[an error occurred while processing this directive]
BBC News
watch One-Minute World News
Last Updated: Tuesday, 24 October 2006, 15:27 GMT 16:27 UK
Amazon river 'switched direction'
Infographic, BBC
The world's largest river, the Amazon, once flowed from the Atlantic Ocean to the Pacific - the opposite of its present direction, a study shows.

Sedimentary rocks in the central part of South America contain ancient mineral grains that must have come from the eastern part of the continent.

Geologist Russell Mapes says this must mean that about 145-65 million years ago, the Amazon flowed east to west.

Mr Mapes will present his findings at a geology meeting in Philadelphia.

The age of rocks on the South American continent differs between east and west.

Rocks as old as 2.5 billion years are found on the eastern side of the continent. Because of continual geological activity in the Andes, on the western side, rocks there are much younger.

Changing landscape

If the Amazon had continuously flowed eastward, as it does now, much younger mineral grains would be found in the sediments, because they would have been washed down from the Andes.

"We didn't see any. All along the basin, the ages of the mineral grains all pointed to very specific locations in central and eastern South America," said Mr Mapes, a graduate student from the University of North Carolina (UNC), US.

He explained that these sediments of eastern origin were washed down from a highland area that formed in the Cretaceous Period, when the South American and African tectonic plates broke away from each other.

That might have tilted the river's flow westward, sending sediment as old as two billion years toward the centre of the continent.

Current course

Afterwards, a relatively low ridge, called the Purus Arch, rose in the middle of the continent, running north and south. This divided the Amazon's flow, so that one half flowed eastward toward the Atlantic and the other westward toward the Andes.

Landsat image of the Amazon river  Image: Nasa
Over millions of years, the Amazon has reversed its flow
In the late Cretaceous, mineral grains younger than 500 million years old began to fill in the basin between the Andes mountains - in the west - and the arch running down the centre of the continent.

After millions of years of build up, the Amazon river finally broke through these sediments and flowed past the Purus arch and into the eastern side of South America. This established the river's current course.

The new data comes from zircons, a type of mineral grain that can be dated in order to determine the age of the sediment.

Previous research has identified a reverse flow, but only in segments of the river. Mr Mapes and his colleague Drew Coleman from UNC traversed about 80% of the Amazon basin, collecting samples of zircon.

Their data supported the previous findings, and illustrate the continent-wide shift of the river's flow over millions of years.

The results will be presented at the Geological Society of America's annual meeting in Philadelphia, which runs from 22-25 October.




SEE ALSO
Amazon forest 'could vanish fast'
25 Jun 01 |  Science/Nature

RELATED INTERNET LINKS
The BBC is not responsible for the content of external internet sites



FEATURES, VIEWS, ANALYSIS
Has China's housing bubble burst?
How the world's oldest clove tree defied an empire
Why Royal Ballet principal Sergei Polunin quit

PRODUCTS & SERVICES

Americas Africa Europe Middle East South Asia Asia Pacific