For UCSB Professor Bradley Hacker, geology isn’t a science. It’s a lifestyle.
“I’ve been on top of six thousand-meter-high mountains in Central Tibet,” Hacker said. “They don’t really have a particular name. Nobody would recognize them. Just features out in the middle of the Plateau. That’s really a kind of special and unusual thing, I think.”
Hacker’s expeditions have taken him to Alaska, Kazakhstan, Norway, eastern China, Mongolia, Siberia, Tibet, Borneo, Nicaragua, Costa Rica, Chile, Australia and Oman.
Not bad for someone who spent years working behind a desk at Apple computers.
“What I’m mostly working on is understanding how the edges of continents, which get buried quite deep in the earth, get back to the surface,” he said.
The process by which the edge of a continent is buried is called subduction. When two continents collide, the edge of one is forced underneath the edge of the other. The finer points of this process became especially perplexing in 1984, when a discovery was made that changed many of scientists’ most basic assumptions about geology.
METEORS TO MOUNTAINTOPS
In 1984, geologists came across samples of a mineral called coesite in Norway and the western Alps. Coesite had been first created in a laboratory in 1953 and later found in asteroids, but the high-pressure conditions necessary to form the mineral were not thought to exist on earth. The find shocked many in the scientific community.
“When these rocks were first found in both Norway and the western Alps, they were found in one place, basically the size of my desk,” Hacker said. “Since that time we now recognize that these features are big and there are something like two dozen localities around the world now. None of them are in North America for some bizarre reason.”
Hacker has followed these and other high-pressure rocks to some of the most remote and inaccessible places on earth. The theory that he and other scientists are working on is a revised model of subduction.
Previously, scientists thought that when the earth’s crust was subducted only the heaviest bottom layer of rock, known as the oceanic lithosphere, was pulled under.
Geologists assumed the continents themselves were never completely subducted. If this were true, continents would never be subjected to the kind of pressure necessary to form coesite.
“That required something like 30 kilobars of pressure, which would be about 30,000 atmospheres and roughly equivalent to burial in the earth at about 120 kilometers depth,” Hacker said.
Somehow, continents were being subducted to over 120 kilometers below the earth and then rising to the surface again. Scientists were forced to revise their theories.
Geologists now think that continents are dragged along with the oceanic lithosphere when it is subducted. The continents are much less dense than the lithosphere and have an inclination to float back to the surface. At a depth of around 200 kilometers, the pressure becomes so great that continents actually tear themselves lose from the lithosphere and rise to the surface.
The whole process is something like taping a piece of cork to a slab of lead and dropping it into a pool. Eventually the cork will free itself from the slab and rise to the surface. Like the cork, continents also rise to the surface much faster than they sink. Rocks that form in the depths can actually make it to the surface before they melt in the pressure-induced heat below the earth.
Hacker’s research has also focused on other areas of continents’ birth and development, such as the formation of continents, which he studied in Alaska.
“The main thing I’m interested in is how fast do things move around on the earth and why,” Hacker said.
THE TIBETAN PLATEAU
Two years ago, Hacker was in northern Tibet. His research team traveled in a caravan of four land cruisers and two six-wheel-drive army trucks carrying fuel and supplies.
“It rained or snowed every day we were there, even in July,” he said. This made it slow going for the research team – even before one of the land cruisers got stuck in quicksand.
“We drove one of these six-wheel-drive trucks out there to try and recover the land cruiser, which we did manage to do. But in the process this one six-wheel-drive truck got stuck. So we were in northern Tibet with basically a couple weeks drive to the nearest anything,” Hacker said. “We did manage eventually to get across that river and into the place.”
In Tibet, Hacker studied how two continents collided and slid over one another to create the thickest region of the earth’s crust.
“The northern part of the Tibetan Plateau is essentially the part that hasn’t been settled in any way. It’s just nomadic herders and so forth. It’s a really unspoiled environment,” Hacker said. “It’s really high and the average elevation is 5,000 meters. You get this impression that the clouds are very close to you because of that. They actually are. You’re about halfway to the clouds.”
The 5-kilometer-high plateau contains the highest elevations on the globe and is sometimes called “the roof of the world.” Its high altitude is due to the extreme width of its crust, which is 75 kilometers thick. In most places on earth the crust is a mere 40 kilometers thick.
Geologists previously thought that such a thick region of the earth’s crust would make it extremely strong. Hacker and other scientists examined volcanic rocks in Tibet and proved that the Tibetan Plateau actually rests on crust that is extremely brittle. This is because the pressure generated by so many layers of stacked rock is causing radioactive decay in the crust, which heats itself up and becomes soupy and brittle as a result.
“Most people thought the lower crust of Tibet was something like 800 or 900 degrees centigrade,” said Hacker. “We showed it was actually as high as 1,200 degrees centigrade.”
Rain and snow have carved the western portion of the plateau into a collection of low valleys and towering mountains, including Everest and K2.
Still, these are the exception and not the rule. Most of the plateau is extremely level.
“It’s like a big hot turd or something like that, that’s spreading out under its own weight,” Hacker said. “Um, you’ll have to use a different analogy,” he added.
Hacker’s research has also proved that the Plateau is 14 million years old, six million years older than previously thought.
ON TOP OF THE WORLD
Hacker has plenty of stories to tell. About the time when he and his colleagues loaded an airplane in Mongolia with so many rocks that it started to tip over. Or about taking the train from China into Mongolia and stopping at a border crossing where porters blasted disco music over the PA system at 11 at night.
His graduate students have stories too.
“I really like Norway because it’s sort of gray and stormy at the top of the world.” said graduate student David Young, who worked in Norway with Hacker. “You can stand on the coastline and think, ‘next stop Greenland, 10,000 kilometers in that direction.'”
“Norway’s a beautiful country,” said grad student Emily Walsh, who also worked there. “And we’re out camping the whole time we’re there.”
Luc Mehl, another of Hacker’s grad students, worked with him in Alaska.
“We went out in a helicopter and got dropped off for 10 days and it was so awesome because we were in this big valley and by the end of each day, we’d be up on a far ridge and be able to look down on a huge glacier. It was great to be able to go out and do geology and then be able to turn around to this awesome view,” said Mehl.
Mehl and the other students were complementary of their advisor. “It’s just nice to be out there with somebody who’s really good at being out there,” said Mehl.
“And he bought us all computers,” said Walsh.
Old habits die hard.