"Snowball Earth" Theory Confirmed


Maroon glacial-deposit layers helped prove this Canadian rock was ice covered and at the Equator during the "snowball Earth" period.

Earth's now steamy Equator was covered with ice 716 million years ago, according to a new study. The discovery hinged on proving that the right rocks had been covered by glaciers in the right place at the right time. Study leader Francis Macdonald, an Earth scientist at Harvard University, and colleagues worked with volcanic rocks in Canada that were found sandwiched between glacial deposits. Such deposits are recognizable by the presence of debris left behind by melting glaciers and sediments deformed by glacial movement.

Using extremely precise uranium-lead mass spectrometry, the researchers determined that both the volcanic rocks and glacial sediments were deposited about 716.5 million years ago—during the purported snowball-Earth period. The team then matched their findings to previous magnetic studies that had found these rocks had formed when Canada was situated near the Equator. Over time the movement of Earth's tectonic plates had pushed the rocks north to Canada's Yukon and Northwest Territories.

There's still plenty of mystery surrounding snowball Earth—aka the Sturtian glaciation—Macdonald said. For example, an icy Equator alone can't tell scientists the extent of ice cover around the world. The continents may have been in a total deep freeze, or the planet may have simply been subjected to a patchwork of constantly moving glaciers or icebergs—or something in between.

And even the "snowball Earth" name might need rethinking.

Earth probably wasn't "just a white ball, but more of a mud ball," Macdonald said. Regular eruptions of ash-spewing volcanoes likely made the continents "dusty messes."

http://news.nationalgeographic.com/news ... al-warming

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