snowball earth, hypothesized condition in which the earth was covered completely by ice during several periods in the Proterozoic. The hypothesis was originally developed to explain evidence of the contribution of glaciation to the formation of siltstone whose layered structure and paleomagnetic composition indicated that its sediments were most likely deposited at sea level in the tropics. In the most extreme form of the hypothesis, glaciation that reached within 30° of the equator (a subtropical latitude) resulted in the reflection of so much sunlight that the earth cooled sufficiently to allow ice sheets to reach the equator, where temperatures were similar to those of present-day Antarctica; the ice sheets completely covered the earth for tens of millions of years. Many proponents of the theory support a slushball earth hypothesis in which glaciation reaches into the tropics but is not complete. The snowball earth hypothesis is controversial. Many critics have questioned if a snowball earth condition would be reversible, and if early life could have survived such extreme conditions. The long-term accumulation of very high levels of carbon dioxide in the atmosphere from volcanic eruptions is generally proposed as the cause of the ice sheets' retreat, and it has been suggested that specialized environments, such as hydrothermal vents, could have enabled life to survive. Despite these disagreements, there is evidence for glaciation several times during the Proterozoic; the Marinoan (650 to 635 million years ago) and Sturtian (750 to 700 million years ago) glaciations are generally considered to be the times when snowball or slushball earth conditions most likely existed.
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