Geologist John Templeton recently spent a year on Norway’s west coast trying to understand how rocks now at the surface made an epic journey deep into Earth’s interior and back during the growth and subsequent collapse of the ancient Caledonian mountains.
The rocks are thought to have started out as volcanic basalt near the surface. Starting about 430 million years ago, earlier versions of Scandinavia and North America collided with each other, thrusting up the Caledonides. In the process, the basalts, caught more or less in the middle, were thrust deep into the mantle. At depths of 60 miles or more, the basalt was cooked into a dazzling rock studded with colorful minerals, called eclogite. Then, in a dramatic reversal called ‘orogenic collapse,’ the forces of collision switched course, and tectonic forces began to tear the continents apart, opening gaps in the original mountain chain. In the geological blink of an eye–maybe 20 million years or so–the eclogite was heaved back to the surface, where it now rests alongside sandstones formed in an ancient river valley as part of the remaining mountain chain.
The collapse of the Caledonides is thought to have been accomplished by a process called extensional faulting, in which the earth’s crust stretches out, and rocks deep below the mountain chain are brought to the surface on huge tectonic faults. This stretching of the now-ancient mountains appears similar to the processes now shaping the Basin and Range Province from Utah to California — repeated rows of steep mountains separated by deep valleys, formed as tectonic forces slowly tear apart the western United States. But extensional faulting remains poorly understood, despite advances in other areas of plate tectonics theory.
Templeton, who is working on his PhD. at Lamont-Doherty Earth Observatory , spent much of his time hiking, and chiseling away rock samples in a mostly roadless region where sheep vastly outnumber people. Now, back in the lab, he is working to analyze his samples for what they can tell us about this unusual form of mountain building and destruction.