It's been a big week for learning about subterranean seas. First Pluto’s subsurface ocean was revealed as probably present from the beginning, then there were positive signs for the possibility of life beneath Europa's icy shell. Now geologists have demonstrated earthquakes that rock the surface of our own planet are driven by the waters of the deep.
When tectonic plates are pushed over and under each other in subduction zones water is carried along, creating an enormous reservoir far beneath our feet. The influence of these vast underground oceans on geology has been suspected for some time, but it is only now, with a paper in Nature, that scientists think they have found confirmation.
“As plates journey from where they are first made at mid-ocean ridges to subduction zones, seawater enters the rocks through cracks, faults, and by binding to minerals,” said Dr George Cooper of the University of Bristol in a statement. “Upon reaching a subduction zone, the sinking plate heats up and gets squeezed, resulting in the gradual release of some or all of its water. As water is released it lowers the melting point of the surrounding rocks and generates magma. This magma is buoyant and moves upwards, ultimately leading to eruptions in the overlying volcanic arc.”
The composition of a volcano’s magma determines its explosive potential; the substances that become gaseous under the right circumstances, known as volatiles, contribute to the danger. “The same process can trigger earthquakes and may affect key properties such as their magnitude and whether they trigger tsunamis or not,” Cooper added.
The influence of volatiles on volcanic eruptions has been extensively studied around the Pacific Ring of Fire, where most of the world's active volcanoes are located. Cooper and co-authors instead went to the Lesser Antilles in the Caribbean, a 750-kilometer (450 miles) volcanic arc where slow spreading of the Atlantic and Caribbean plates are producing subduction. The authors expected this would mean even more water captured far beneath, but also greater diversity in the amount of water at different locations, making it possible to make comparisons that reveal the water's effects.
The effects of water and other volatiles on earthquakes is not simple, however. When a tectonic plate sinks into the mantle its water is forced out of major cracks, which geologists call fracture zones, causing dehydration. In the area studied the highest concentration of small earthquakes is in areas where fracture zones are dehydrated. Previously dehydrated locations have more volcanoes and thicker crust, leading to larger islands.
Cooper and his co-authors hope the work will eventually lead to the capacity to not only understand the causes of earthquakes, but predict their timing, but acknowledge they are not there yet.