Earth may once have frozen over, all the way to the equator.
About 700 million years ago, glaciers seem to have reached sea level in the tropics. Many geologists think the whole planet froze into a 'snowball' for millions of years, and that volcanoes eventually thawed it. Others argue it was more of a slushball. The argument is still running.
The Snowball Earth hypothesis proposes that during certain ancient ice ages, Earth's surface froze almost entirely, leaving little or no open water. It is most closely tied to the Cryogenian period and its two great glaciations, the Sturtian, roughly 717 to 660 million years ago, and the Marinoan, about 650 to 635 million years ago.
The clue that started it all was rock laid down by glaciers in places that appear to have been near the equator at the time. When sediments form, magnetic minerals in them line up with Earth's magnetic field, recording the latitude where they were deposited. In 1964 W. Brian Harland used such data to argue that glacial deposits in Svalbard and Greenland had formed in the tropics. In the 1960s the Soviet climatologist Mikhail Budyko showed why that matters: ice reflects sunlight, so once ice sheets spread far enough, cooling feeds on itself until the planet is covered. Budyko concluded it had never happened, because his model offered no way back out.
Joseph Kirschvink, who coined the term 'snowball Earth' in 1992, proposed the escape route. Volcanoes keep pumping out carbon dioxide regardless of the ice. Over millions of years the gas builds up into an extreme greenhouse that finally melts the ice, possibly within as little as a millennium once it starts. Paul Hoffman and colleagues boosted the idea in 1998 with 'cap carbonates', odd layers of limestone lying directly on top of glacial deposits, which fit rapid chemical weathering in a hot, acidic aftermath.
Critics question whether the glacial deposits really formed in the tropics at the same time, whether ice-covered oceans are physically plausible, and whether the cap carbonates mean what supporters say. A popular middle ground is a 'slushball' with a band of open water near the equator. Intriguingly, these frozen episodes came shortly before complex multicellular life flourished, and some researchers suggest the last one may have helped trigger it.
Source: Wikipedia — Snowball Earth · Text summarised from Wikipedia (CC BY-SA 4.0)
