The Hadean is the first geologic eon of Earth and represents the “genesis” in a non-Biblical since of the physical space body called Earth. It was during this eon that the Solar System and Earth was formed. The term “Hadean”, was coined by geologist Preston Cloud for the pre-Archean period (or pre-Isuan sequence) whose record does not appear to physically exist but which can be approximated from meteorites and lunar rocks. The name Hadean is derived from “Hades”, the ancient Greek god of the underworld in reference to the hellish conditions on Earth at the time. The planet had just formed and was still very hot due to high volcanism, a partially molten surface, and frequent collisions with other Solar System bodies. In the truest sense, the Hadean is not actually a “geologic Epoch” because there were no rocks until the conclusion of subsequent Epochs of the Archean Eon. It is believed that the non-oxygenated atmosphere was very similar to that of Venus and/or Jupiter.
Tectonics and Paleoclimate
At the beginning of the Hadean, the Solar System was forming in what was probably a large cloud of gas and dust around the sun, called an “accretion disc” which was most likely created when an old, immense star exploded (i.e., supernova). As the sun formed, it shrank in on itself due to gravitational compaction until it began to undergo nuclear fusion and give off light and heat. Surrounding particles began to coalesce by gravity into larger lumps (a.k.a. “planetesimals”) which continued to aggregate into planets.
One theory holds that the Earth and Moon were formed from a collision between two older planets roughly the size of Mars. And because collisions between large planetesimals release a lot of heat, the Earth and other planets would have been molten at the beginning of their histories. The atmosphere was most likely a mixture of methane, ammonia, carbon dioxide, hydrogen, sulfur oxides, nitrous oxides, and other gases. Rocks from the Hadean have not yet been discovered. If rocks existed, they were most likely destroyed through erosion and plate tectonics.
Flora & Fauna
To date, there is no evidence of plant or animal life during the Hadean.
Meteorite Impacts on Earth
I included a list of meteorite impacts relevant to this time period as a point of reference since many of the explanations for mass extinctions throughout Earth’s history include meteorite impact(s) as a possible cause. The meteorite impact information below was obtained from the ‘Earth Impact Database’ maintained by the Planetary and Space Science Centre, University of New Brunswick, Fredericton, New Brunswick, Canada ( www.passc.net/EarthImpactDatabase).