View source for Earth:Slab gap hypothesis

{{Short description|Theory in plate tectonics}}
In [[Earth:Geology|geology]], the '''slab gap hypothesis''' is one of the explanations put forward to explain several instances of [[Earth:Crust (geology)|crustal]] extension that occur inland near former subduction zones.

Standard plate tectonic theory holds that once a [[Earth:Oceanic trench|trench]] is closed by an overriding plate reaching a [[Earth:Mid-ocean ridge|rift/spreading center]], the plate that has just been fully consumed continues to descend beneath the overriding plate for some time, transmitting compressive pressures to the overriding plate above as well as occasional volcanism.  Meanwhile, the descending plate leaves behind it a "window" of inactivity.  In this view, there is no [[Earth:Mantle (geology)|mantle]] upwelling, so once the crustal [[Earth:Rift|rift]] is overridden, the only residual effects are from the remnant descending plate slab.  However, actual observations of the crust in western North America where the [[Earth:Farallon Plate|Farallon Plate]]'s trench and rift was snuffed out millions of years ago by the westward movement of the [[Earth:North American Plate|North American Plate]], and replaced by the San Andreas Fault, show not compression inland, but extension.  This is most evident in the Great Basin where the [[Earth:Continental crust|continental crust]] and [[Earth:Lithosphere|lithosphere]] as a whole is becoming increasingly thin.  The crust here is also warming from below.

The slab gap hypothesis proposes that instead of a "window", the descending slab leaves behind a "gap" through which the [[Earth:Asthenosphere|asthenospheric mantle]] of the former spreading zone continues to act beneath the overriding plate.  This hypothesis then assumes that a crustal spreading zone is also underpinned by a corresponding asthenospheric mantle spreading zone or upwelling of warmer material.  The gap is created because instead of the old subducted plate continuing to sink, it quickly melts, allowing the asthenospheric upwelling zone to act directly on the underside of the overriding plate, heating it and causing it to spread apart.  The fast melt is because the portion of the subducted plate nearest the spreading zone is thin and still warm from its recent creation. 

The slab gap hypothesis goes on to state that the upwelling can form very deep cracks, which in turn lets very fluid [[Earth:Basalt|basalt]] [[Earth:Lava|lava]] quickly spread over the land surface forming [[Earth:Shield volcano|shield volcano]]es and vast volcanic plains called "[[Earth:Flood basalt|flood basalt]]s". If, however, the extension is spread over a very large area then these flood basalt events may not occur. 

This idea has been used to explain the extension and very large flood basalts that occurred in what is now southern {{wipe|Washington (state)}}, Oregon and northern {{wipe|California}} about 17 million years ago (see Columbia River Plateau). Slab gap has also been used to help explain the earlier creation of the [[Earth:Basin and Range Province|Basin and Range Province]].

==See also==
*{{annotated link|Earth:Slab window}}

==External links==
*[http://www.colorado.edu/GeolSci/Resources/WUSTectonics/SanAndreas/SlabGap.html Slab Gap web page at University of Colorado]

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[[Category:Plate tectonics]]

{{Sourceattribution|Slab gap hypothesis}}