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About Subduction

What happens when a lithospheric plate must go down

Subduction is a scientific Latin word meaning "carried under." Basically, it happens when one plate meets another—that is, in convergent zones—and one of them goes down into the mantle.

Continents are made of rocks that are too buoyant to be carried much farther than about 100 kilometers deep. So when a continent meets a continent, no subduction occurs (instead, something else starts to give way). When oceanic lithosphere meets continental lithosphere, the continent always wins and the oceanic plate subducts. That is, subduction happens only to oceanic lithosphere.

When two oceanic plates meet, the younger plate wins. Here's why. Oceanic lithosphere is formed hot and thin at the mid-ocean ridges, and as it moves away from the ridge it cools and grows thicker as more rock hardens underneath it. It also sits lower, because like all rocks it shrinks as it cools. With age, the cold oceanic lithosphere becomes denser than the hot asthenosphere beneath. Therefore when two plates meet the younger, higher plate has an edge.

It's a mistake to picture oceanic plates as floating like ice on water—they are more like sheets of paper on water, ready to sink as soon as one edge can start the process. They are gravitationally unstable.

Once a plate begins to subduct, gravity takes over. A descending plate is usually referred to as a "slab". In the western Pacific where very old seafloor is being subducted, the slab falls almost straight down. In the eastern Pacific where very young plates are being subducted, the slab descends at a shallow angle. (Another theory explains this through motions of the underlying mantle, not slab density.) Subduction, in the form of gravitational "slab pull," is thought to be the largest force driving plate tectonics.

At a certain depth the high pressure turns the basalt in the slab to a denser rock, eclogite (that is, a feldspar-pyroxene mixture becomes garnet-pyroxene). This makes the slab even more eager to descend.

U.S. Geological Survey image
Where the subducting slab bends downward, a deep-sea trench forms. Trenches capture a lot of sediment from nearby land masses, much of which is carried down along with the slab. In about half the world's trenches, some of that sediment is instead scraped off. It remains on top as a wedge or prism of material, like snow in front of a plow, and slowly the trench is pushed offshore as the upper plate grows by accretion.

Once subduction begins, the materials on top of the slab—sediments, water, delicate minerals—are carried down with it, then the water, thick with dissolved minerals, rises into the upper plate. There this chemically active fluid enters an energetic cycle of volcanism and tectonic activity, sometimes called the subduction factory. The rest of the slab keeps descending and leaves the realm of plate tectonics. (more on the death of plates)

It's a mistake to picture subduction as a sumo match, a battle of plates in which the top plate forces the lower one down. In many cases it's more like jiu-jitsu: the lower plate is actively sinking as the bend along its front edge works backwards (slab rollback), so that the upper plate is actually sucked over the lower plate. This explains why there are often zones of stretching, or crustal extension, in the upper plate at subduction zones.

Back to Plate Tectonics in a Nutshell

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