Driving forces of Plate activities
Does mantle convection journey plate tectonics?
After the discovery of radioactivity by Marie Curie and Henri Becquerel in 1896 it came to be clear that the Earth"s internal would be heated by radioactive decay in the mantle and crust. Absent is a bad conductor of heat. If there were not some other system to cool the inner the Earth"s internal would be greatly molten. Yet the growing seismic data from about the revolve of the and very first two decades of the 20th century showed that the crust and also mantle to be solid. Holmes proposed that the interior was cooled through convection the the solid, ductile mantle. By 1928 Holmes used mantle convection together a device to create continental drift as questioned by Wegener and also DuToit. But the principle of a convecting solid mantle wasn"t an extremely palatable come the scientific mainstream of the time.
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In the 1960s v the key tectonics revolution, very early hypothesis because that a driving pressure for seafloor dispersing was ago to mantle convection. In fact, convection practically certainly does occur in the mantle. Seismic tomography of the mantle shows areas of lower seismic velocities - lower thickness (hotter) mantle rock which presumably are rising and other areas of higher seismic velocity - greater density (not together hot) absent which need to be sinking. However some bowl are an extremely wide, thousands of kilometers native the ridge to the trench, and also it doesn"t it seems ~ plausible to have actually a convection cell that is really wide (thousands of kilometers) in comparison to its depth (less than 670 km). Seismic tomography shows that climbing mantle material beneath ridges only extends down 200 come 400 km. Moreover, translate of geoid anomalies, based upon variation in gravity produced by upwelling and downwelling regions, shows that there space multiple convection cell beneath the Pacific plate. The merged effect would not be one that would drive the movement of the bowl - some cells would help, some would certainly hinder. Additionally, the asthenosphere is weak sufficient that relatively little pressure would be applied to the basic of the oceanic lithosphere by convecting mantle.
So, if not convection, then what?
The most essential forces: gravity = "ridge push" and also "slab pull" (Forsyth and also Uyeda, 1975)
"ridge push" The lithosphere thickens v distance (and time) far from the midocean ridge. This is because it cools as it moves away from the ridge and also the boundary in between the heavy lithosphere and slightly molten asthenosphere becomes deeper - the boundary in between the lithosphere and asthenosphere is essentially a temperature boundary. The an outcome of this thickenning with distance indigenous the ridge is that the lithosphere/asthenosphere boundary slopes away from the ridge. The weight of the lithosphere on this sloping surface produces a downslope force. And since the asthenosphere is weak, the load of the lithosphere close to the ridge sliding down the "slippery slope" the the asthenosphere "pushes" the older component of the key in front of it. Keep in mind that as the lithosphere slides under away native the ridge, tensional forces and also normal fault earthquakes take place at the ridge axis where two plates are sliding (pulling) apart.
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"slab pull" as lithospheric plates relocate away native midocean ridges they cool and become denser. They ultimately become an ext dense than the underlying hot mantle. ~ subducted, cool, dense lithosphere sinks right into the mantle under its very own weight. This helps to pull the rest of the bowl down through it.
resisting forces The main result of the basic mantle is to develop a shearing or frictional force resisting the movement of lithospheric plates. And also for newly subducting slabs, the resisting force of the mantle together the slab tries to pressure its means through is stronger than the shearing ~ above the top and also bottom of the diminish slab. Friction in between the converging key and additionally the pressure required to bend a plate withstand the motion of the plate at subduction zones.
It seems likely that heaviness drives the plates and the motions of the plates help to line the mantle, fairly than the convection that the mantle control the plates.
However, mantle circulation may be important locally with respect come the movement of continents with deep keels (thickened lithosphere, particularly in old orogenic belts), however not because that oceanic lithosphere.