We tend to think of mountains as features that grow very, very slowly. Think again, says one group of scientists. They claim the Andes jumped by as much as 2.5 kilometres in just 4 million years – a geological blink of an eye.

Carmala Garzione of the University of Rochester in New York State, US, and colleagues say the sudden rise was caused by a huge layer of dense rock dropping off the underside of the crust that forms the mountain chain.

Previously, the rock had been dragging the relatively light crust down. Liberated of its ball and chain, the mountain chain underwent a growth spurt.

The theory is controversial, but Garzione reckons she can back it up. She and her colleagues have looked at the chemistry of the sedimentary rocks in the Andes and say it reveals what the mineral levels were when the mountains formed.


Rain proof

Sediment formation depends on reactions that are modulated by temperature and humidity – two factors that vary with altitude. In particular, heavy isotope molecules tend to precipitate out of the atmosphere at low altitudes.

This means that as moisture moves up the sides of mountains, it sheds the heavy isotopes it contains in rain and snow – the higher the altitude, the greater the proportion of light isotopes.

Garzione and her colleagues looked at isotope ratios of certain atoms inside Andean rocks to see what altitude they were at when the rocks were formed.

The data showed that the Andes rose slowly for tens of millions of years, but then, between 10 and 6 million years ago, suddenly shot up by between 1.5 and 2.5 kilometres.

To the researchers, this is proof for a controversial geological theory called “delamination”.

Floating cork

Mountains are believed to form slowly when two tectonic plates push against each other. Along the western coast of Latin America, the ocean shelf pushes against the less dense continental shelf, and slowly gets squeezed under it. As this happens, the continental crust buckles and gets thicker.

Traditional tectonics suggests that, as the continental crust thickens, the denser rock at the base is slowly eroded away by heat into the semi-molten mantle below.

Delamination theory, however, holds that this denser rock can detach from the underside of the crust like thick blobs of molasses, allowing the lighter crust above to pop up like a cork. If this is true, then mountains could indeed be born in sudden bursts.

Garzione and colleagues believe their isotopic data proves mountains are formed by delamination. They say the uplift seen in the Andes would have taken tens of millions of years to arise from the gradual melting of the lower crust – the rapid removal a large section of this crust best explains such a growth spurt.

Others, though, would like to see more work done before declaring themselves convinced. Todd Ehlers of the University of Michigan in the US points out that as the Andes rose, they will have changed the local climate and rain cycle. This could affect the way the isotopic data should be interpreted.

Journal reference: Science (vol 320, p 1304)