How can massive volumesof absent move roughly the surface of the earth? In order to properly recognize this question, the pressures that journey platemotion must very first be understood. This is no as an easy as it might seem. There has been much controversy over theyears around the specific mechanisms that drive the lithospheric plates. Presentedbelow are currently the many widely accepted mechanisms.

All the forces actingon the plates deserve to be categorised right into two main classes: driving and resistingforces.

Driving:

- Slab traction

- Slab (Trench)Suction

- Ridge push

Resisting:

- Slab Resistance

- ContinentalResistance

- TransformFault Resistance

- Basal drag

Driving Forces

Slab pull

This pressure occursas a subducting plate sinks into the warm mantle beneath it. The subductingplate, typically basalt, is denser than the product it is subducting into,purely due to its distinction in temperature. Asthe key sinks into the mantle, that acts to traction the remainder of the key behindit. This pressure is taken into consideration by part to be the primary pressure driving platemotion at collisional area (Wilson,1993). However, there room some plates whereby there is small or no subduction arising such together the Antarctic Plate. This plate activity needs to be explainedby a different mechanism.

The slab traction forceonly works as soon as the subducting slab is well attached to the plate the is pullingbehind it. When the slab is no well attached the may collection up additional circulationpatterns in the mantle that offer to suck the bowl downwards. That is alsointeresting to keep in mind that plates v a slab subducting right into the mantle movefaster in the direction of the subduction zone than do plates there is no a slab. This isthought to it is in primarily as result of the slab pull exhilaration on the plate. This facttends to support the idea that slab traction is indeed a dominant force in platemotion (Conrad and also Lithgow-Bertelloni, 2002).

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Fig.3.1. Slab Pull and Collisional Resistance

Slab (Trench) Suction

This force occurs between two colliding plates whereby one is subducting beneaththe other. As one bowl subducts, that sets up convection currents in the uppermantle the "exert a network trenchward pull" ie, acts to suck both the platestogether (Wilson, 1993).
Associated with the slab suction pressure is the idea of trench roll-back. Together a slab ofoceanic tardy subducts right into the mantle, the hinge of the bowl (the pointwhere the plate begins to subduct) often tends to regress away from the trench.This occurs because there is effectively no force to organize the hinge in onelocation.
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Fig.3.2 Diagram depicting Trench Suctionand Trench Roll-back

Ridge Push

Before the nineties, this force was considered tobe the top contributor because that driving the plates roughly the earth. However,this idea has actually now readjusted with the main mechanism established to it is in slab pull.However, ridge push is still thought about to it is in of significance, especiallywhere over there is little or no slab pull acting on the bowl (eg the AntarcticPlate mentioned above). There have actually been two key models the ridge push proposedby geologists. Bott (1991) states that the two contending models space thatof "gravity wedging" and also "gravity sliding". Recently, heaviness sliding hasemerged together the dominant model. The name provided to this pressure is actually rather decieving and also has brought about amisundertanding that this process. The ridges are not propelled apart at theiredges together is generally thought from the heaviness wedging model. The "pushing"on the plates is actually due to a distinction in gravitational potentialenergy in between a plate at its dispersing centre and also subduction zone. That isknown the mid-oceanic ridges rise thousands that metres above the s floor.When brand-new sea floor is created, that is warm and reasonably thin, as well asbeing much higher in elevation than the abyssal plains and trenches. Together therock moves away from the dispersing centre, the rocks proceed to cool. Additionalmaterial is pasted top top the basic of the crust indigenous the mantle below. Thismeans that together a plate moves far from the spreading zone it gets denser,heavier and thicker. beneath the lithosphere is a zone of soft "plastic" material referred to as the aesthenosphere.This product is less dense than the key riding above it and also acts as amassive shear zone for the over-riding plate. The plate will certainly effectivelyslide down the slope of the aesthenosphere because of the weight difference betweenthe plate in ~ its dispersing centre and subduction zone. Because the plate getsthicker and denser the further away native the spreading centre, the ridgepush force will increase towards the subduction zone.


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Fig.3.3. Diagram showing the ridge push pressure
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