2023-07-11 10:39:00
“If you walk along the Cascadia area, you can see significant gas emissions from the depths,” explains the geologist. “There is a lot of seismic activity off the coast of Washington, Oregon,” and numerous studies have identified nearly “350 underwater liquid emission points, [lesquels] were mapped using sonar. […] We weren’t expecting anything in particular at the time,” he added.
However, as the ROV approached the emission zone, the device began to detect a decrease in salinity and an increase in water temperature. Which is not normal. At the ocean ridges, there are emissions of liquids at very high temperatures, which come from magmatic activity. In addition, they contain salt, which is a very permeable mineral. “However, in a subduction zone, we do not expect to see such events. There are no magmatic chambers there,” says Evan Solomon.
The leaks observed are made up of substances with very low permeability. However, the subducted plate is covered with sediments, which contain many “pores”, where sea water is trapped. As the ocean shelf dips, the upper plate crushes the sediments. With this significant pressure, the water that was lodged there, “then tries to get out, like when you squeeze a sponge”, explains Evan Solomon. “But since the sediments are impermeable, the water has difficulty getting out.” There are indeed no interconnected spaces allowing the water to rise. “It’s a bit like a ball,” he describes. But some liquids manage to make their way to the surface.
Water locked in the subduction zone is able to escape because it is located on the “forearc” region, a term referring to a region in a subduction zone between an oceanic trench and the associated volcanic arc. At Cascadia, the escape zone includes sediments that are more permeable than those of the rest of the oceanic shelf. The high pressure improves the porosity of the rocks, which allows liquids to rise from the depths to the surface. However, only small amounts of water escape each year. They come out cold given the long distances traveled along plate boundaries. However, “in the case of Cascadia, the fluids are much hotter than the ambient temperature because the output flow is very high”.
The fluids seem to come from the boundary between the oceanic shelf and the continental shelf and migrate vertically, over a fairly long distance. The discovery of this leak reflects the fact that it is “the pressure of the water which allows [aux deux plaques engagées dans une interaction de subduction] which allows them to slide more easily”.
It appears that the pressure exerted on the interstitial fluids plays a role on the mechanics of the faults. The process of subduction, during which the oceanic plate plunges under the continental plate, causes a thrust effect which is regulated by a balance between the water inputs which are integrated with the subducted plate, and the outflow of the fluids. These interactions of liquids play a regulatory role in the pressure of the fluids at the level of the subduction zones. This discovery therefore provides details as to the functioning of plate tectonics, but cannot in any case, at least for the moment, make it possible to prevent an earthquake.
1689074406
#Discovery #ocean #leaks #explain #plate #tectonics
