As the cloud rotated, it became a flat disc perpendicular to its axis of rotation. Most of the mass concentrated in the middle and began to heat up. Meanwhile, the rest of the disc began to break up into rings, with gravity causing matter to condense around dust particles. Small fragments collided to become larger fragments, including one collection about from the center: this would become the Earth. As the Sun condensed and heated, nuclear fusion started, and the solar wind cleared out most of the material in the disc, which had not yet condensed, into larger bodies.
The Earth's relatively large natural satellite, the Moon, is unique. During the Apollo program, rocks from the Moon's surface were brought to Earth. Radiometric dating of these rocks has shown the Moon to be 4527 ± 10 million years old, about 30 to 55 million years younger than other bodies in the solar system. New evidence suggests the Moon formed even later, 4.48±0.02 Ga, or 70–110 Ma after the start of the Solar System. Another notable feature is the relatively low density of the Moon, which must mean it does not have a large metallic core, like all other terrestrial bodies in the solar system. The Moon has a bulk composition closely resembling the Earth's mantle and crust together, without the Earth's core. This has led to the giant impact hypothesis: the idea that the Moon was formed during a giant impact of the proto-Earth with another protoplanet.
The impactor, sometimes named Theia, is thought to have been a little smaller than the current planet Mars. Theia finally collided with Earth about 4.533 Ga.
Models reveal that when an impactor this size struck the proto-Earth at a low angle and relatively low speed (), much material from the mantles (and proto-crusts) of the proto-Earth and the impactor was ejected into space, where much of it stayed in orbit around the Earth. This material would eventually form the Moon. However, the metallic cores of the impactor would have sunk through the Earth's mantle to fuse with the Earth's core, depleting the Moon of metallic material. The giant impact hypothesis thus explains the Moon's abnormal composition. The ejecta in orbit around the Earth could have condensed into a single body within a couple of weeks. Under the influence of its own gravity, the ejected material became a more spherical body: the Moon.
The radiometric ages show the Earth existed already for at least 10 million years before the impact, enough time to allow for differentiation of the Earth's primitive mantle and core. Then, when the impact occurred, only material from the mantle was ejected, leaving the Earth's core of heavy elements untouched.
The impact had some important consequences for the young Earth. It released an enormous amount of energy, causing both the Earth and Moon to be completely molten. Immediately after the impact, the Earth's mantle was vigorously convecting, the surface was a large magma ocean. The planet's first atmosphere must have been completely blown away by the enormous amount of energy released. The impact is also thought to have changed Earth’s axis to produce the large 23.5° axial tilt that is responsible for Earth’s seasons (a simple, ideal model of the planets’ origins would have axial tilts of 0° with no recognizable seasons). It may also have sped up Earth’s rotation.
At the beginning of the Archean, the Earth's heat flow was nearly three times higher than it is today, and was still twice the current level by the beginning of the Proterozoic. Thus, plates and volcanic activity were considerably more active than they are today; the Earth's crust was not only thinner than is today, but probably broken up into many more tectonic plates, with numerous hot spots, rift valleys, and transform faults. The existence of plate tectonics in this eon is disputed: it is an active area of modern research.
There were no large continents until late in the Archean; small protocontinents were the norm, prevented from coalescing into larger units by the high rate of geologic activity. These felsic protocontinents probably formed at hot spots rather than subduction zones, from a variety of sources: mafic magma melting more felsic rocks, partial melting of mafic rock, and from the metamorphic alteration of felsic sedimentary rocks.
The Archean atmosphere apparently lacked free oxygen. Temperatures appear to have been near modern levels, although astronomers think that the sun was about one-third dimmer. This is thought to reflect larger amounts of greenhouse gases than later in Earth history.
The oldest rock formations exposed on the surface of the Earth are Archean or slightly older. Archean rocks are known from Greenland, the Canadian Shield, western Australia, and southern Africa. Although the first continents formed during this eon, rock this age makes up only 7% of the world's current cratons; even allowing for erosion and destruction of past formations, evidence suggests that only 5-40% of the present continental crust formed during the Archean.
In contrast to the Proterozoic, Archean rocks are often heavily metamorphized deep-water sediments, such as greywackes, mudstones, volcanic sediments, and banded iron formations. Greenstone belts are typical Archean formations: they are made of alternating high and low-grade metamorphic rocks. The high-grade rocks come from volcanic island arcs, and the low-grade metamorphic rocks come from deep-sea sediments. These sediments eroded from the island arcs and ended in a forearc basin. In short, "greenstone belts show where protocontinents were stuck together".
Fossils of cyanobacterial mats (stromatolites) are found throughout the Archean—becoming especially common late in the eon—while a few other probable bacterial fossils are known from chert beds. In addition to bacteria, microfossils of the extremophilic archaea have also been identified.
There are no known eukaryote fossils. No fossil evidence yet exists for viruses.
The geologic record of the Proterozoic is much better than that for the preceding Archaean. In contrast to the deep-water deposits of the Archean, the Proterozoic features many strata that were laid down in extensive, shallow epicontinental seas; furthermore, many of these rocks are less metamorphosed than Archean-age ones, and plenty are in fact unaltered. Study of these rocks show that the eon featured rapid continental accretion (unique to the Proterozoic), supercontinent cycles, and mountain-building (orogeny) activity.
The first known glaciations occurred during the Proterozoic. One ice age began shortly after the beginning of the eon. There were at least four during the Neoproterozoic, climaxing with the "Snowball Earth" of the Varangian glaciation.
The Great Oxygenation Event was one of the most important events of the Proterozoic. Though oxygen was undoubtedly released by photosynthesis well back in Archean times, it could not build up to any significant degree until chemical sinks--unoxidized sulfur and iron—had been filled; until roughly 2.3 billion years ago, oxygen was probably only 1 to 2% of its current level.(Stanley, 323) Banded iron formations, which provide most of the world's iron ore, were also a prominent chemical sink; most accumulation ceased after 1.9 billion years ago, either due to an increase in oxygen or a more thorough mixing of the oceanic water column.
Red beds, which are coloured by haematite, indicate an increase in atmospheric oxygen after 2 billion years ago; they are not found in older rocks.(Stanley, 324) The oxygen build-up was probably due to two factors: a filling of the chemical sinks, and an increase in carbon burial, which stored organic compounds which would have otherwise been oxidized by the atmosphere.