State of the planet· The columns of time
Sturtian Snowball Earth
717 Ma → 660 Ma
A planetary glaciation lasting more than fifty million years, reaching equatorial latitudes.
What the evidence shows
The ice reaches the tropics, and its coming and going are dated with a precision that is rare this deep: onset at 716.5 million years give or take 200,000, and a globally synchronous deglaciation at 660.98 give or take 740,000, fifty-six million years later. The synchrony is the striking part, because it means the ice retreated everywhere at once, which is what a planetary rather than a regional event looks like.
Bounds and conventions
The dates are unusually tight for this depth of time, and the precision does not come from the glacial debris, which is hard to date, but from black shales above and below it, whose rhenium and osmium act as a clock. What the shales cannot say is how thick the ice was, or whether any open water survived: the clock fixes the ice in time without describing it.