The subduction zone of the Juan de Fuca plate and the North American plate.

A subduction zone is an area in the lithosphere where one tectonic plate dives (subducts) under another. This happens when two plates converge and one of the two has a higher density than the other and is therefore heavier. At least the heavier of the plates is oceanic. The heavier plate then slips under the lighter one, usually creating an oceanic trench right at the plate boundary and a volcanic arc on the lighter plate parallel and close to the boundary. If two continental plates collide, as for instance along the northern boundary of the Indian plate, no subduction will happen. The subducting plate forms a Wadati-Benioff zone with unusual geothermal behavior. Frequent earthquakes occur along subduction zones mostly on or below the lighter plate, with hypocenters getting deeper the further away from the plate boundary the earthquake hits.