Andes Rose Rather Rapidly: Analyses of Sediments Suggest New Scenario Explaining South American Mountains' Height

Article excerpt

South America's Andes reached their staggering heights after a sudden growth spurt millions of years ago, new evidence suggests.

The central part of the Andes, one of the world's longest and tallest mountain chains, is home to some of the Earth's thickest crust: In spots, the crust extends to depths of 70 kilometers (SN: 1/15/05, p. 45). Previous studies have suggested that the slow, steady collision between the Nazca Plate, made of dense oceanic crust, and the South American plate, of lighter, continental crust, gradually lifted the Andes. But new analyses of South American sediments cast doubt on that steady-growth scenario, says John Eiler, a geochemist at Caltech in Pasadena.

Eiler and his colleagues looked at the mix of rare chemical isotopes in sediments found in the Altiplano, a high-altitude region of Bolivia and Peru that lies between parallel chains of Andean peaks. When sediments are deposited at low temperatures, atoms of some rare isotopes are more likely to end up near each other in the resulting crystal structure, Eiler explains. The higher the environmental temperature, the more random the distribution of those atoms. Using such mineralogical analyses, plus measurements of oxygen isotope ratios affected both by temperature and elevation, the researchers could estimate the elevation at which the sediments were deposited. …