Deep water cycle
However, in addition to the surface cycle, water also plays an important role in geological processes reaching down into the crust and mantle.
Seawater percolates into oceanic crust and hydrates igneous rocks such as olivine and pyroxene, transforming them into hydrous minerals such as serpentines, talc and brucite.
Despite this, changes in the global sea level over the past 3–4 billion years have only been a few hundred metres, much smaller than the average ocean depth of 4 kilometres.
Water carried into the mantle eventually returns to the surface in eruptions at mid-ocean ridges and hotspots.
[16] The transition zone is also composed of at least 40% majorite, a high pressure phase of garnet;[17] this only has capacity of 0.1 wt% or less.
[18] The storage capacity of the lower mantle is a subject of controversy, with estimates ranging from the equivalent of three times to less than 3% of the ocean.
Results may be biased upwards by hydrous mineral inclusions and downwards by a failure to maintain fluid saturation.
Models of the outer core predict that it could hold as much as 100 oceans of water in this form, and this reaction may have dried out the lower mantle in the early history of Earth.
)[20] Mid-ocean ridge basalts (MORBs) are commonly classified by the abundance of trace elements that are incompatible with the minerals they inhabit.
[20] Another common classification, based on analyses of MORBs and ocean island basalts (OIBs) from hotspots, identifies five components.
[9] If these sources sample all the regions of the mantle, the total water depends on their proportion; including uncertainties, estimates range from 0.2 to 2.3 oceans.
Of the thirteen ice-VII instances found, eight have pressures around 8–12 GPa, tracing the formation of inclusions to 400–550 km.
[26] Both sudden decreases in seismic activity and electricity conduction indicate that the transition zone is able to produce hydrated ringwoodite.
The USArray seismic experiment is a long-term project using seismometers to chart the mantle underlying the United States.
Using data from this project, seismometer measurements show corresponding evidence of melt at the bottom of the transition zone.
This reaction would be possible with the interaction of the subduction of minerals containing water and the extensive supply of iron in the Earth's outer core.
At a depth of about 180 km, where the pressure is about 6 gigapascals (GPa) and the temperature around 600 °C, there is a possible "choke point" where the stability regions just meet.
[33][34] An imbalance in deep water recycling has been proposed as one mechanism that can affect global sea levels.
