Raising the Andes: The View beneath South America Shows Some Surprises

Article excerpt

Seen from space, the curved hook of the Andes Mountains looks a bit like a walking cane ready to head westward on a jaunt across the Pacific Ocean. If you could peer outward from the center of Earth, however, a different pattern would emerge: The same arcing shape would turn into a continent-size question mark.

The question that needs answering is why the Andes exist at all, contend geo-physicists Raymond M. Russo and Paul G. Silver, who claim that the classical theory of plate tectonics offers no explanation.

World-class mountain ranges are formed, in theory, only when two continents run into each other, as India and Asia are doing today. That continental shoving match has over the last 40 million years compressed the landscape as though it were an accordion, raising up the Himalayas and the Tibetan plateau. Similarly, the Andes show massive compression in places and possess a high plateau like Tibet's.

But whereas India is plowing into Asia, South America has no continental opponent to push against. So what forces have erected the Andes, the highest peaks in the Western Hemisphere, ask Russo and Silver, former colleagues at the Carnegie Institution of Washington, D.C. (Russo has since moved to the University of Montpellier II in France.)

Instead of focusing their attention on the tectonic plates covering Earth's surface, the two scientists used seismic waves to look deeper into the planet, probing the hidden reaches of the mantle. Their findings led to a far-reaching theory that seeks to explain not only the Andes, but also the Rocky Mountains and a host of other peculiar features on Earth.

"If it's true for South America, then it's probably true for North America. And if it's true for North America, then it's probably true for the rest of the Atlantic as well," Silver told a meeting of the American Geophysical Union (AGU) in Baltimore in May.

He and Russo did not start out with such grand ambitions. Their theory arose only after they noticed something unusual about the earthquake waves passing beneath South America.

The two researchers were trying to track the movement of mantle rock a few hundred kilometers under the western edge of the continent. Although the mantle is solid, the high pressure and temperatures deep inside Earth cause rock to flow slowly, like a soft plastic. Seismologists can discern the direction of mantle flow by looking for anisotropy - differences in the speed of earthquake waves depending on their direction and polarization. This technique works because rock crystals tend to align themselves in the direction of flow, changing the speed at which they transmit seismic waves.

Seismic wave studies thus gave the geophysicists a way to see through solid rock to the subterranean streams in the mantle underlying South America. Before starting their study, Russo and Silver had expected that mantle currents would mirror the movement of the tectonic plates on Earth's surface. One of these, a patch of the Pacific seafloor called the Nazca plate, slides eastward underneath South America at about 6 centimeters per year, in a process geophysicists call subduction. Meanwhile, South America moves westward at about half that speed.

According to theory, mantle currents should flow with the eastward-moving Nazca plate. But the seismic data collected by Russo and Silver pointed to a north-south movement instead. The region of the mantle beneath the Nazca plate apparently flows parallel to the west coast of South America, at right angles to the anticipated orientation.

To explain the discrepancy, Silver invokes what he calls the "bad-boat analogy." Think of a square-fronted boat trying to push its way through the sea. The awkward ship can advance only by forcing water to flow around its wide bow.

This is how Russo and Silver picture the undersides of South America. As the continent moves westward, it bulldozes through the mantle, forcing rock to flow parallel to the coastline until it can swing around the continent's northern and southern tips. …