Period· The columns of time

Rhyacian

2.3 Ga → 2.05 Ga

Oxygen overflows, then withdraws; magma pours into the crust in quantities never seen again, and something large and flat may already be living.

What the evidence shows

In what is now South Africa, close to a million cubic kilometres of magma pushed into the crust between 2.06 and 2.055 billion years ago, pulse after pulse rather than as a single filling, and cooled slowly enough for its minerals to settle out by density, building the layered stack that holds some of the world's richest seams of platinum. The period is named for flows of lava, and it earns the name. The chemistry of the ocean swings just as violently: marine carbonates carry the largest positive carbon isotope excursion in the record, the Lomagundi event, read conventionally as carbon buried on an unprecedented scale, since every carbon atom buried instead of being left to rot leaves a molecule of oxygen behind. Then the excursion ends, oxygen falls back, and stays low for more than a billion years. In Gabon, in rocks of 2.1 billion years, lie flattened discs up to twelve centimetres across whose biological nature is still argued.

Bounds and conventions

The two bounds are conventional round numbers. What the period contains is dated far better than what defines it: the Bushveld intrusion to within five million years, the Yarrabubba impact to 2.229 billion years. The biological reading of the Gabonese fossils, on the other hand, is genuinely contested, and the rocks that hold them are not. Nor is the Lomagundi excursion still read by everyone as a global signal: a recent compilation finds its highest values confined to nearshore and coastal settings, which would make it a record of where the carbonate formed rather than of the whole carbon cycle.

Sources

1. Cohen, K. M., Finney, S. C., Gibbard, P. L., Fan, J.-X. (2013). The ICS International Chronostratigraphic Chart. Episodes 36(3), 199–204, mise à jour continue, voir Cohen et al. (2025), Episodes.
2. Scoates et al. (2021), Journal of Petrology · Dating the Bushveld Complex: timing of crystallization, duration of magmatism, and cooling
3. Prave et al. (2021), Journal of the Geological Society · The grandest of them all: the Lomagundi-Jatuli Event and Earth's oxygenation
4. Bekker & Holland (2012), Earth and Planetary Science Letters · Oxygen overshoot and recovery during the early Paleoproterozoic
5. El Albani et al. (2010), Nature · Large colonial organisms with coordinated growth in oxygenated environments 2.1 Gyr ago
6. Erickson et al. (2020), Nature Communications · Precise radiometric age establishes Yarrabubba as Earth's oldest recognised meteorite impact structure