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Earth of fire

Actualité volcanique, Article de fond sur étude de volcan, tectonique, récits et photos de voyage

Seismologists at the University of Liverpool recount , in an article in the journal Geology, their finding of marine faults areas in depth, that can carry much more water to the upper mantle than previously anticipated .

The water is fed to the mantle through zones of faults that penetrate deep sea oceanic plate at its curvature in a subduction zone. When an oceanic plate dives under another plate, it produces large earthquakes,  that can cause tsunamis, and other earthquakes occurring much deeper .

Map of Sendai Earthquake 2011Epicenters of the earthquake of March 11, 2011 / Pacific coast of Tohoku ( or Sendai ) / Japan , and many replicas - this earthquake was the cause of a tsunami and the Fukushima collateral disaster - doc . www2.demis.nl


Seismic waves travel more slowly in fault zones than in the rest of the subducting plate ... this is due to the reaction between seawater percolating through faults and oceanic rocks , which form the serpentinite (* ), a hydrated mineral .

2011 Sendai Subd zone Subduction Zone NE Japan - Virtualupp
2011 Sendai area Subduction Zone NE Japan - Relative velocity of seismic wave-P in the subduction zone - doc . virtualuppermantle .

Subduction Benioff - freerepublic (450x227)
Summary of subduction zone structure inferred for waveform modeling of dispersed P-wave arrivals. Hydration of slab mantle is described by von Kármán function, which provides large-scale elongate structures representing normal fault structures inferred. Smaller scale structures are required to provide observed P-wave coda. Fault depth increase with distance from outer rise is due to downward forcing of fluids (Faccenda et al.,2009, 2012).

Black dots show Wadati-Benioff zone (northern Japan) seismicity. Approximate depth at which mineral-bound water is released is shown by white balloon shapes.

Some mineral-bound water is delivered to mantle transition zone (e.g., Rüpke et al., 2004; Savage, 2012), indicated by white arrow. Depth of penetration of these hydrated fault zone structures is controlled by temperature at which serpentinite is stable. As slab is heated at depth, stability fi eld of serpentinite becomes restricted to cool core of slab -  doc. freerepublic


 A portion of the water line to the mantle through these fault zones is released into the atmosphere, following the fusion it causes in depth at the subducting plate ... which leads to a possible eruption at the level of the associated volcanic arc . On the other hand, the water is headed deeper into the mantle to be stored .

subduction Geol B10General known scheme of the subduction of an oceanic plate under a continental plate, and the volcanism induced in a volcanic arc - doc.Geol B10

The team from the University of Liverpool has found that the fault zones formed in the deep ocean trench northeast Japan reach depths of more than 160 kilometers . These hydrated areas can carry large quantities of water, suggesting that the subduction zones convey much more water than scientists previously believed .

This study provides the first direct measurements of the hydration of the lower lithosphere, at intermediate depths , and suggests that the regasification of the mantle is stronger than previously proposed.

This also fuels the theory of storing large amounts of water in the depths of the earth.

(* ): The serpentinite is a metamorphic rock . Its name derives from its similar appearance to the scales and causes a special feeling to the touch, which could evoke the skin of the snake. It should not however be confused with the serpentine , which is a generic name that covers several mineral species. It is a yellowish to greenish rock (or dark green ) or with greenish inclusions ( porphyry form) . Especially incorporated ( over 75 %) with antigorite ( phyllosilicate magnesium) . This rock come from an alteration of peridotite in the presence of water .



Sources :

- Geology - Order of magnitude increase in subducted H2O due to hydrated normal faults within wadati-Benioff zone - by Tom Garth & Andreas Rietbrock - 01.2014 - link

- International Business times - Deep dea earthquakes release water into Earth's mantle; process"more vigourous" than tought. - link

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