The title is a bit misleading,
but what we learn is that quartz acts as a mechanism for stress release in
rocks by releasing and I suspect reabsorbing water, thus shifting in and out of
a viscous state.
Perhaps, it certainly provides a
mechanism for healing rocks back together.
It is noteworthy that quartz often jumps through structures and this is
often seen as replacement, while simple intrusion is just as satisfactory. The actual fluids are the water based
solutions that often are associated with quartz structures.
Thus stressed rocks crack and are
then rehealed by viscous quartz. This can happen repeatedly to form a large
quartz rich structure.
Thus we gain a useful model for copper
porphyry. An intrusive quartz stockwork
rises into a host rock inducing heavy cracking while providing hot mineralized
fluids that precipitate throughout the fracturing. The flow of mineral is
ongoing through the quartz over geologic time frames and it precipitates into
the halo of fractured rock as the chemistry changes. This nicely ends my confusion over the proper
genesis of such deposits.
The bigger the better as that
means a larger heat source and longer life.
Thus any attempt to understand
geological ‘viscosity’ begins rightly with the quartz content. Making new quartz is another important
matter.
Viscous Cycle: Quartz Is Key To Plate Tectonics
by Staff Writers
Quartz may play a major role in the movements of continents, known as
plate tectonics. Credit: USGS
More than 40 years ago, pioneering tectonic geophysicist J. Tuzo Wilson
published a paper in the journal Nature describing how ocean basins opened and
closed along North America's eastern seaboard.
His observations, dubbed "The Wilson Tectonic Cycle,"
suggested the process occurred many times during Earth's long history, most
recently causing the giant supercontinent Pangaea to split into today's seven
continents.
Wilson's ideas were central to the so-called Plate Tectonic Revolution,
the foundation of contemporary theories for processes underlying
mountain-building and earthquakes.
Since his 1967 paper, additional studies have confirmed that
large-scale deformation of continents repeatedly occurs in some regions but not
others, though the reasons why remain poorly understood.
Now, new findings by Utah State University
geophysicist Tony Lowry and colleague Marta Perez-Gussinye of Royal Holloway, University of London
"It all begins with quartz," says Lowry, who published
results of the team's recent study in the March 17 issue of Nature.
The scientists describe a new approach to measuring properties of the
deep crust.
It reveals quartz's key role in initiating the churning chain of events
that cause Earth's surface to
crack, wrinkle, fold and stretch into mountains, plains and valleys.
"If you've ever traveled westward from the Midwest's Great
Plains toward the Rocky Mountains , you may
have wondered why the flat plains suddenly rise into steep peaks at a
particular spot," Lowry says.
"It turns out that the crust beneath the plains has almost no
quartz in it, whereas the Rockies are very
quartz-rich."
He thinks that those belts of quartz could be the catalyst that
sets the mountain-building rock cycle in motion.
"Earthquakes, mountain-building and other expressions of
continental 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 one
location to another."
Wilson's theories provide an important clue, Lowry says, as scientists
have long observed that mountain belts and rift zones have formed again and
again 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 these
factors and answer long-standing questions."
Since 2002, the National Science Foundation (NSF)-funded Earthscope
Transportable Array of seismic stations across the western United States
"We've combined Earthscope data with other geophysical measurements
of gravity and surface heat flow in an entirely new way, one that allows us
to 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 American
West.
"This intriguing study provides new insights into the processes
driving large-scale continental deformation and dynamics," says Greg
Anderson, NSF program director for EarthScope. "These are key to understanding
the assembly and evolution of
continents."
Seismic velocity describes how quickly sound waves and shear waves
travel through rock, offering clues to its temperature and composition.
"Seismic velocities are sensitive to both temperature and rock
type," Lowry says.
"But if the velocities are combined as a ratio, the temperature
dependence drops out. We found that the velocity ratio was especially sensitive
to quartz abundance."
Even after separating out the effects of temperature, the scientists
found that a low seismic velocity ratio, indicating weak, quartz-rich crust,
systematically occurred in the same place as high lower-crustal temperatures
modeled independently from surface heat flow.\
"That was a surprise," he says. "We think this indicates
a feedback cycle, where quartz starts the ball rolling."
If temperature and water are the same, Lowry says, rock flow will
focus where the quartz is located because that's the only weak link.
Once the flow starts, the movement of rock carries heat with it and
that efficient movement of heat raises temperature, resulting in weakening of
crust.
"Rock, when it warms up, is forced to release water that's
otherwise chemically bound in crystals," he says.
Water further weakens the crust, which increasingly focuses the
deformation in a specific area.
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