CHRIS BUTLER / SCIENCE PHOTO LIBRARY CHRIS BUTLER / SCIENCE PHOTO LIBRARY
Snowball Earth. Computer artwork of the Earth frozen in snow and ice some 590 million years ago. The continents are in different positions due to tectonic plate movements. One theory suggests that an unusual amount of equatorial land disturbed the carbon cycle. This land was free of ice, and so large amounts of carbonate rock could form as rain washed carbon dioxide out of the atmosphere. Carbon dioxide helps keep Earth warm by a natural greenhouse effect, and as levels fell Earth cooled and runaway glaciation formed kilometre-thick global sea ice that lasted millions of years. As the continents moved away from the equator, and as volcanoes restored the carbon dioxide, a warm climate was restored. Most life was killed, but the survivors exploded in evolutionary diversity.
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