The fate of water
What happens to the water during its cycle into the deep Earth? Is it all fluxed back to the surface or is it lost into the deep Earth?
Water is the one of most important elements of our planet. It covers 71% of the Earth's surface and it forms the oceans, the rivers, the rain and the lake (also called the hydrosphere). Under its solid state (ice), water also forms the snow that covers the mountains during winter. Under its vapor state (water vapor), water forms clouds, fog and steam (also known as the atmosphere). Water is also the major constituant of most of the living forms (such as plants, animals, human beings). Hence, water is essential to the life on Earth and it makes our planet habitable. The Earth is also called the "blue planet" because most of its surface is covered by oceans. In simple words, without water our planet would look different and life as we know it would not exist. Because water is so essential to every living forms on Earth, we need to understand what happens to the water during its cycle into the deep Earth.
Sketch showing the deep water cycle at subduction zones, inspired from the Subduction Factory. (1) Water is trapped in minerals because of seafloor alteration during plate formation. (2) As pressure and temperature increase during subduction, some of the water is released to trigger melting of the hot mantle. (3) Water is processed as a melt that erupts as lavas or form gas (4) in the volcano factory.
Water is recycled into the deep Earth and returned back to the surface via subduction zones. Subduction zones are places where one plate goes underneath another plate. Because (1) plates are hydrated during their formation on the seafloor (as they are in contact with seawater), they can release their bounded water as the plate goes down (2), as pressure and temperature increase. Just think of it as cooking rice. When you cook rice, you need to add water to it. As your rice cooks, it trapps water (the water is not free anymore but is bounded to rice). If you put your cooked rice into the oven, and you increase the pressure and the temperature, it will lose its water (or dehydrates) and dry out ! This is exactly what happens to the oceanic plate during its formation on the seafloor (i.e., we add water to it) and its dehydration (i.e., we remove water from it) during subduction. Water released from the downgoing plate (slab) then interacts with the hot and viscous mantle and triggers melting. Melts carry the slab-derived water (3) and they erupt to the surface as water-rich lavas. Water can also be recycled to the surface as volcanic gasses (4). Some of this water is not recycled to the surface, but lost ("wasted") into the deep Earth.
One of the main questions that still needs to be addressed is how much water is processed through the volcano factory (recycled to the surface) or lost during subduction? One part of my research is to answer to this fundamental question.