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Snowball Earth
c. 717–635 million years ago· Extinction or crisis· Earth, climate & extinctions
Strong evidenceGood evidence; dates or details still being refined.
Two near-global ice ages, about 717 to 635 million years ago, when ice may have reached the equator.
See it on the Life map →What happened
In the Cryogenian Period, the Sturtian glaciation (about 717–660 million years ago) and the shorter Marinoan (about 650–635 million) covered much or all of the oceans with ice. A likely trigger was the breakup of the supercontinent Rodinia, which boosted rock weathering that pulls carbon dioxide from the air. Volcanoes kept venting CO₂, and over millions of years it built up until the ice melted rapidly, leaving a hot greenhouse.
Why it matters
The extreme freeze and thaw reshaped ocean chemistry and nutrient supply; large algae and animals appear soon after, though a causal link is unproven.
The evidence (fossils, genes, rocks)
Glacial deposits laid down near the equator (shown by magnetic minerals), thick 'cap carbonate' layers on top recording rapid warming, and precise uranium-lead dates. Joseph Kirschvink named it in 1992; Paul Hoffman's team expanded it in 1998.
Still debated
Hard snowball or 'slushball'? Whether tropical oceans stayed partly open is debated, as is how life survived: perhaps in meltwater pools, thin ice or hot springs.
Inside this (1)
Connections (5)
▸The 'Boring Billion' gave way to Snowball EarthStrong evidence
The connection
The long stability ended as the supercontinent Rodinia broke apart from about 800 million years ago. Rifting, huge basalt eruptions and more coastline increased rock weathering, which removes carbon dioxide from the air. The cooling may then have tipped Earth into runaway ice cover around 717 million years ago.
When and how
Worldwide, about 800–717 million years ago; the trigger is often linked to the Franklin eruptions of Arctic Canada, about 720 million years ago.
How sure are we?
Precise dates show the Franklin eruptions and the first glaciation nearly coincide, and rock chemistry suggests increased weathering of fresh basalt.
Debates and open questions
The trigger is still debated: weathering, volcanic sulfur haze, or changes in life such as spreading algae all have supporters.
▸Snowball Earth may have boosted Neoproterozoic Oxygenation EventDebated
The connection
Glaciers grinding over continents produce fine rock flour rich in phosphorus, a key nutrient. When the ice melted, that flood of nutrients may have fed algae blooms; buried algae locked away carbon, leaving more oxygen in the air. Some researchers link the oxygen rise directly to the Snowball glaciations.
When and how
After each Cryogenian glaciation, about 660 and 635 million years ago; Noah Planavsky and colleagues proposed the phosphorus link in 2010.
How sure are we?
Phosphorus is abundant in iron-rich rocks deposited around the glaciations, and some oxygen indicators rise afterward, but the timing is imprecise.
Debates and open questions
Oxygen may have begun rising before the Snowball, so the glaciations might be part of a wider change rather than its cause.
▸Snowball Earth was followed by Ediacaran biotaStrong evidence
The connection
Large Ediacaran organisms appear within about 60 million years of the last Snowball glaciation. The thaw left warmer seas, altered nutrient supply and possibly more oxygen, and the Ediacaran Period itself is defined as beginning at the end of the Marinoan glaciation, 635 million years ago.
When and how
Oceans worldwide, from about 635 to 575 million years ago; the period's starting point is marked at a cap carbonate in South Australia's Flinders Ranges.
How sure are we?
The sequence of events is well dated by uranium-lead ages; the causal link is inferred from timing and chemistry, not proved.
Debates and open questions
Did the glaciations help trigger complex life, or did life recover on its own schedule? A later ice age, the Gaskiers about 580 million years ago, complicates things.
▸Huronian glaciation was an earlier version of Snowball EarthStrong evidence
The connection
The Huronian ice ages, about 2.4–2.2 billion years ago, may have covered much of the planet. About 1.7 billion years later, the Snowball Earth glaciations froze Earth nearly pole to pole. Both may have followed rises in oxygen, and both show how shifts in greenhouse gases can tip climate into a deep freeze.
When and how
Huronian deposits in Ontario, Canada; the Snowball Earth idea was named by Joseph Kirschvink in 1992 and developed by Paul Hoffman and Daniel Schrag in 1998.
How sure are we?
Similar glacial deposits, dropstones and magnetic evidence of ice near the equator in both periods, though the Huronian record is older and patchier.
Debates and open questions
Whether the Huronian was a true global snowball or several regional ice ages; South Africa's Makganyene glaciation is the strongest candidate.
▸Snowball Earth features as a possible trigger in Why the Cambrian explosion happenedDebated
The connection
Some researchers argue the Snowball glaciations reset ocean chemistry: meltwater delivered nutrients, oxygen may have risen, and survivors found open niches. In this view the deep freeze helped set the stage for complex animals, though roughly 100 million years separate the last Snowball from the explosion.
When and how
Glaciations about 717–635 million years ago; championed by Paul Hoffman and colleagues, who studied Namibian rocks in the 1990s.
How sure are we?
The order of events fits, and post-glacial rocks show nutrient spikes, but a direct cause is hard to test.
Debates and open questions
The long gap before the Cambrian makes a direct trigger doubtful; the glaciations may have mattered more for the Ediacaran organisms.