New study examines rare sedimentary rocks atop some of the planet's oldest large land masses.
The rocks in this image are part of the Singhbhum Craton in India, which emerged from the ocean more than 3 billion years ago. — Earth's first continents may have emerged from the oceans roughly 750 million years earlier than previously thought, rising from the seas in a manner completely unlike modern continents. These early masses of solid rock may have floated buoyantly atop magma welling up from below, a new study finds.
When and how Earth's earliest continents, the cratons, first rose above the oceans remains uncertain. Previous research suggested this emergence began roughly 2.5 billion years ago and was driven by plate tectonics -- the drifting, crashing and diving of the giant plates of rock that now make up Earth's surface.
In contrast, the new study focused on exceptionally rare sedimentary rocks deposited in rivers and beaches on top of the earliest cratons. The ages of microscopic zircon grains in these ancient stones "provide perhaps the most direct and unambiguous constraint on the timing of early continental emergence," Chowdhury noted.
The chemical breakdown of continental rock traps the global warming gas carbon dioxide. "The emergence and weathering of the earliest landmasses on the Earth between 3.3 billion and 3 billion years ago would have sequestered carbon dioxide from the atmosphere and lead to global cooling," Chowdhury said. "Indeed, this hypothesis is supported by the first appearance of glacial deposits in the geological record around 3 billion years ago.
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