杏吧原创

NOT all mountains are created equal 鈥� especially when it comes to size. The Himalayas take the gold for their impressive stature, towering more than 5 kilometres on average above sea level. The Andes get a respectable silver for their stunning 4 kilometre average loft. By comparison the Alps are pitifully small, the Atlas Mountains mere pipsqueaks and the Rockies hardly worth a mention. They, like all the world鈥檚 other big mountain ranges, top out at an average of just 2 or 2.5 kilometres.

Why do mountains get so big, and no bigger? And what makes two ranges so much bigger today than all the rest? Surprisingly, even geologists don鈥檛 have all the answers. In particular, they have struggled to explain why the Andes are so unexpectedly high. But now Simon Lamb from the University of Oxford believes he has an explanation. The Andes, he thinks, owe their massive size to the ocean.

There鈥檚 more to mountain building than meets the eye. A few isolated mountains are volcanic blips, or hot spots, where hot magma from inside the Earth bursts up like an unpopped bubble. Some lofty peaks and plateaux, such as those in the American West, exist because they are buoyed up by hot rock in the mantle beneath. Other mountains are lumps of unusually thick, light crust that float on the rock beneath like icebergs in water, their tips bobbing up into the sky. And some stand tall simply because of the type of rock they鈥檙e built on; hard, solid foundations can support more weight than softer more fluid ones. If it鈥檚 a truly majestic mountain range you鈥檙e after, you can鈥檛 beat crashing a couple of tectonic plates together. The faster the collision and the stronger the rock, the bigger the peaks produced.

But there are also plenty of natural forces working to stunt any mountain鈥檚 growth. In areas where the rock is soft or radioactive, a pointed peak can melt into a spreading puddle. And the effects of wind and rain can be dramatic. Since the Andes were born 50 million years ago, the elements have shorn off an astonishing 15 kilometres of rock from the peaks, according to Lamb. The other big leveller is gravity. The bigger the mountain, the more gravity tries to pull it down.

At almost 9 kilometres, Everest is about as high as any mountain on Earth can get before the rock weighs too much to be supported by the ground below. Like the rest of the Himalayas, it was pushed up that high by a colossal upward thrust 鈥� the Indian plate slamming head-on into Asia. The collision is happening at a blistering 5 centimetres per year, and all the rock it crumples up goes into building the mountains. Also, the Indian plate is made of incredibly strong rock, so it can hold up the mountains like Atlas with a globe on his shoulders.

Unlike the Himalayas, the Andes are formed by an oceanic plate slipping down under a continental plate. Normally, such subductions don鈥檛 form great mountains. But the Andes鈥� highest peak, Aconcagua, reaches nearly 7 kilometres. 鈥淚t鈥檚 an age-old problem,鈥� says Rick Allmendinger with the Andes research group at Cornell University in New York. Oceanic plates are so dense in comparison to continental ones that they usually slip down without much fuss 鈥� aside from the odd earthquake. This shouldn鈥檛 generate enough force to push up a range the size of the Andes.

Geologists have used computer models to work out how high the Andes should be 鈥� balancing the force building the range which comes from the movement of the plate, against the force it takes to squish rocks up into hills and hold them there against gravity. Some models, like those of Seth Stein from Northwestern University in Illinois and Mian Liu from the University of Missouri-Columbia, do correctly predict their height. But others come up very short. Calculations made by Paul Davis from the University of California, Los Angeles give an answer of just 2 kilometres, half the height required. So Davis needs to invoke some other force to keep those mountains propped up.

鈥淭he key to keeping mountains up high is focusing the forces that create them,鈥� says Lamb. In the Himalayas, that鈥檚 done simply by geography 鈥� since much of the Indian plate is under water, all the mountain-building forces concentrate on the narrow bit where continent grinds on continent 鈥� a strip just 2000 kilometres long. In the Andes, Lamb thinks that focusing comes from a trick of the oceans. The cold Peruvian Current sweeps up from the Antarctic past the west coast of Peru and Chile. There鈥檚 little evaporation from these frigid waters, which helps explain why the Atacama Desert, just in front of the Andes, is the driest place on Earth. No rain means no rivers, and no bits of soil and clay washed into the Pacific, reasons Lamb. Oceanographers have long known that there is very little sediment inside the trench where the oceanic plate slips down into the Earth just between 15掳 and 32掳 south. And that happens to match up perfectly with the part of the Andes that, on average, is particularly tall.

Lamb鈥檚 idea is that in most subduction zones bits of detritus produce a slushy sediment that acts like an injection of oil, helping the plates to slide past one another. With only bare rock in the trench off the coast of the Andes, there鈥檚 a lot more friction. And that could focus the tectonic forces into a narrow band, letting the mountains reach greater heights.

Lamb and Davis are now grinding out the maths to see if they can predict the right height for the Andes based on this focusing. But even if they can, the idea will be very difficult to prove. Lamb hopes to get his hands on information from the Ocean Drilling Program, which has cores showing how sediments have changed in the area over time. He鈥檒l then look for a correlation between the amount of silt and the rate of growth of the Andes over the past 50 million years.

Meanwhile, most geologists find Lamb鈥檚 speculations intriguing. Even Stein, whose models match the height of the Andes, has come up with a similar idea. This April, he and David Hindle from Michigan State University suggested that the growth of the Andes might have accelerated in recent years as the amount of sediment in the trench decreases (Geophysical Research Letters, DOI: 10.1029/2001GL013757. Stein doesn鈥檛 invoke ocean currents, however. Instead, he argues that the higher the Andes become, the greater the barrier for any water-laden air sweeping in from the east. That makes the Atacama Desert even drier, so less sediment reaches the sea, creating a feedback loop where the ocean bed gets more and more barren and the hills ever higher.

Allmendinger is more cautious. He agrees that friction caused by lack of sediments probably plays a part in propping up the Andes. But how big a part 鈥� is Lamb making a mountain out of a molehill? 鈥淚t鈥檚 so hard to say,鈥� he comments. 鈥淚f there was some other trench and the only difference between it and Peru was that it had more sediment, then you could tell. At the moment we have one experiment, and no control.鈥�