When Earth Froze Solid

Snowball Earth is a geohistorical hypothesis proposing the planet was nearly entirely frozen. Little liquid water remained exposed.

This global glaciation is most commonly associated with the Cryogenian Period. That era included two major glacial episodes.

In 1964, W. Brian Harland presented palaeomagnetic data showing glacial tillites were deposited at tropical latitudes. Tillites make terrible tourists.

A single reliable tropical tillite datum existed.

Soviet climatologist Mikhail Budyko developed an energy-balance climate model. Advancing ice sheets increased reflectiveness, causing further cooling.

Joseph Kirschvink coined the term snowball Earth in a short paper published in 1992. Volcanoes finally clocked in.

Snowball Earth episodes occurred before sudden radiations of multicellular bioforms known as the Avalon and Cambrian explosions. Multicellularity arrived late.