Wednesday, June 29, 2011

Amazon flow reversed by continental tilt

Amazon River Once Flowed Other Way
National Geographic News: The entire Amazon Basin, the world's largest river drainage system, once flowed in the opposite direction... Geologists in the United States and Brazil say the discovery that the river previously flowed east-to-west was accidental. The team was studying how swiftly sediment travels in the Amazon Basin from its headwaters in the Andes Mountains of Peru to the Atlantic Ocean...

'All the current indicators in the ancient sediments' -- including ripple marks and telltale mineral traces -- 'showed that the current, the river flow, was from the east to the west,' said study author Drew Coleman, a geologist at the University of North Carolina at Chapel Hill...

The Amazon was flowing west-to-east, as it does now, when the shift occurred. That reversal is 'almost certainly tied' to the breakup of the South American and African continents, which began about 130 million years ago, Coleman said. 'When that happened, the east coast [of South America] was uplifted -- and the whole river flowed the other way.'..

When the Andes Mountains started growing at about the end of the Cretaceous period (around 65 million years ago), the geologic tide began to shift again in favor of the Amazon's current west-to-east course... 'It flip-flopped pretty quickly,' he said, adding that the study reveals these shifts can occur on a 'continental scale.'... The study's findings highlight that 'the surface of the Earth is very transient.' [said] study co-author Russell Mapes, a UNC graduate student.
Image source here.