High concentrations of quartz, one of the most common minerals found at Earth's surface, may cause weak spots in the crust that determine where mountain ranges form and continents rift apart, a new study suggests.
On average, quartz—a chemically stable mineral particularly common in granites—makes up about 12% of Earth's crust. Quartz takes many forms, including sand grains and semiprecious gems such as agate and amethyst. These crystalline minerals are typically hard at Earth's surface, but they readily soften and flow at the temperatures and pressures found deep within Earth's crust—characteristics that make quartz one of the weakest minerals in that veneer. Accordingly, lab studies show that the abundance of quartz in a rock is one of the key factors influencing its tendency to flow under heat and pressure.
Now, a study published online today in Nature suggests a link between the quartz content of crustal rocks and large-scale deformations such as mountain ranges.
In the new research, geophysicist Anthony Lowry of Utah State University in Logan and colleague Marta Pérez-Gussinyé of Royal Holloway, University of London, analyzed seismic data gathered in the western United States. In particular, they looked at the speed of compressive P-waves, the seismic equivalent of sound waves in air, emanating from an earthquake. They compared those speeds with that of the temblor's S-waves, which move more slowly and transmit shear forces.
After removing the effects of temperature and density of crustal rocks on the speeds of seismic waves, the researchers found that at sites where the crust was stable, such as areas well east of the Rocky Mountains, P-waves traveled more than 1.8 times faster than S-waves. But at sites where the continental crust was deformed—all along the Rockies, for example, or in the Basin and Range province of central Nevada, where Earth's crust is stretching and thinning—the speed ratio falls below 1.8. This threshold offers a way to identify quartz-rich rocks deep in the crust, the researchers contend.
Besides explaining ongoing deformations, high concentrations of quartz may help explain why Earth's crust has apparently ripped apart at the same seams time after time, as supercontinents collided and then split again, while other regions have remained relatively unscathed, the researchers add. Once a quartz-weakened patch of crust has begun stretching or folding, increased heat flow from underlying rocks helps drive water out of the crustal minerals, a process that weakens the rocks even further, ensuring that large-scale deformations take place there and not elsewhere.
The team's results "are pretty convincing," says geophysicist Roland Bürgmann of the University of California, Berkeley. Although the researchers make a good argument that the presence of quartz starts the rock-weakening process, he notes, other factors, such as large-scale imperfections that formed when Earth's crust first cooled, could have helped determine where ancient crustal deformations originated. Similar analyses using seismic data gathered in the eastern United States should provide scientists with the raw material to test the team's quartz-weakening hypothesis, Bürgmann notes.
The new findings are "very intriguing and reasonably plausible," says seismologist Don Forsyth of Brown University. Like Bürgmann, however, he notes that other factors may be at play. For example, he suggests, some of the features that Lowry and Pérez-Gussinyé ascribe to the crust may actually be related to rocks deeper in Earth's mantle. "Their results are very dependent upon their assumptions, and they've made a lot of assumptions," he notes.