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The name Hadean refers to a geologic period: the time prior to 3,800,000,000 years before the present -- 3,800MA. The geologist Preston Cloud coined the now somewhat outdated term in 1972, originally to label the period before the earliest known rocks. In the last decades of the 20th century geologists identified a few Hadean rocks from Western Greenland, Northwestern Canada and West Australia.

The term Hadean seldom occurs. W. B. Harland has coined an almost synonymous term: the "Priscoan period".

The oldest known rock formations comprise somewhat altered sediments from Greenland dated around 3800MA by a volcanic dike that penetrated the rocks after they were deposited. Individual zircon crystals redeposited in sediments in Western Canada and the Jack Hills region of West Australia are much older. The oldest dated zircons date from about 4400MA - very close to the hypothesized time of the Earth's formation . The Greenland sediments include banded iron beds. They contain possibly organic carbon and quite possibly indicate that photosyntheticPhotosynthesis is a biochemical process in which plants, green algae, and some bacteria use the energy of light to combine water and carbon dioxide into oxygen and energy. It nourishes nearly all living things directly or indirectly, making it vital to li lifeAlternate uses: see Life (disambiguation) and Living (disambiguation Life is a multi-faceted concept with no simple definition; this article is confined to the primary meanings in biology; articles on life in other senses are included in the article life had already emerged at that time. The oldest known fossilFor other uses of the term, see Fossil (disambiguation Fossils are the mineralized remains of animals or plants or other artifacts such as footprints. The totality of fossils and their placement in rock formations and sedimentary layers is known as the fos (from Australia) date from a few hundred million years later.

The etymology of the name "Hadean" (from HadesHades ( Greek: ‘ Haides or ‘δης Hades ("unseen") means both the ancient Greek abode of the dead and the god of that Underworld. Haidou was the genitive form of the word, meaning "the house of Hades"; its nominative form, Haides was origin) suggests underworldFor other meanings of the word "underworld" see Underworld (disambiguation In the study of mythology and religion, the underworld is a generic term approximately equivalent to the lay term afterlife, referring to any place to which newly-dead souls go.-like heat and confusion.