Content provided by Becky Oskin, OurAmazingPlanet
A soaring mountain range as tall as the Himalayas once towered over the
U.S. East Coast. Some 20 miles (32 kilometers) of rock have since
transformed into sand and mud, exposing an outcrop of the most extreme
rocks in America.
Banded with colorful, unique garnets, the gneiss — a form of
metamorphic rock — was pushed as far as rock can go before it melts, to
1,800 degrees Fahrenheit (1,000 degrees Celsius), geologists report in
the Dec. 13 issue of the journal Geology.
The find is the first discovery of ultrahigh temperature metamorphic rocks
in the United States, said Jay Ague, a professor at Yale University and
lead author of the study. The next hurdle is figuring out how they
formed, he said.
"The fact that these rocks are there at all challenges all the existing models for mountain building
in the area," Ague said. "These ultrahot (rocks) are becoming an
important part of how we think mountain belts form," he told
OurAmazingPlanet.
In appearance, the gneiss looks "quite ordinary," Ague said — it
doesn't have the classic collection of minerals found in ultrahigh
temperature rocks in Canada or other continents. In fact, Ague, who
specializes in metamorphic rocks, walked over outcrops of the gneiss for
nearly 20 years without recognizing its unique history. He even brought
samples back to his lab at Yale, but only for the interesting minerals
in veins crossing the gneiss.
Finally, in connection with a separate study, Ague and his colleagues recently examined the gneiss under a microscope. (Gallery: Hidden Rainbows in Ordinary Rocks)
Garnets in the gneiss had patterned inclusions of rutile, a mineral
that also gives star sapphires their special appeal. "They are really
quite beautiful," Ague said. "Those features in garnets are really only
associated with really extreme temperature or pressure conditions, so
right there we knew there was something special about these rocks," Ague
said. "It was a completely serendipitous discovery."
The outcrops discovered so far stretch along Interstate 84 through
northeast Connecticut toward the Massachusetts border. They are from the
Brimfield Schist, part of the Acadian metamorphic belt. One quarry in
the region uses the rock for road gravel, Ague said. Hikers along the
Nipmuck Trail might also find a piece.
"The rocks are not super unique-looking, so the possibility exists that
there is a lot more of this out there to find," Ague said.
The researchers now plan to figure out when the rocks formed — their
current guess is about 400 million years ago — and what drove them to
such extreme temperatures.
Their origin lies in the Acadian orogeny, when a chain of volcanic islands collided with North America.
"We can see back through all that history to unravel the earliest
beginnings of how this rock formed," Ague said. "These rocks began as
basically muds on ancient ocean floors in very chilly conditions. Then
they were buried and completely recrystallized up to 1,000 degrees (Celsius), and then subject to two more metamorphic and deformational
events, then ultimately brought back to the surface so we can see them
here today."
Based on minerals in the gneiss,
the rocks formed at a minimum depth of about 20 miles (35 km) below the
surface, Ague said. Theories abound as to how to nearly melt rocks at
that depth: having extremely radioactive crust radiate excess heat; a
subduction zone allowing hot magma to heat the rocks; or intense
mountain-building that produces thick crust,
such as in the Himalayas. Another idea: Severely twisting rocks during
mountain-building can produce heat, like bending a paper clip
back-and-forth makes it feel warm, Ague said.
"These are all hypotheses we can test," he said. "The nice thing is that no matter what the answer is, it's very interesting."
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