CLIMATE × EARTH

The First Ice Age

~2.9 billion years ago

Archean · Mesoarchean

Boulder beds in South Africa hold stones scratched and faceted by moving ice, and, in the fine layered mud nearby, single stones that could only have been dropped from ice floating overhead. They are the oldest glacial deposits known, laid down half a billion years before the first global glaciations.

Why it matters

Under a Sun a fifth fainter than today's, the surprise is not that ice appeared; it is that it appeared so rarely. Something was keeping the planet warm, and this bed of stones is the exception that measures it.

Dating & uncertainty

The evidence is a jumbled boulder bed in the Pongola Supergroup, with stones scratched and planed flat by ice, stones dropped from floating ice into fine layered mud, and a weathering chemistry typical of glacial debris. A single basin is a thin base for a global claim, and how far the ice reached is unknown.

Sources

1. Young et al. (1998), The Journal of Geology · Earth's oldest reported glaciation: physical and chemical evidence from the Archean Mozaan Group
2. von Brunn & Gold (1993), Journal of African Earth Sciences · Diamictite in the Archaean Pongola sequence of southern Africa
3. Catling & Zahnle (2020), Science Advances · The Archean atmosphere

Earth history as one day

08:40:1015 h 20 min before midnight

if Earth’s 4.54 billion years were compressed into a single day