when massive sheets of ice nearly engulfed the entire landmass.The positions of
the continents also had a tremendous influence on the initiation of ice ages.
Lands wandering into the colder latitudes enabled the buildup of glacial ice.
Global tectonics, featuring extensive volcanic activity and seafloor spreading,
might have triggered the ice ages by drawing down the level of oxygen in the
ocean and atmosphere. This would have preserved more organic carbon in the
sediments so that living organisms could not return it to the atmosphere.
Plate tectonics also began to operate more vigorously. Plate subduction
thrust carbonaceous sediments deep inside Earth along with the underlying
oceanic crust. The growing continents stored large quantities of carbon in
thick deposits of carbonaceous rocks such as limestone. The elimination of
carbon dioxide in this manner caused Earth to cool dramatically. Besides high
rates of organic carbon burial, iron deposition and intense hydrothermal activ-
ity associated with plate tectonics furthered global cooling. Although this was
the first ice age the world had ever known, it was not the worst.
The burial of carbon in Earth’s crust might have been the key to the
onset of another glacial period just before the appearance of recognizable ani-
mal life near the end of the Proterozoic about 680 million years ago, called the
Varanger ice age, named for the Varanger Fjord in Norway. Massive glaciersHistorical Geology
TABLE 5 Chronology of THE MAJOR ICE AGESTime (years) Event10,000–present Present interglacial
15,000–10,000 Melting of ice sheets
20,000–18,000 Last glacial maximum
100,000 Most recent glacial episode
1 million First major interglacial
3 million First glacial episode in Northern Hemisphere
4 million Ice covers Greenland and the Arctic Ocean
15 million Second major glacial episode in Antarctica
30 million First major glacial episode in Antarctica
65 million Climate deteriorates, poles become much colder
250–65 million Interval of warm and relatively uniform climate
250 million The great Permian ice age
700 million The great Precambrian ice age
2.4 billion First major ice age