E9 Planetary Geodynamics and Lithospheric Cycle

Topic

Planetary geodynamics describes the movement of tectonic plates and the evolution of the lithospheric cycle over timescales of millions of years.

The primary driver of plate tectonics is slab pull: the cold—and therefore dense—oceanic lithosphere sinking into a subduction zone pulls the rest of its plate into the mantle with a force of approximately ten trillion newtons per meter of slab.

A standard thermal model of oceanic lithosphere predicts that both the bathymetric depth of the seafloor and the thermal thickness of the lithosphere increase in proportion to the square root of its age; however, at ages exceeding eighty to one hundred million years, this gradual subsidence levels off because the oceanic lithosphere begins to behave like a plate of finite thickness resting on a base maintained at a constant temperature, rather than continuing to cool indefinitely.

The complete cycle of an ocean basin opening and closing begins with continental rifting, which subsequently generates a mid-ocean ridge; new lithosphere is created at this ridge, cools, and moves away from the ridge over time, and eventually, this same lithosphere subducts and closes the very ocean it helped open—much like the Atlantic Ocean continues to widen today while the Pacific Ocean slowly closes.

In a continental collision—such as the ongoing interaction between the Indo-Australian and Eurasian plates that has uplifted the Himalayas and the Tibetan Plateau—the continental crust doubles in thickness and develops a deep orogenic root extending into the mantle; in a state of isostatic equilibrium, for every kilometer of mountain rising above sea level, there are approximately 5.8 kilometers of root extending below the mantle reference level. Rifting is the process opposite to this continental collision: it thins the lithosphere rather than thickening it and causes subsidence instead of uplift; if the rifting persists long enough, it can culminate in the opening of a new ocean, thereby completing the full lithospheric cycle—from the breakup of a continent to the collision of two continents—with which this description of planetary geodynamics began.

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