Reconstructed setting· The columns of time

Landscape of the Sturtian glaciation

717 Ma → 660 Ma

This backdrop illustrates the Sturtian glaciation: ice to the horizon, narrow channels of open water rather than an open ocean, and nothing on the ice.

Backdrop representing the Sturtian glaciation: ice to the horizon, with narrow channels of open water rather than an open ocean, and nothing depicted on the ice.

What the evidence shows

The Sturtian glaciation began 716.5 million years ago and ended 660.98 million years ago. Ice then reached equatorial latitudes, which distinguishes this glaciation from an ordinary ice age. The narrow channels are depicted deliberately: whether the tropical ocean froze entirely, the hard snowball hypothesis, or retained a band of open water, the slushball hypothesis, remains the principal unresolved question about these episodes, and depicting either state would settle it by image. Nothing is depicted on the ice, complex life then being marine and microscopic, animals appearing only later.

Bounds and conventions

The dating is exceptionally precise for this chronological depth; the mechanism is not. The principal difficulty is to account for the rewarming of a planet covered in ice that reflects most of the radiation it receives, the effect that drives the ice-albedo feedback. The backdrop therefore leaves open a question the data do not settle.