April Editorial      

Quartz - the weakest link

As two tectonic plates converge, one plate is pushed underneath the other plate, and sinks into the Earth’s mantle. This process is known as subduction, and it does not happen smoothly. Subduction zones (where two tectonic plates move towards one another and one slides under) are noted for their dramatic geological activity. Earthquakes, tsunamis, active volcanism and orogenesis (mountain building) happen vigorously in a subduction zone.

Mountain building happens when one plate pushes against the other, causing the edges to rear up, just as a rug does when pushed by a foot - except in this case the process results in the deformation of large parts of a continent, or the creation of an island chain. However, not all plate collisions result in mountain ranges, and until now it has not been possible to determine why not. Or to put it simply why are mountains where they are, and why do others not appear in similar circumstances?

The authors of a recent Nature paper think they found the answer and it is crustal quartz.

The authors of the paper are two researchers, Anthony Lowry and Marta Pérez-Gussinyé. Their research started from a simple and previously known observation - any mountain range has an abundance of quartz but the crust beneath most plains or prairie has almost no quartz in it. It began to appear that quartz, which is the weakest mineral in the Earth’s crust, may be important in mountain formation. But until recently there was no efficient way of testing this theory. However, the Earthscope Transportable Array of Seismic Stations across the western United States was founded in 2002 by the National Science Foundation, and this institution provided the necessary help. Earthscope supplied remote sensing data which had information about the thickness and the seismic velocity ratio of continental crust in the American West.

Seismic velocity describes how quickly p-waves and shear waves travel through rock, offering clues to its temperature and composition. P-waves (or primary waves) are seismic waves which can travel through gasses solids and liquids. S-waves (shear or secondary) waves can only travel through solids (so for example they can’t travel through the molten outer core of the Earth) because fluids do not support shear stresses, and are slower than P-waves.

The velocity of seismic waves depends on both the temperature and the composition of the material they travel through, but when they are analysed as a ratio (vP/vS) the figures become temperature independent, allowing scientists to focus on rock composition. The results show that low seismic velocity ratio is constant with quartz-rich mountainous areas. Surprisingly this also correlated well with the higher lithospheric temperature observed in these regions.

To explain the importance of these findings we can use Lowry’s own words:

‘We think this indicates a feedback cycle, where quartz starts the ball rolling. If temperature and water are the same, 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. Water further weakens the crust, which increasingly focuses the [upward] deformation in a specific area.”

This indeed gives us an elegant model for mountain formation.

Reference

Anthony R. Lowry and Marta Pérez-Gussinyé. The role of crustal quartz in controlling Cordilleran deformation. Nature, 2011, 471, pp353–357