Hadaikum

Hadaikum, or the Hadean Eon, is the earliest division of Earth's geologic history, spanning roughly 4.6 to 4.0 billion years ago. The name, derived from Hades, the Greek god of the underworld, reflects the extreme and largely unknown conditions of the time. The eon immediately precedes the Archean.

During the Hadean, Earth formed by accretion from the solar nebula and experienced pervasive melting, frequent volcanic activity, and large-scale impacts. A global magma ocean is believed to have existed early in the eon, and the surface was repeatedly reworked by collisions with planetesimals. The formation of the Moon, inferred to result from a giant impact near 4.5 billion years ago, profoundly influenced the planet’s early evolution.

The atmosphere and hydrosphere were outgassed from the solidifying planet and were likely reducing in composition, rich in water vapor, carbon dioxide, nitrogen, and other gases. As the planet cooled, conditions may have allowed the formation of oceans, though surface environments remained volatile and transient.

Evidence for the Hadean comes largely from ancient minerals rather than full rock records. Detrital zircon grains found in the Jack Hills of Western Australia have ages up to about 4.4 billion years, providing constraint on early crustal growth and crystallization. Overall, rocks from this eon are sparse, and the Hadean is inferred from geochemical and geophysical modeling rather than continuous rock records.

The Hadean ends around 4.0 billion years ago, marking the transition to the Archean, when the oldest substantial crustal rocks begin to appear more reliably in the geological record.