A new study published in the journal Nature reveals some mysterious complexities about our planet’s water cycle—in particular, how much ocean water gets trapped in the earth’s interior via plate tectonics. According to the study, about three times as much as we thought.
The earth’s mantle consists of an interlocking puzzle of tectonic plates. When those plates collide and one slides beneath the other, water gets pulled into the subduction zone. Through a combination of heat and pressure, the water chemically transforms into “wet rocks,” a hydrous mineral that gets locked inside the plate and pulled deeper into the earth’s crust. Douglas Wiens, professor of earth and planetary sciences at Washington University in St. Louis, wanted to find out just how much water is absorbed this way. He began in the Mariana Trench, the deepest natural formation on the planet at seven miles below sea level.
Using seismographs placed along the trench, researchers were able to listen to underwater seismic activity and essentially map sections of the earth’s interior by “tracking the relative speeds of types of rock that have different capabilities for holding water,” according to The Source, a university publication. After a full year of listening to the rumblings of the Mariana and Pacific plates 20 miles below the sea floor, they found that the mantle contains four times the amount of water than formerly thought. To put this in perspective, that’s as much water as all the oceans on earth combined. Blue planet indeed.
While some trenches around the globe exhibit more faulting and others less, we can nonetheless extrapolate that they’re all sucking down a heck of a lot of ocean—three times more, says Wiens, than estimated. If you think of the earth like a vacuum, what goes in must come out—or more accurately, up. Scientists believe that most of the water consumed at subduction zones gets expelled as water vapor during volcanic eruptions.
But here’s the rub: These recent estimates from the Washington University study reveal a stark imbalance in the intake and the outflow. Geologically speaking, the more water transferred to the mantle should mean less water on the surface. And yet, Wiens points out, for the past 550 million years or so, oceans have looked much like they do now. New research in Alaskan and Central American trenches will likely shed some light on this mystery.
Whether you’re fascinated by the slow geological grind or not, it’s a mystery worth unravelling. Understanding how water works on, in, and around our planet will not only illuminate how we live here on earth now, but how we might survive on some other planet. Someday.