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About 720 to 635 million years agoPeer-reviewed · 2 sourcesDebated

Snowball Earth: the planet freezes over

On the timeline · around About 720 to 635 million years ago · Toward the Rise of AnimalsToward the Rise of AnimalsSnowball Earth: the planet freezes over800 Ma775 Ma750 Ma725 Ma700 Ma675 Ma650 Ma625 Ma600 Ma575 Ma

What happened

Twice during the Cryogenian period, roughly 720 to 710 million years ago and again around 640 million years ago, geological evidence shows glaciers reaching all the way to the equator, each episode lasting on the order of 10 million years. NASA's Goddard Institute for Space Studies points to glacial deposits found at tropical latitudes as the core evidence: if ice reached the tropics on land, the logic runs, it likely wrapped the whole planet, a hypothesis known as Snowball Earth. The most recent of these glaciations ended around 635 million years ago, just as complex life was beginning to take shape. How it ended is genuinely unresolved. The standard explanation holds that volcanic carbon dioxide slowly built up beneath the ice until it thawed the planet, but NASA's own researchers note there is no geologic evidence for atmospheric carbon dioxide ever reaching the level that explanation requires, which has pushed some scientists toward a milder 'Slushball Earth' with open water at the equator instead of a solid ice shell.

Why it matters

A planet-wide freeze followed almost immediately by the Ediacaran biota's first large, complex organisms is too close a sequence to ignore. Many researchers treat the Cryogenian glaciations as an evolutionary bottleneck and reset, one that emptied ecological niches and may have helped trigger the burst of animal complexity that followed.

How we know

The glaciations are dated from glacial sediments, tillites and dropstone-bearing marine rock, sandwiched between layers laid down at low latitudes, plus the distinctive 'cap carbonates' that sit directly on top of the glacial deposits worldwide. The unresolved part, exactly how the ice age ended, is honestly flagged as an open problem by NASA's own account rather than papered over with false certainty.

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Part of a timelineEarly Life on Earth9 events · How a single microbial ancestor became a planet run by oxygen, complex cells, and eventually animals, from LUCA to the doorstep of the Cambrian explosion.View all →
Snowball Earth: the planet freezes over · Early Life on Earth · SourcedStory