The Hadean Eon is the first eon in Earth’s history as a planet. It begins at the time when the planet’s crust formed, and extends to about 3.8-3.5 billion years ago. Early in the Hadean the oceans formed as the atmosphere cooled and water condensed. Late in the Hadean life had begun, and left traces in forms of particular forms of carbon isotopes deposited in the oldest sedimentary rocks (from about 3.7-3.9 billion years ago). The Apex cherts from Pilbara Supergroup (northwest Australia) have fossils of what is presumably cyanobacteria in rocks that are 3.47 billion years old, among the oldest fossil records of life on Earth. During the Hadean Eon there was probably intense ultraviolet radiation hitting the planet’s surface since there was little oxygen in the atmosphere (and thus, very little or no ozone layer).

People used to believed that during the Hadean Eon the earth was mostly molten rock, and so Earth was a big ball of lava during most of the Hadean Eon. However, this is no longer the prevailing theory; on the contrary, the Earth probably cooled down and had a mostly solid surface within a hundred million years of planetary formation, which means that for most of the Hadean Eon the earth had a solid surface. It seems possible that the Late Heavy Bombardment (a period of about 300 million years between 3.8 and 4.1 billion years before present) caused so much destruction that the earth's crust was essentially re-melted toward the end of the Hadean era. It seems early simple life survived the LHB and evolved in the Archean Eon that followed the Hadean.

1. Kevin Hefferan’s page about the Hadean Eon and the oldest rocks.
2. The Idaho Museum of Natural History’s page about geologic time.
3. U. Cal. Berkeley’s excellent page on Hadean time.
4. Palaeos has an excellent page about the Hadean Eon.

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This work by Eric & Chun-Chih Hadley-Ives is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License.