snowball earth

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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 ventshydrothermal vent,
crack along a rift or ridge in the deep ocean floor that spews out water heated to high temperatures by the magma under the earth's crust. Some vents are in areas of seafloor spreading, and in some locations water temperatures above 350°C; (660°F;) have
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, 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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In particular, this mechanism may have contributed to Earths entry into a long period of global glaciation around 700 million years ago, a theory known as the Snowball Earth hypothesis.
For a very cold Earth, an extra cooling perturbation can trigger a runaway regime and Earth can eventually be totally covered by snow: this is the so-called snowball Earth hypothesis (Stern et al.
Beyth, 2006: Evidence for the snowball Earth hypothesis in the Arabian-Nubian Shield and the East African Orogen.
The snowball earth hypothesis has suggested the land and oceans of our planet were thrown into a deep freeze, the like of which has never been seen before or since.
The Snowball Earth hypothesis is the subject of considerable media interest on a par with global warming.
and Schrag, D.P., 2002, The snowball Earth hypothesis: testing the limits of global change: Terra Nova, v.
The snowball Earth hypothesis germinated in 1964 in the mind of W.
Some scientists have taken to referring to the snowball Earth hypothesis as the slushball Earth hypothesis.
Narbonne, a paleontologist at Queen's University in Kingston, Ontario, says the Snowball Earth hypothesis seems to explain the available evidence.
Hoffman has already started to test the Snowball Earth hypothesis by looking for geological evidence that the oceans were ice-covered--and cut off from the atmosphere--for millions of years.
The controversial Snowball Earth hypothesis posits that the Earth was covered from pole to pole by a thick sheet of ice lasting, on several occasions, for millions of years.
This is contrary to the Snowball Earth hypothesis, which envisages a fully frozen Earth that was locked in ice for many millions of years as a result of a runaway chain reaction that caused the planet to cool.