Oceans-Worth of Water Miles Under US
One of the special things about Earth that has enabled it to support life as we know it is the abundance of water on its surface. Approximately seventy-one percent of the surface is covered with water, with the majority of that being from the oceans, but as much as that is, there may be another, larger water reserve. Researchers at Northwestern University and the University of New Mexico have recently used seismological data to find a region in the mantle that is likely holding water, hundreds of miles beneath us.
For some time it has been speculated that water has been getting trapped within the mantle, through the course of plate tectonics. More specifically, in an area between the lower and upper mantle, between 250 and 450 miles beneath the surface, that stretches across most of the United States called the transition zone. The reason for this speculation is that we can see melting happening there with seismographs, but most melting in the mantle happens just 50 miles below the surface. Water would explain that melting, but not in its usual solid, liquid, or gaseous forms. That deep in the mantle, the temperatures and pressures are so extreme that the water molecules (H2O) are being broken apart into hydroxyl radicals (OH) and bounding with minerals called ringwoodite. The water cannot survive higher pressures though, so if the rocks try to enter the lower mantle, they have to first melt enough to lose the water, trapping in the transition zone.
According to experiments that synthesized ringwoodite, the mineral could be as much as 1% water by weight, which may sound small, but with how much of it is in the mantle, this water reserve could be three times greater than the oceans on the surface. Now armed with this information, researchers can really start to approach a whole-Earth water cycle that follows the molecule above and deep below the surface.
Source: Northwestern University