Did you know there’s a mineral that may prevent catastrophic earthquakes and sequester CO2 from Earth’s atmosphere? A new study explores this link.
By Melissa Scruggs, PhD, @VolcanoDoc
Citation: Scruggs, M., 2022, Carbon-storing rocks may lubricate the San Andreas, Temblor, http://doi.org/10.32858/temblor.280
Visible from space, the San Andreas Fault is one of the largest fault zones on Earth. Since 1906, its large, destructive earthquakes have made it one of the most famous faults in the world. Yet, some sections of the San Andreas are oddly quiet. There, slip along faults usually happens without noticeable jolts.
A new study shows how chemical reactions happening deep in the fault zone create the minerals needed to induce movement without large earthquakes. These same reactions store carbon dioxide in their mineral products, and have been proposed by scientists as a way to remove significant amounts of carbon dioxide from the atmosphere.
Tiny quakes move plates
Typically, an earthquake occurs because adjacent pieces of Earth get stuck as they slide past one another along faults, causing stress to build up. Earth’s tectonic plates are always on the move. As the plates continue to shift, stress increases along these stuck points. When these locked sections of a fault eventually break, slip releases some of the stress in the form of an earthquake.
But, some faults — including the San Andreas, Calaveras and Hayward faults in California — can release that stress without triggering catastrophic earthquakes. In certain places, these faults creep at a relatively steady pace by producing small, repeating quakes that limit the faults’ abilities to build up enough stress to cause large earthquakes, says Diane Moore, a U.S. Geological Survey (USGS) research geologist who studies the San Andreas.
Scientists have been monitoring creep along these faults since the 1960s. Early mechanisms proposed to explain the phenomenon include triggering by deeper earthquakes, the lubrication of a fault with slippery rocks such as serpentinite (a rock formed by alteration of Earth’s mantle), differences in fault geometry and elevated fluid pore pressures in fault sediments.
An intriguing discovery
Decades later, scientists are still wrestling with understanding why creep occurs along certain faults, but slippery rocks seem to play a key role.
The link between talc (a soft mineral sometimes found in baby powder) and creep was first made in 2007, says Moore, when scientists from a drilling project designed to explore a creeping segment of the San Andreas Fault found talc and serpentinite crossing the fault about three kilometers below the surface.
Earth’s mantle is mostly made up of olivine, a magnesium-rich mineral that is unstable at the surface and easily weathers to serpentinite.
As a marine geologist who studies the alteration of mantle rocks, Frieder Klein of the Woods Hole Oceanographic Institution says he was examining maps of serpentinite along the California coast when he noticed that deposits of magnesite (a mineral similar to calcite that incorporates magnesium instead of calcium) were often located near sections of faults that don’t produce large earthquakes.
When serpentinite is exposed to fluids rich in carbon dioxide, a chemical reaction turns the greasy green rock into soapstone – a mixture of magnesite and talc. Continued exposure to carbon dioxide-rich fluids changes soapstone into a rock made of quartz and additional magnesite. Curiously, said Klein, these mineral deposits and the locations of carbon-dioxide-rich springs “all lined up along fault zones.” But when Klein and his colleagues went to collect surface samples from the San Andreas Fault, they found magnesite and quartz, but no talc. Klein and his coauthors reasoned that because magnesite and quartz were present at the fault surface, the missing talc must be forming at greater depths in the fault zone, where it could not be seen.
Understanding mineral behavior using computer models
Because Klein and his colleagues had no way to sample rocks deep along the fault, they used models of how minerals behave at different conditions within Earth. The model results showed that talc could indeed be produced by the carbon dioxide-rich fluid alteration of mantle rocks at the same pressures and temperatures the rocks would experience in the fault zone.
However, mantle rocks can be transformed to talc in other ways, such as shearing within a fault zone or alteration caused by silica-rich fluids, says Moore. Although scientists can’t say exactly why creep happens, this study suggests that the formation of talc deep in fault zones could be essential to facilitating creep. To decrease friction generated as segments of Earth’s surface move past each other, rocks along these fault boundaries need to be relatively slippery. Talc has “mechanical properties that allow it to deform… without promoting earthquakes,” Klein says.
Storing carbon dioxide while preventing big earthquakes
Using geochemical reactions in mantle rocks as a way to store carbon was first proposed in the early 1990s. Altering mantle rocks to soapstone could be a naturally-occurring way to store carbon dioxide in rocks and minerals, and scientists have proposed accelerating this chemical reaction to remove carbon from Earth’s atmosphere and help lessen the effects of climate change. Accelerating this process at slow-spreading mid-ocean ridges where mantle rocks can be exposed at Earth’s surface, may also naturally reduce the amount of carbon dioxide dissolved in our oceans, says Klein.
As “societies are getting desperate…people are taking climate change more seriously and are willing to consider exploring possible opportunities to solve the problem,” says Klein, and the carbonation of mantle rocks appears to be a promising methods for sequestering carbon on an industrial scale.
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