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Geologists redefine the Earth's "forbidden zone"

To medieval scholars, Hell was like a fiery furnace a hot, forbidding place deep in the bowels of the Earth.

According to modern geologists, there really is an underground "forbidden zone" not the biblical version of Hell but a subterranean region whose very existence would seem to be prohibited by the laws of nature.

"It is an accepted tenet of geology that very low temperatures at high pressures constitute a 'forbidden zone' never realized in Earth," write Professor Juhn G. Liou and research associate Ru-Yuan Zhang of the Department of Geological and Environmental Sciences.

But now, say the authors, new geological evidence "casts doubt on this paradigm," with major implications for understanding the forces that shape our planet.

Liou, Zhang and geologist Bradley R. Hacker of the University of California-Santa Barbara describe their findings in a recent issue of the journal Science.

The authors point to a fundamental law of geophysics: The farther you go underground, the hotter it gets and the more intense the pressure.

While it may be a pleasant 75 F three miles below the surface, at extreme depths you would encounter hellish temperatures and pressures so intense that ordinary carbon is transformed into precious diamonds.

Geologists calculate that, for every mile you dig beneath the Earth's surface, the temperature rises 15 F and the pressure increases simultaneously at a rate of about 7,300 pounds per square inch.

Violations of the 15-degrees-per-mile rule are unknown and constitute the notorious forbidden zone.

At least that's what geologists used to think.

But researchers around the globe are finding unusual rocks that violate geological dogma.

Embedded in the rocks are microscopic bits of a mineral known as garnet that crystallized millions of years ago under extreme conditions.

Chemical analysis revealed that the garnet fragments formed 120 miles underground, where the pressure is more than 800,000 pounds per square inch and the temperature is normally 2,200 F.

But Liou and Zhang determined that the garnet probably crystallized at 1,620 F still hot by most standards but nearly 600 degrees cooler than the expected temperature at that depth.

How is it possible to have such a relatively low temperature occur at such an ultrahigh pressure?

Perhaps the forbidden zone is not so forbidding after all, say the authors, especially when plate tectonics is involved.

Plate tectonics is the theory that the Earth's crust consists of large, rigid plates that move horizontally and collide into one another, creating earthquake faults and causing mountains to rise.

It turns out that the garnet samples discovered in the Dabie Mountains of central China were part of an enormous chunk of Earth called the Yangtze continental plate (see illustration).

When the Yangtze plate collided with the equally enormous Sino-Korean plate about 250 million years ago, the Yangtze got pushed underground in a process called "subduction."

Because subducted plates are part of the Earth's crust, they are much cooler than the hot, rocky material found below the surface.

Geologists have long speculated that, as plates are pushed underground, they stay cooler even at ultrahigh pressures like those found 120 miles below the surface.

Now there is proof.

"In subduction zones, rocky material is carried down faster than it can heat up," says Hacker, which explains why garnet crystals could form so deep beneath the surface at such relatively low temperatures.

The garnet-imbedded rocks not only formed at great depths, they also "bounced back to the surface by some unknown mechanism," says Hacker. "Geologists thought the temperature increased at a rate of at least 5 C per kilometer (15 F per mile) underground," he adds, "but we're saying that rocks in some subduction zones heat up at a slower rate. The only way we can walk on these rocks today is if they remained 'cold' during their trip toward the center of the Earth."

Studies have shown that continental plates can be shoved 30 miles underground without being destroyed in the fiery mantle below. But the Chinese garnets provide "the first evidence that continental rocks have been subducted to depths of 200 kilometers (120 miles)," according to the authors.

"We now have proof that these cold subduction zones are going down deeper and coming up faster than we realized," says Hacker, although "faster" is a relative term.

It probably took the garnets five million years to travel from the depths of the Earth to the surface relatively fast by geological standards.

Understanding the mechanism that causes continental plates to sink so far underground and then bounce back to the surface will provide geologists with a better understanding of the tremendous forces that build mountains and create destructive, unpredictable earthquakes and volcanoes.

The discovery of rocks from the forbidden zone, write the authors, "now provides a revolutionary new window into the subduction of continental margins."



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