Underground 'forbidden
zone' may not be as hot as Hades
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.
Geologists believe that
the Earth's crust consists of large, rigid landmasses
called plates. Millions of years ago, the Yangtze Plate
(green) collided with the Sino-Korean Plate (yellow) in
central China. The force of that collision gradually
drove a portion of the Yangtze crust underneath the
Sino-Korean Plate in a process called subduction. The
subducted crust (gray) remains cooler than the
surrounding rocks below the surface (pink), which
explains why garnet crystals can form so deep underground
at such relatively low temperatures. The subducted crust
shown here was pushed at least 120 miles underground,
then bounced back to the surface millions of years later,
bringing the garnet crystals with it. (Credit: Bradley R. Hacker/UCSB)
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
75o 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 15o 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 special rock known as garnet
peridotite 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 2,200o
F.
But Zhang and Liou
determined that the garnet probably crystallized at 1,620o
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 central China were part of
an enormous chunk of Earth called the Yangtze continental
plate (see illustration above).
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 5o C per kilometer (15o
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." SR
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