We experience the cycle of water through the Earth's surface and the atmosphere in everyday life. Clouds build up in the sky. Precipitation falls as rain, hail or snow, seeps into the ground, fills seas, lakes and rivers. Under the influence of heat, the water evaporates again. The water vapour rises into the atmosphere, where it cools down, condenses, and forms new clouds or fog. However, the hydrological cycle of the Earth is not limited to the Earth's surface and atmosphere. Water also flows at great depths in the Earth's crust and mantle.
How does water penetrate deep into the ground?
At subduction zones, two tectonic plates move towards each other and collide. The lighter continental plate poses an obstacle for the approaching, heavier oceanic plate. As a result of the collision, the oceanic plate bends upwards - like a piece of paper pushed over a table top and up against an adjacent wall. In this bulge area, about 200 km off the coast, the upper part of the lithosphere is subjected to an extensional regime. The resulting tensional stress can cause faulting, and vertical offset of the pieces lead the front fragments to sink downwards towards the deep sea trench. These so-called bend-faults provide a pathway for marine sediments and sea water to enter the slab. In some subduction zones of the Earth, such as to the west of Nicaragua in Central America, these faults are very deep. They extend down into the mantle.
When the oceanic plate with its bend-faults descends further, it is progressively subjected to higher temperatures and pressures. Part of the water on the surface of the oceanic plate is only relatively loosely bound to clay minerals, the very fine-grained portion of the marine sediments. At shallow depths in the forearc of the subduction zone, this water is squeezed out of the plate and leaves the system. However, when the water in the deep faults of the subducting oceanic plate comes into contact with the earth mantle, chemical reactions produce stable, hydrous minerals. Olivine, the mineral that forms the main constituent of the mantle, reacts with water, generating the mineral serpentine. Serpentine withstands the rising temperatures and pressures associated with the subduction of the plate to depths of over 100 km. Only then does its crystal lattice become unstable. A portion of the water particles are extracted from the mineral and are released into the overlying mantle. Here, the water promotes melting of the mantle wedge. Magmas are generated, they rise towards the surface and lead to volcanism. The other part of the water is transported further down into greater depths. In this way, water also penetrates into areas of the mantle that are far away from a subduction zone.
What does the Earth's mantle have to do with life on earth?
Water is an incompatible component in the Earth's mantle. “Incompatible" in this context describes the chemical behaviour of a substance: its preference to reside in a particular material more than in another. In this case it means that the mantle endeavours to expel the water. When new magma forms in the mantle wedge, the water tends to accumulate in the melt rather than remaining in the mineral part of the mantle. It is then transported to the surface with the ascending magma, where it supplies the living world and becomes part of the water cycle on the Earth's surface. Through rain and rivers it flows back into the ocean. If a subduction zone with bend-faults is located beneath the ocean, part of it can be transported back into the deep system in a large-scale cycle. If the Earth's mantle did not behave as it does, there would be no life on Earth. The Earth's mantle permits the biosphere to be.
