• Geological Eon Hadean, 4.5 Ga
  
• Paleontologist and Geographer Preston Ercelle Cloud, Jr., 1912
  

Since few geological traces of this period remain on Earth there is no official subdivision. However, the Lunar geologic timescale embraces several major divisions relating to the Hadean and so these are sometimes used in a somewhat informal sense to refer to the same periods of time on Earth.

The Lunar divisions are:

• Pre-Nectarian, from the formation of the Moon's crust up to about 3.92 Ga
• Nectarian ranging up to about 3.85 Ga, in a time when the Late Heavy Bombardment, according to that theory, was in a stage of decline.

There is a recently proposed alternative scale published in 2010 by Solid Earth, a new open access journal. The article proposes the addition of the Chaotian and Prenephelean Eons preceding it and divides the Hadean into three eras with two periods each. The Paleohadean era consists of the Hephaestean (4.5 - 4.4 BYA) and the Jacobian periods (4.4 - 4.3 BYA). The Mesohadean is divided into the Canadian (4.3 - 4.2 BYA) and the Procrustean periods (4.2 - 4.1 BYA). The Neohadean is divided into the Acastan (4.1 - 4.0 BYA) and the Promethean periods (4.0 - 3.9 BYA).

In the last decades of the 20th century geologists identified a few Hadean rocks from Western Greenland, Northwestern Canada, and Western Australia. Rock formations in Greenland comprise sediments dated around 3.8 Ga and are somewhat altered 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 Western Australia are much older. The oldest dated zircons date from about 4.0 Ga — very close to the hypothesized time of the Earth's formation.

The Greenland sediments include banded iron beds. They contain possibly organic carbon and imply some possibility that photosynthetic life had already emerged at that time. The oldest known fossils (from Australia) date from a few hundred million years later.

A sizeable quantity of water would have been in the material which formed the Earth. Water molecules would have escaped Earth's gravity more easily when it was less massive during its formation. Hydrogen and helium are expected to continually leak from the atmosphere.

Part of the ancient planet is theorized to have been disrupted by the impact that created the Moon, which should have caused melting of one or two large areas. Present composition does not match complete melting and it is hard to completely melt and mix huge rock masses. However, a fair fraction of material should have been vaporized by this impact, creating a rock vapor atmosphere around the young planet. The rock vapor would have condensed within two thousand years, leaving behind hot volatiles which probably resulted in a heavy CO₂ atmosphere with hydrogen and water vapor. Liquid water oceans existed despite the surface temperature of 230 °C (446 °F) because of the atmospheric pressure of the heavy CO₂ atmosphere. As cooling continued, subduction and dissolving in ocean water removed most CO₂ from the atmosphere but levels oscillated wildly as new surface and mantle cycles appeared.

Study of zircons has found that liquid water must have existed as long ago as 4.4 Ga, very soon after the formation of the Earth. This requires the presence of an atmosphere. The Cool Early Earth theory covers a range from about 4.4 Ga to 4.0 Ga.

A September 2008 study of zircons found that Australian Hadean rock holds minerals that point to the existence of plate tectonics as early as 4.0 Ga. If this is true, the time when Earth finished its transition from having a hot, molten surface and atmosphere full of carbon dioxide, to being very much like it is today can be roughly dated to about 4.0 billion years ago. The action of plate tectonics and the oceans traps vast amounts of carbon dioxide, thereby eliminating the greenhouse effect and leading to a much cooler surface temperature and the formation of solid rock, and possibly even life.

It is unlikely that life could have formed and established itself in the extreme, volatile conditions of the Hadean. If life had begun to form at this time, it most likely would have been destroyed several times, being forced to start over again. It is probable, however, that the building blocks necessary for life as humans know it were formed at some point during this time. Life would be granted a true start in the succeeding Archean Eon, after conditions on Earth began to stabilize.

Despite the difficulties of finding employment and getting into a four year college during the Great Depression, Preston paid for his own first semester at The George Washington University in Washington, D.C..

An influential figure in Cloud's life was Dr. Ray Bassler, a professor and the curator of paleontology at the National Museum of Natural History. Bassler noticed Cloud's interest in his work, and he arranged for the student to work at the museum. Later on, he worked with G. Arthur Cooper, a paleontologist and stratigrapher. Under Cooper, Preston learned much about fossils, especially those of brachiopods. He worked full time at the museum, but he graduated in 1938, earning a Bachelor of Science degree. Cooper financially allowed for Preston to attend Yale University, which he did.

In 1961, Preston focused on academia. He was the chairman of the geology department at the University of Minnesota for four years. As the professor of biogeology, he spent three years at the University of California at Los Angeles and six years at the University of California at Santa Barbara.

In 1972, Preston coined the term "Hadean" for the Hadean geologic eon, referring to the Earth's magma state of constant heat by using the Greek word for the Underworld.

Cloud was a member of the National Academy for thirty years, he was chairman of the Geology Section and occupied positions in its Council and Executive Committee.

In graduate school, Preston was engaged to Mildred Porter. When Preston resigned from Harvard, they were divorced. In his time at Washington, he married Frances Webster, with whom he had three children, Karen, Lisa and Kevin. As Preston left for Minnesota, they were divorced, but kept in touch because of the children.

In Santa Barbara, Preston met and married Janice Gibson, an opera singer and mother of three children herself.