Geologists had estimated that the mountain system, which extends the length of western South America, rose gradually over the past 40 million years.

Mountains that grow this fast might affect climate and evolution, said Carmala Garzione, an associate professor of geology at the University of Rochester in New York.

The 4,400-mile-(7,000-km)long range is tall and jagged in many parts, with its highest peak reaching 22,800 feet (6,900 metres) and an average height of 13,000 feet (4,000 metres).

It was formed through plate tectonics, specifically the subduction of a large sheet of crust called the Nazca plate beneath the South American plate. This process, still ongoing, causes earthquakes and volcanic eruptions.

Writing in the journal Science, Garzione and colleagues said they had studied sediments to determine that the Andes rose slowly for tens of millions of years, then suddenly lifted much more quickly between 10 million and 6 million years ago.

This suggests a process called delamination had formed the Andes, Garzione said.

When oceanic and continental plates come together, general theory holds that the continental crust buckles, forming a mountain range.

Beneath the crust, the buckling creates a heavy "root" that anchors the crust, Garzione said.

Conventional theory says that movement of the fluid mantle deep in the Earth slowly erodes this heavy root, allowing mountains to rise gradually as the crust shortens and thickens.

But Garzione proposes an alternative theory -- that delamination causes the root to heat up and ooze downward like a drop of thick syrup, abruptly breaking free and sinking into the hot mantle. The mountains above, suddenly free of the root, then spring up.

Her team is now looking to see evidence of what effects the sudden appearance of such a tall mountain range would have had on climate and evolution. (Reporting by Maggie Fox; Editing by Will Dunham and Xavier Briand)