M1 Plate Tectonics

Topic

Plate tectonics is the dominant dynamic regime of the solid Earth: its rigid lithosphere is divided into approximately fifteen major plates that move over a viscous asthenosphere at speeds ranging from one to twenty centimeters per year. This constant movement redistributes mass, heat, and stress on a planetary scale, and ultimately controls where earthquakes occur, where volcanism emerges, where mountain ranges rise, and where ocean basins form.

The primary driver of this movement combines three distinct forces: the pull of the cold, dense subducting slab, which drags the rest of its plate into the mantle; the push from the mid-ocean ridge, where the elevation laterally pushes the rest of the plate originating there; and the viscous drag exerted by the convecting mantle itself on the base of the plate. The balance of these three forces acting on each plate determines both its speed and direction of movement; this movement can be described as the rotation of the entire plate around a fixed pole of rotation at a constant angular velocity, provided the driving forces remain unchanged.

At mid-ocean ridges, the adiabatic upwelling of hot mantle material generates new basaltic crust at rates ranging from two to eighteen centimeters per year, depending on the specific ridge.

In subduction zones, the oceanic plate—being denser than the continental plate or the other oceanic plate it collides with—sinks beneath that neighboring plate; it is precisely this subduction process that generates the majority of the planet's seismic activity, as well as the volcanism characteristic of a volcanic arc. The expected behavior of this entire system is the global conservation of the total plate area: the rate at which new crust is created at mid-ocean ridges equals, on average, the rate at which old crust is destroyed in subduction zones, thereby closing the complete cycle of lithosphere creation and destruction through which plate tectonics redistributes mass, heat, and stress across the planet, just as was outlined at the beginning of this description.

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