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Snowball Earth
c. 720–635 million years ago· Event· Universe & Earth
Mostly agreedGood evidence; some details, dates or numbers uncertain.
Earth froze over, nearly from pole to pole, at least twice between about 720 and 635 million years ago.
See it on the History map →What happened
Two great glaciations, the Sturtian and the Marinoan, sent ice down to the tropics. Cooling fed on itself: ice reflects sunlight, so more ice meant more cold. Volcanoes kept releasing carbon dioxide, and over millions of years enough built up to melt the ice suddenly. Whether the oceans froze over completely ('snowball') or kept open water ('slushball') is debated.
Why it matters
The deep freezes and thaws reshaped ocean chemistry and may have raised oxygen levels just before large animals appeared.
How we know
Glacial rocks in places that lay near the equator then; thick 'cap carbonate' rocks laid down during the sudden warming; magnetic records in the rocks. Joseph Kirschvink coined the term in 1992.
Still debated
How complete the freeze was, what triggered it, and whether it helped spark animal evolution remain open questions.
Connections (2)
▸Snowball Earth may have spurred Ediacaran biota: the first large animalsDebated
The connection
As the last great freeze ended about 635 million years ago, melting glaciers washed nutrients into the oceans, and oxygen levels may have risen. Some scientists argue these changes let larger, oxygen-hungry organisms appear soon after, in the Ediacaran Period.
When and how
In the oceans after the Marinoan glaciation ended, about 635–575 million years ago.
How sure are we?
The sequence fits: big Ediacaran fossils follow the thaw. Chemical evidence for an oxygen rise exists but is disputed, so the link is unproven.
Debates and open questions
Whether the freeze-thaw cycles caused, helped, or merely came before the rise of large life is debated.
▸Snowball Earth may have set the stage for Cambrian explosionDebated
The connection
The Snowball Earth episodes reshaped ocean chemistry and may have raised oxygen levels. Some scientists argue this set up conditions, more oxygen and nutrients, that later let animals grow larger and more complex during the Cambrian explosion.
When and how
Following the glaciations of about 720–635 million years ago, roughly 100 million years before the Cambrian began.
How sure are we?
The order of events fits, but about 100 million years separate them, so a direct causal link is speculative.
Debates and open questions
Whether Snowball Earth was a key trigger or simply an earlier event is debated; other causes, such as new genes and predation, are also proposed.