South America’s Andes reached their staggering heights after a sudden growth spurt millions of years ago, not gradually as many studies have proposed, 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 two tectonic plates — the Nazca Plate, made of dense oceanic crust, and the South American plate of lighter, continental crust — crumpled the crust and gradually lifted the Andes. But new analyses of South American sediments cast doubt on that steady-growth scenario, says John Eiler, a geochemist at the California Institute of Technology in Pasadena.
Eiler and his colleagues looked at the mixture and distribution of rare chemical isotopes in sediments deposited in geological basins found in the Altiplano, a high-altitude region of Bolivia and Peru that lies between parallel chains of peaks in the central Andes. When sediments are deposited under low-temperature conditions, atoms of some rare isotopes are, on average, more likely to end up near each other in the resulting crystal structure, Eiler explains. The higher the temperature of the environment is, the more random the distribution of those atoms becomes. Using the results of such mineralogical analyses, plus measurements of the ratio of oxygen isotopes found in the rocks – which can be affected by both temperature and elevation — the researchers could estimate the elevation at which the sediments were deposited.
