(June 26, 2014 at 11:03 am)Welsh cake Wrote:(June 13, 2014 at 11:02 am)Chuck Wrote: If plate tectonics does not stop, then the subduction process will likely consume all the oceans on the surface of the earth long before the sun goes red giant.Wouldn't that just turn it into steam?
No. As the water in waterlogged sediments on the bottom of the ocean gets dragged down into the subduction zone, increasing pressure keeps the water from flashing into steam until the sediments reach anywhere from 50-200 miles deep. At that depth the pressure and temperature becomes so high the most of water chemically combines with the mantle rocks to form hydrated minerals. Only a small portion of the this water then come back up as part of the material that makes up volcanic magma. If the magma erupts onto the surface as lava, then the water will boil out of the lava as steam. The the steam is a tiny percentage of the total water that was originally dragged down into the mantle.
So plate tectonics is in effect gradually mopping up the water in the ocean, and squeezing the water into a essentially bottomless, and only slightly leaky, reservior in the mantle.
