View source for Earth:Outer trench swell

{{Short description|Seafloor ridge near an oceanic trench}}
[[File:Cross-section of a subduction zone and back-arc basin.jpg|thumb|right|300px]]

The '''outer trench swell''', '''outer trench high''', or '''outer rise''' is a subtle [[Earth:Ridge|ridge]] on the seafloor near an [[Earth:Oceanic trench|oceanic trench]], where a descending plate begins to flex and fault in preparation for its descent into the [[Earth:Earth's mantle|mantle]] at a subduction zone. The [[Earth:Lithosphere|lithosphere]] is bent upwards by plate stresses, and is not in [[Earth:Isostasy|isostatic]] equilibrium (distinguish from the "outer ridge" of a [[Earth:Forearc|forearc]]).

==Characteristics==
Typically, the [[Physics:Gravity|gravity]] field over the outer swell is about 50 [[Physics:Gal (unit)|mGal]] (0.5 [[Metre per second squared|mm/s²]]) higher than expected from isostasy, while gravity over the [[Earth:Oceanic trench|trench]] is about 200 mGal (2&nbsp;mm/s²) less than that expected from isostatic considerations.

The bending of the plate is associated with tension in the upper 20&nbsp;km, and shallow [[Earth:Earthquake|earthquake]]s, caused by tensional failure induced by the downward bending of the oceanic plate are common; about 20 extensional outer rise earthquakes with magnitude 5 or greater occur annually. Most tension axes are perpendicular to the trench, independent of the direction of relative motion between the two plates, indicating that failure is controlled by bending stresses in the plate. Plate bending also causes deeper (down to 50&nbsp;km) earthquakes due to compression.

The [[Wavelength|wavelength]] and [[Physics:Amplitude|amplitude]] of this flexure can be used to constrain the state of [[Physics:Stress (mechanics)|stress]] across the plate boundary. The width of the outer rise is directly related to the [[Physics:Flexural rigidity|flexural rigidity]] of the lithosphere. The thickness of the elastic lithosphere varies between 20 and 30&nbsp;km for most trench profiles. Faulting related to plate bending and stair-stepping of the descending slab into the trench may allow seawater to infiltrate deep into the crust and perhaps upper mantle.  This may lead to large scale formation of [[Earth:Serpentinite|serpentinite]] in the upper [[Earth:Mantle (geology)|mantle]] of the downgoing plate (Ranero et al., 2003).

[[Earth:Fault (geology)|Faulting]] of the downgoing plate results in a [[Earth:Horst (geology)|horst]] and [[Earth:Graben|graben]] structure that allows sediment that reaches the trench to be deposited in graben and carried downward.  This faulting also breaks up [[Earth:Seamount|seamount]]s as they approach the trench. The principal mechanism of frontal erosion may reflect combined effects of seamount tunneling, mass wasting and transport to the trench, deposition in a graben on the downgoing plate, and descent into the mantle.

Outer trench swells are geoscientific frontiers and much remains to be learned about them.  Recent [[Earth:Volcano|volcano]]es have been discovered on ~135-million-year-old [[Earth:Pacific Plate|Pacific Plate]] east of Japan (Hirano et al., 2006).  These small alkalic volcanoes are [[Earth:Partial melting|small percent melts]] of [[Earth:Asthenosphere|asthenosphere]] that exploit bending-related lithospheric faults to reach the seafloor.  Hirano et al., (2006)  proposed that these small volcanoes erupted along lithospheric fractures in response to plate flexure during subduction.  If bending-related faulting and serpentinization is an important process beneath outer trench swells, there are probably also abundant low-[[Physics:Temperature|temperature]] [[Earth:Hydrothermal vent|hydrothermal vent]]s on the swells, similar to those of the Lost City (hydrothermal field).

==References==
*Hirano, N., Takahashi, E., Yamamoto, J., Abe, N., Ingle, S.P., Kaneoka, I., Hirata, T., Kimura, J.-I., Ishii, T., Ogawa, Y., Machida, S., and Suyehiro, K., 2006. "Volcanism in Response to Plate Flexure: Science 313, 1426, {{doi|10.1126/science.1128235}}
*Ranero, C., Morgan, J. P., McIntosh, K., and Reichert, C., 2003. "Bending-related faulting and mantle serpentinization at the Middle America trench".  Nature 425, 367–373
*Stern, R.J., 2002. "Subduction Zones"  Reviews of Geophysics, 40, {{doi|10.1029/2001RG000108}}

{{physical oceanography|expanded=other}}

[[Category:Plate tectonics]]
[[Category:Physical oceanography]]

{{Sourceattribution|Outer trench swell}}