Volcanoes triggered the deep freeze that has covered polar regions with ice for millions of years, say US geologists. For years, researchers have debated the causes of the latest series of ice ages, which began roughly 2.6 million years ago. Among the prime suspects were periodic changes in the tilt of the Earth's rotation axis and its orbit. Another contender was tectonic changes. The raising of the Himalayan Plateau was among several such changes that could have triggered off changes in air and water circulation, redistributing heat around the Earth. However, David Rea and Libby Prueher of the University of Michigan, usa , say that while these factors readied the planet for eventual glaciation, volcanic eruptions pushed the climate "over the edge". Ocean cores from the North Pacific show glaciation started within 3,000 years, much faster than possible if the trigger was orbital or tectonic. Also, there was ash in the cores. "All around the North Pacific, volcanoes started firing off," says Rea ( Geology , Vol 26, p 1027).
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