difficult. Stromatolite-like structures have been shown to be as old as 3.5 billion years, but it can be debated whether they were made by living organisms, or natural forces (hydrothermal vents). The earliest conclusive radiometric markers of life (such as O-12 uptake, or the first evidence of photosynthesis, for example), date to about 2.7 billion years old. However, it is widely believed that the first life appeared much earlier, possibly around the beginning of the Archean, around 3.8 billion years ago, or even in the Hadean. The earliest chemical markers of life are dated to about 3.8 billion years, but this is not the same as finding microfossils. [EDIT: the oldest conclusive evidence of life has been pushed back to about 3.43 billion years old, at Strelley Pool in Western Australia.] The first organisms were likely non-photosynthetic, utilizing methane, ammonia or sulfates for their energy needs. Photosynthesis became common with the cyanobacteria, perhaps as early as 3.5 billion years ago. The oxygen produced by these bacteria went into oxidizing rocks on the Earth and the iron in the oceans, so there was no increase in atmospheric oxygen for a very long time. Atmospheric oxygen did not begin to rise significantly until billions of years after photosynthesis first began. The Archean was the period in which continent formation first began. The surface of the Earth had started to solidify in the Hadean, with the presence of liquid water as early as 100 million years after the formation of the Earth. But the early crust was unstable, and was continually eroded, recycled and re melted. During the Archean these areas of land increased in size and during the middle Archean the first continent sized expanses of land first appeared. These proto continents no longer exist, but their remnants are sometimes found in cratons, areas of ancient rock that survive on some of the continental shields today. Cratons typically appear when the overlying rock (mostly volcanic igneous rock) is buried deep, but not deep enough to be re melted. Instead, the heat and pressure converts it into metamorphic rock. These are areas where the crust has thickened, with fresh igneous rock on top and metamorphic rock beneath (though folding of the crust can obscure this relationship). For reasons that are not well understood, there were extensive cratonization events towards the last third of the Archean, which have never been repeated in the history of the Earth. However, continents as we know them today, with continental plates and plate tectonics did not appear until the...
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