When two plates converge, one of them, the one with the highest density, subducts under the other. As a result, ocean trenches are produced , large depressions on the seabed, which can reach a depth of more than 11,000 meters below sea level.
An example of an oceanic trench is the Peruvian-Chilean trench, as a result of the collision between the South American continental plate and the Nazca oceanic plate.
As a result of the friction between the plates, the subducting plate generates earthquakes associated with the subduction plane, called the Benioff plane.
When one plate subducts under another, in addition to producing an oceanic trench, part of the lithosphere merges and a volcanic island arc originates, as for example occurs in many archipelagos in the Pacific Ocean.
If one of the plates that converges drags continental crust, between the volcanic arc that forms and the coast of the continent, a marginal sea will remain, as is the case of the Sea of Japan.
Sometimes, marine sediments accumulate in the pit until they cover it, forming an accretion prism that later becomes part of an orogen next to the island arch.
The main geological processes linked to ocean trenches are:
- Seismicity. The friction between the plates causes numerous earthquakes.
- Volcanism. The subducting plate partially melts, and the ejected magmas originate arcs from volcanic islands, such as the Aleutians, the Philippines or Japan.
- Formation of orogens. When one of the two plates drags a continent, the accretion prism and the volcanic arc join together forming a marginal orogen like the Andes. If the two plates carry continental crust, when the two continents collide, a colliding orogen like the Himalayas will form.