EARTH

A Billion Years Torn Out of the Record

~720 to 539 million years ago (cause disputed)

Proterozoic · Neoproterozoic · Cryogenian

In the Grand Canyon you can put a hand on the join: Cambrian sandstone above, basement rock a billion years older below, and nothing in between. The same gap opens on continent after continent, which is why it has a name of its own: the Great Unconformity. Where the erosion has been measured, three to five kilometres of crust were stripped away, and the record of a billion years went with them.

Why it matters

The largest gap in the Earth's diary falls immediately before animals appear in it, and we still argue about who tore the pages out. In North America, thermochronology reads the cooling of the rock as glaciers scouring the continents during the snowball glaciations; the same method applied in North China puts the deepest erosion between about 2.1 and 1.6 billion years ago, under the assembly of the first supercontinent. What is at stake is whether the ice that nearly killed the biosphere also cleared the ground for the animals that followed.

Dating & uncertaintyDisputed

Nobody disputes the missing rock; what removed it is contested. The same thermochronology has been read as glacial stripping during the Cryogenian, as a much older exhumation tied to the assembly of Columbia, and as a patchwork of episodes spread through the Neoproterozoic as supercontinents came together and broke apart. Each reading has been answered in print by the others.

Sources

1. McDannell et al. (2022), PNAS · Thermochronologic constraints on the origin of the Great Unconformity
2. McDannell et al. (2022), PNAS · Reply to Flowers et al. on the thermochronologic constraints
3. Zhan et al. (2026), PNAS · Tectonism rather than snowball Earth glaciation is responsible for the Great Unconformity

Earth history as one day

20:11:373 h 48 min before midnight

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