Rock Cycle and Plate Tectonics Not Directly Linked by Rock Weight

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The plate tectonic cycle produces chemically distinct mid-ocean ridge basalts and arc volcanics, with the latter enriched in elements such as Ba, Rb, Th, Sr and Pb and depleted in Nb owing to the water-rich flux from the subducted slab. Basalts from back-arc basins, with intermediate compositions, show that such a slab flux can be transported behind the volcanic front of the arc and incorporated into mantle flow.

Data on high-pressure mineralogical phase assemblages (4) show that mid-ocean ridge basaltic composition is significantly denser than harzburgite down to 660 km depth but less dense than harzburgite and ambient mantle from 660 to 720 km depth. This density crossover at 660 km depth is thought to promote basalt accumulation near the base of the mantle transition zone (MTZ), but the high viscosity of cold and stiff subducted slabs may prevent efficient separation (segregation) of the basaltic crust from the subducting slab (5, 6).

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As the oceanic plate moves farther and farther away from the active, hot spreading ridge, it gradually cools down. The colder the plate gets, the denser (“heavier”) it becomes. Eventually, the edge of the plate that is farthest from the spreading ridges cools so much that it becomes denser than the asthenosphere beneath it. As you know, denser materials sink, and that’s exactly what happens to the oceanic plate—it starts to sink into the asthenosphere!

The uplift and sinking of land, ... of interactions and stress due to the movement of the plates. Plate motion may seem slow, but over millions of years plate tectonics shapes the distribution of continents and oceans and mountain ranges that shape diverse ecosystems and influence global climate. This Earth system model is one way to represent the essential processes that are related to plate tectonics, including Earth’s internal heat and the rock cycle...

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Conversely, dense oceanic crust does not “float” as high—forming lower-lying ocean basins. As oceanic crust cools, it becomes denser and ultimately sinks back into the mantle under its own weight after about 200 million years.

Larger particles like rocks and sand settle out very near the shore while smaller particles settle out further away. Since small particles sink slowly, ocean currents can transport lithogenous sediments over a long distance. Small particles (< 4 micrometers) known as abyssal clay make up a large portion of the sediment on the ocean floor. Prior to the theory of plate tectonics, early scientists suggested that since continental erosion was continually taking place, lithogenous sediments should constantly be filling in ocean basins resulting in a very thick layer of sediment.