Why subduction occurs

It happens when one lithospheric plate meets another—that is, in convergent zones —and the denser plate sinks down into the mantle. Continents are made up of rocks that are too buoyant to be carried much farther than about kilometers deep.

So when a continent meets a continent, no subduction occurs instead, the plates collide and thicken. True subduction happens only to oceanic lithosphere. When oceanic lithosphere meets continental lithosphere, the continent always stays on top while the oceanic plate subducts.

When two oceanic plates meet, the older plate subducts. Oceanic lithosphere is formed hot and thin at mid-ocean ridges and grows thick as more rock hardens underneath it. As it moves away from the ridge, it cools. Rocks shrink as they cool, so the plate becomes more dense and sits lower than younger, hotter plates. Therefore, when two plates meet, the younger, higher plate has an edge and does not sink. Oceanic plates do not float on the asthenosphere 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. 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.

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 backward 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. Where the subducting slab bends downward, a deep-sea trench forms. The deepest of these is the Mariana Trench, at over 36, feet below sea level. 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.

Employees in the News. Emergency Management. Survey Manual. EarthWords is an on-going series in which we shed some light on the complicated, often difficult-to-pronounce language of science.

Think of us as your terminology tour-guides, and meet us back here every week for a new word! Subduction occurs when an oceanic plate runs into a continental plate and slides beneath it. Cartoon showing cross section through subduction zone before an earthquake above and during an earthquake below. This also causes the overriding plate to warp in response, such that the surface of the earth goes down near the trench, and the surface of the earth goes up farther inland.

When an earthquake occurs, the locked zone ruptures, causing uplift near the trench and subsidence farther inshore. The Pacific Plate is thinner and denser, so it is being thrust underneath the North American plate. This subduction zone has generated many large, devastating earthquakes, including the second largest earthquake ever recorded: the magnitude 9.

Not only did ground-shaking and landslides associated with this earthquake devastate Anchorage and other Alaskan towns, the resulting tsunami caused deaths as far away as California.