<P> During Hadean time (4.6--4 Ga), the Solar System was forming, probably within a large cloud of gas and dust around the sun, called an accretion disc from which Earth formed 4,500 million years ago . The Hadean Eon is not formally recognized, but it essentially marks the era before we have adequate record of significant solid rocks . The oldest dated zircons date from about 4,400 million years ago . </P> <P> Earth was initially molten due to extreme volcanism and frequent collisions with other bodies . Eventually, the outer layer of the planet cooled to form a solid crust when water began accumulating in the atmosphere . The Moon formed soon afterwards, possibly as a result of the impact of a large planetoid with the Earth . Some of this object's mass merged with the Earth, significantly altering its internal composition, and a portion was ejected into space . Some of the material survived to form an orbiting moon . More recent potassium isotopic studies suggest that the Moon was formed by a smaller, high - energy, high - angular - momentum giant impact cleaving off a significant portion of the Earth . Outgassing and volcanic activity produced the primordial atmosphere . Condensing water vapor, augmented by ice delivered from comets, produced the oceans . </P> <P> During the Hadean the Late Heavy Bombardment occurred (approximately 4,100 to 3,800 million years ago) during which a large number of impact craters are believed to have formed on the Moon, and by inference on Earth, Mercury, Venus and Mars as well . </P> <P> The Earth of the early Archean (4,000 to 2,500 million years ago) may have had a different tectonic style . During this time, the Earth's crust cooled enough that rocks and continental plates began to form . Some scientists think because the Earth was hotter, that plate tectonic activity was more vigorous than it is today, resulting in a much greater rate of recycling of crustal material . This may have prevented cratonisation and continent formation until the mantle cooled and convection slowed down . Others argue that the subcontinental lithospheric mantle is too buoyant to subduct and that the lack of Archean rocks is a function of erosion and subsequent tectonic events . </P>

The earliest period in earth's history is