The Andes’ Mountainous Paradox

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When asked if mountains grow slowly and steadily versus in rapid spurts, most people intuitively gravitate to the “slow and steady” model. Mountains, we are taught, take an incomprehensively long time to build up their scads of boulders, jagged peaks and high-altitude plateaus.

In fact, most known mountain building processes do require large amounts of time to complete their skyward climb. But for every rule there is an exception. Consider the Himalaya and Andes mountains—despite their relative geologic youth, these mountain belts rank among the world’s tallest peaks.  And therein lies the mountainous paradox: How do geologically young mountains grow extremely tall in extremely short time periods?

Conventional geology tells us that as the earth’s tectonic plates collide and dive beneath one another, and these actions cause the earth’s skin to crumple and fold. For a superficial visual effect, pinch together an inch or two of your forearm skin. Just as your skin crumples into peaks and valleys under pressure from your fingers, deforming tectonic pressures cause the earth’s crust to shorten and thicken into crenulations and folds, which alpinists yearn to climb and landscape photographers strive to capture on film. But below the surface, mountains have deep roots where dense material accumulates over time, often from the action of one tectonic plate diving beneath another whereby material is scraped off of one onto the other. It was previously thought that a gradual erosion of this root by the more plastic asthenosphere resulted in the gradual rise of the crust (see figure right).

But a new study tracking the uplift of a central portion of the massive Andes Mountains in South America shows that mountain building—what geologists term “orogeny”—may actually occur in much faster fits and spurts than previously realized due to the rapid loss of large amounts of material from the mountain’s root.

Carmala Garzione, lead author of the paper, with one of her graduate students, prepares to cross a river in the Andes.

Photo by Bruce MacFadden

While conventional theory would predict that the Andes Mountains rose gradually and in sync with the scrunching of the Nazca plate beneath the South American plate, which scientists know has caused dense material to accumulate millennia after millennia up to 70 kilometers below South America’s western coast, Florida Museum of Natural History paleontologist Bruce MacFadden said that this is not what happened after all. MacFadden is a co-author of the study published June 6 in the journal Science.

“Instead of the Altiplano rising little by little each year, we found two phases of spasmodic or punctuated uplift interspersed by millions of years of stability,” MacFadden said.

The authors assert that as the crustal layer, or lithosphere (which floats above the mantle) was squeezed under deforming pressures, earth processes caused large parts of the accreted dense material to plummet downwards into the more plastic upper mantle layer, also known as the athenosphere. This loosening of the root load caused the surface crust layer to rise, buoyed upward like a released cork, by the excision of massive extra weight below.

“Our findings will force geologists to acknowledge that removal of lower lithosphere material could be an important process that causes rapid surface uplift in different mountain belts worldwide and over geologic time,” said lead author Carmala Garzione, a geologist at the University of Rochester. “The subduction process may cause shortening and thickening of the mantle lithosphere and dense lower crust that accumulates at depth until that dense material is removed rapidly—either by downward dripping, which is a convective process, or by another process called delamination.”