Some Norwegian rocks are telling a racy tale. They're the survivors of a mountain growth spurt a mere 13 million years long--outrageously speedy compared to the tens or hundreds of millions of years geologists thought mountains needed to grow. The new evidence suggests that fast growth of young ranges may not be unusual, and it could force geologists to reevaluate how long it took older mountain ranges such as the Appalachians to sprout.
When two plates of Earth's crust collide, the land buckles and folds. Rocks near the surface are forced down into the depths, where the high pressure squeezes, heats, and sometimes even melts the stones. The heat and pressure can cook the rocks so much that all of their gases bubble out. Since one technique for dating these rocks involves measuring the amount of argon gas they contain, cooked rocks are effectively "reborn" when a mountain forms. As such, the age of the rocks accurately reflects the age of the mountain.
So geochronologist James Lee of Queen's University in Kingston, Ontario, and colleagues were perplexed when a pair of billion-year-old rocks were reported in the 425-million-year old Caledonian mountain range in Norway. The only thing that could explain this discrepancy, the researchers argue, is if some of the original argon gas had remained in the rocks when the mountains formed. This would mean that the mountains must have formed very quickly--not giving the earth enough time to heat the stones. By the team's calculations, described in tomorrow's Nature, the mountains must have formed in 13 million years, at least three times faster than most ranges were assumed to grow.
"They've shown that the mountain building rates of old mountain ranges are the same as those of younger mountain ranges that are growing right now," says Kip Hodges, a geologist at Massachusetts Institute of Technology in Cambridge. Previously, some geologists thought young mountains such as the Himalayas, which seem to have grown in just 25 million years, were freakish instead of average.