Viscous Cycle of Quartz

The title is a bit misleading,but what we learn is that quartz acts as a mechanism for stress release inrocks by releasing and I suspect reabsorbing water, thus shifting in and out ofa viscous state.

Perhaps, it certainly provides amechanism for healing rocks back together. It is noteworthy that quartz often jumps through structures and this isoften seen as replacement, while simple intrusion is just as satisfactory.  The actual fluids are the water basedsolutions that often are associated with quartz structures.

Thus stressed rocks crack and arethen rehealed by viscous quartz. This can happen repeatedly to form a largequartz rich structure.

Thus we gain a useful model for copperporphyry.  An intrusive quartz stockworkrises into a host rock inducing heavy cracking while providing hot mineralizedfluids that precipitate throughout the fracturing. The flow of mineral isongoing through the quartz over geologic time frames and it precipitates intothe halo of fractured rock as the chemistry changes.  This nicely ends my confusion over the propergenesis of such deposits.

The bigger the better as thatmeans a larger heat source and longer life.

Thus any attempt to understandgeological ‘viscosity’ begins rightly with the quartz content.  Making new quartz is another importantmatter.

Viscous Cycle: Quartz Is Key To Plate Tectonics

by Staff Writers

Washington DC (SPX) Mar 18, 2011

Quartz may play a major role in the movements of continents, known asplate tectonics. Credit: USGS

More than 40 years ago, pioneering tectonic geophysicist J. Tuzo Wilsonpublished a paper in the journal Nature describing how ocean basins opened andclosed along North America's eastern seaboard.

His observations, dubbed "The Wilson Tectonic Cycle,"suggested the process occurred many times during Earth's long history, mostrecently causing the giant supercontinent Pangaea to split into today's sevencontinents.

Wilson's ideas were central to the so-called Plate Tectonic Revolution,the foundation of contemporary theories for processes underlyingmountain-building and earthquakes.

Since his 1967 paper, additional studies have confirmed thatlarge-scale deformation of continents repeatedly occurs in some regions but notothers, though the reasons why remain poorly understood.

Now, new findings by Utah State Universitygeophysicist Tony Lowry and colleague Marta Perez-Gussinye of Royal Holloway, University of London, shed surprising light on theserestless rock cycles.

"It all begins with quartz," says Lowry, who publishedresults of the team's recent study in the March 17 issue of Nature.

The scientists describe a new approach to measuring properties of thedeep crust.

It reveals quartz's key role in initiating the churning chain of eventsthat cause Earth's surface tocrack, wrinkle, fold and stretch into mountains, plains and valleys.

"If you've ever traveled westward from the Midwest's GreatPlains toward the Rocky Mountains, you mayhave wondered why the flat plains suddenly rise into steep peaks at aparticular spot," Lowry says.

"It turns out that the crust beneath the plains has almost noquartz in it, whereas the Rockies are veryquartz-rich."

He thinks that those belts of quartz could be the catalyst thatsets the mountain-building rock cycle in motion.

"Earthquakes, mountain-building and other expressions ofcontinental tectonics depend on how rocks flow in response to stress,"says Lowry.

"We know that tectonics is a response to the effects of gravity,but we know less about rock flow properties and how they change from onelocation to another."

Wilson's theories provide an important clue, Lowry says, as scientistshave long observed that mountain belts and rift zones have formed again andagain at the same locations over long periods of time.

But why?

"Over the last few decades, we've learned that high temperatures,water and abundant quartz are all critical factors in making rocks flow more easily,"Lowry says. "Until now, we haven't had the tools to measure thesefactors and answer long-standing questions."

Since 2002, the National Science Foundation (NSF)-funded EarthscopeTransportable Array of seismic stations across the western United States has provided remotesensing data about the continent's rock properties.

"We've combined Earthscope data with other geophysical measurementsof gravity and surface heat flow in an entirely new way, one that allows usto separate the effects of temperature, water and quartz in the crust,"Lowry says.

Earthscope measurements enabled the team to estimate the thickness,along with the seismic velocity ratio, of continental crust in the AmericanWest.

"This intriguing study provides new insights into the processesdriving large-scale continental deformation and dynamics," says GregAnderson, NSF program director for EarthScope. "These are key to understandingthe assembly and evolution ofcontinents."

Seismic velocity describes how quickly sound waves and shear wavestravel through rock, offering clues to its temperature and composition.

"Seismic velocities are sensitive to both temperature and rocktype," Lowry says.

"But if the velocities are combined as a ratio, the temperaturedependence drops out. We found that the velocity ratio was especially sensitiveto quartz abundance."

Even after separating out the effects of temperature, the scientistsfound that a low seismic velocity ratio, indicating weak, quartz-rich crust,systematically occurred in the same place as high lower-crustal temperaturesmodeled independently from surface heat flow.\

"That was a surprise," he says. "We think this indicatesa feedback cycle, where quartz starts the ball rolling."

If temperature and water are the same, Lowry says, rock flow willfocus where the quartz is located because that's the only weak link.

Once the flow starts, the movement of rock carries heat with it andthat efficient movement of heat raises temperature, resulting in weakening ofcrust.

"Rock, when it warms up, is forced to release water that'sotherwise chemically bound in crystals," he says.

Water further weakens the crust, which increasingly focuses thedeformation in a specific area.

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