Young, Grant M. Department of Earth Sciences, University of Western Ontario, Ontario, Canada.
Last reviewed:May 2016
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A major interval of geologic time between about 4600 and 541 million years ago (MYA), encompassing most of Earth history. The Precambrian is divided into the three eons: Hadean, Archean, and Proterozoic (Fig. 1). The Earth probably formed around 4600 MYA and was then subjected to a period of intense bombardment by meteorites so that there are few surviving rocks older than about 4000 MYA. The Hadean includes the first several hundred million years that are missing from the rock record during which the Earth was melted and formed a magma ocean. Ancient rocks are preserved exclusively in continental areas. All existing oceanic crust is younger than about 200 million years, for it is constantly being recycled by the processes of sea-floor spreading and subduction. Development of techniques for accurate determination of the ages of rocks and minerals that are billions of years old has revolutionized the understanding of the early history of the Earth. See also: Dating methods; Geologic time scale; Hadean; Rock age determination
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