Researchers believe they have found the Earth’s largest water reservoir around 643 km underneath North America, inside mantle rock.
Though not in the familiar liquid form – the ingredients for water are bound up in rock deep in the Earth’s mantle, according to researchers from Northwestern University and the University of New Mexico.
Northwestern geophysicist Steve Jacobsen and University of New Mexico seismologist Brandon Schmandt have found deep pockets of magma located about 643 km beneath North America, a likely signature of the presence of water at these depths.
The discovery suggests water from the Earth’s surface can be driven to such great depths by plate tectonics, eventually causing partial melting of the rocks found deep in the mantle. Scientists have long speculated that water is trapped in a rocky layer of the Earth’s mantle located between the lower mantle and upper mantle, at depths between 400 km and 660 km.
The study is the first to provide direct evidence that there may be water in this area of the mantle, known as the ‘transition zone,’ on a regional scale. The region extends across most of the interior of the US.
The study combined Jacobsen’s lab experiments in which he studies mantle rock under the simulated high pressures of 643 km below the Earth’s surface with Schmandt’s observations using vast amounts of seismic data from the USArray, a dense network of more than 2,000 seismometers across the US.
Their findings have converged to produce evidence that melting may occur about 643 km deep in the Earth. H2O stored in mantle rocks, such as those containing the mineral ringwoodite, likely is the key to the process, researchers said.
If just one per cent of the weight of mantle rock located in the transition zone is H2O, that would be equivalent to nearly three times the amount of water in our oceans, the researchers said.
This water is not in a form familiar to us – it is not liquid, ice or vapour. This fourth form is water trapped inside the molecular structure of the minerals in the mantle rock, researchers said.
The findings are published in the journal Science.