Oct. 25, 2006 — Even the most reliable geographic facts — like the direction of the world's largest river — can change dramatically over geologic time.
Millions of years ago, a new study suggests, the Amazon River flowed from east to west — a 180-degree difference from its current eastward path to the Atlantic Ocean.
The evidence for this continent-wide reversal comes from tiny, dark, virtually indestructible minerals called zircons that are perpetually recycled in the sandstones of the Amazon Basin.
"People use the zircons in ancient sands to reconstruct continents, to understand what might have been upstream," said geologist Russell Mapes, a doctoral candidate at the University of North Carolina in Chapel Hill.
Very often zircons are the only mineral remnants of long-gone mountain ranges.
Geologists know this thanks to new technology that measures the age of the minerals by the minute quantities of uranium and lead isotopes within them. The isotopes reveal how long it has been since the zircon solidified from molten rock.
In the case of the Amazon zircons, Mapes and his advisor Drew Coleman, along with their Brazilian colleagues Afonso Nogueira and Angela Maria Leguizamon Vega, discovered that the minerals don't come from the relatively young Andes at all.
Instead, they are from 1.3 to 2.1 billion years ago. That strongly suggests the zircons must have eroded out of some mountain range that bordered eastern South America back when it was still attached to Africa — before there was an Atlantic Ocean at all.
Yet the zircons were found to the west of the region where they were born, so the river must have once flowed in that direction, the researchers concluded.
"The rocks we saw on the river suggested this," said Mapes. "But when I got the actual data back, I was happy."
