Subduction

See related animations:
Seafloor spreading
Other animations related to plate tectonics

Animation Source:
Earthguide at Scripps Institution of Oceanography

Key points

Subduction is a geological process in which the edge of a lithospheric plate slides underneath the edge of an adjacent plate.
Subduction is one of the two major processes of plate tectonics, the other being seafloor spreading.
Trenches, accretionary wedges (prisms) and volcanic or island arcs are key surface features produced by subduction.
A subduction zone is the entire area of subduction between the trench and the volcanic arc.
Depending on the particular subduction zone, the angle of descent varies. This affects the distance between the trench and volcanic or island arcs.
The trench marks the line where subduction begins.
The place where subduction begins can move backward relative to the motion of the subducting plate, if the overriding plate moves forward.
Subduction allows oceans to close (get smaller) even as it grows new seafloor at the same time. The Pacific is concurrently closing overall and growing new seafloor at the East Paicific Rise. The Atlantic is growing in overall area and growing new seafloor.
As a descending lithospheric plate warms up at depth, it will eventually lose its ability to sustain the earthquakes that allow us to distinguish that plate from the rest of the mantle.
Trenches are the very narrow and deepest features of the Earth's surface that mark the place where plates begin their descent. Many occur unexpectedly close to the edges of continents.
Subduction produces lines of volcanoes over the subducting plate, parallel to the trench and perpendicular to the direction of plate motion. The types of volcanoes associated with subduction are composite volcanoes, also called stratovolcanoes They occur high in elevationl as parts of major mountain ranges. Volcanic activity is violent and hazardous. Volcanic activity continues for the duration of subduction over very long periods of geologic time. The volcanoes associated with subduction produce many kinds of igneous rocks, but andesite and diorite are special associations.
When the volcanic mountain ranges produced by subduction occur under the edges of continents, they are called volcanic arcs. Volcanic ranges produced by subduction under the edges of oceanic crust are called island arcs. The Andes and Cascades are volcanic arcs. Japan and the Philippines are island arcs.
Accretionary wedges (prism) are folded and faulted piles of material that have been scraped off the tops of the subducting lithospheric plates.

Questions for thought

What subducts - lithosphere, crust, or mantle?
Why is there much less subduction around the Atlantic than the Pacific?
What is a typical rate of subduction?