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Sometime in our planet’s history—around 700 million years ago, the Earth's surface was covered with ice from pole to pole. This “Snowball Earth” theory—not surprisingly a controversial one—suggests that plate tectonics caused a group of continents to drift to the equator. There the hot and rainy climate enhanced the weathering of the continental crust and "pulled" carbon dioxide out of the atmosphere, leading to global cooling and the eventual freezing over of the planet. About ten million years later, the "snowball" melted as the action of plate tectonics led to volcanic eruptions which released carbon dioxide into the atmosphere. The greenhouse effect began building up again.
Supporters of the Snowball Earth theory point to evidence of glaciers at sea level at the equator. The sediment record shifts abruptly from evidence of glacial cold to tropical heat, and thick layers of Bahamas-like limestone directly overlay the glacial deposits. Most intriguingly, the very first fossils show up in the geological record less than five million years after the "snowball" melted off for the final time, suggesting that this massive climate perturbation might have somehow triggered the rise of multicellular organisms.
Callan Bentley, associate professor of geology at Northern Virginia Community College, discusses the Snowball Earth theory, a paradigm-shifting concept that addresses modern climate change and our own existence on Earth.
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