About 700 million years ago, ice may have covered the entire planet. Glaciers reached the sea near the equator, the oceans froze into ice hundreds of metres thick, and seen from space the Earth would have been a white and grey ball. It happened at least twice, in two great glaciations of a period that geologists have named the Cryogenian, from the Greek for “cold” and “birth”. How complete the freeze was is still argued over, but the evidence that ice reached the tropics is hard to dismiss.
Ice at the equator
A key clue came in 1964, when the geologist Harland showed from magnetic data that rocks deposited by glaciers in Svalbard and Greenland had formed in the tropics. Glacial deposits of this age are now known from 22 ancient continents and microcontinents. Many of them are capped directly by layers of carbonate rock, known as cap carbonates, along with banded iron formations and sharp swings in the chemistry of carbon. The name “snowball Earth” was coined by Kirschvink in 1992, and in 1998 a team led by Hoffman set out the case in detail from rocks in Namibia.
There were two such glaciations. The Sturtian began about 717 million years ago and lasted 57 to 59 million years. After a warmer interval of perhaps 9 to 19 million years came the Marinoan, which ended about 635 million years ago. In 2025 a study that mapped rocks in Namibia with drones and dated layers of volcanic ash concluded it lasted only about 4 million years.
How to freeze a planet
Ice reflects sunlight. Once it spreads far enough towards the equator, the planet reflects so much of the Sun’s heat that it cools further, and the ice spreads further still. A 2024 study traced what pushed the Earth over that edge the first time: the amount of carbon dioxide coming out of mid-ocean ridges had fallen to an all-time low, while a huge field of fresh volcanic rock in Canada was being weathered by rain, a process that pulls carbon dioxide out of the air. Its level in the air dropped below about 200 parts per million, and the volcanic supply stayed low for the whole glaciation.
A world of ice
In the hard-snowball picture, the oceans were covered by a sea glacier several hundred metres thick that flowed slowly towards the equator. The equator was as cold as Antarctica is today. In the tropics the ice did not melt so much as evaporate straight into the dry air, leaving behind dust that darkened its surface. Volcanoes kept erupting through it.
Not everyone accepts a frozen ocean. In the slushball or waterbelt models, a band of open water survived near the equator. A 2026 study of about 2 600 annual layers in rocks from islands off Scotland found that they recorded seasons, solar cycles and El Niño-like swings during the Sturtian. In models most such cycles die out under complete ice but can resume if about 15 per cent of the ocean is free of it, which the authors see as a short-lived exception within a frozen world. Another 2026 study suggested that the long Sturtian may have been a series of freezes and thaws.
How it ended
Under the ice, rain and rock weathering no longer removed carbon dioxide, but volcanoes kept adding it. It built up, perhaps to between 1 and 10 per cent of the air, until the planet was warm enough for the ice to give way. Once the tropics opened, models suggest the ice could have retreated in only about 2 000 years, leaving a layer of meltwater on the sea that may have reached 40 to 60 °C at the surface.
Life survived. Fossil stromatolites, layered structures built by microbes, suggest that it carried on in shallow water at least, and the proposed refuges include meltwater ponds, holes melted by dark dust and hot springs on the seafloor.
