Convergent Boundaries


Converging plates contain either oceanic or continental crust at their leading edge.

When oceanic crust encounters continental crust the more dense oceanic crust is subducted (Fig. 17.5B).

When ocean crust encounters ocean crust, the older (and denser) crust is subducted. If this crust is extremely old (i.e., early Cretaceous/late Jurassic), the subduction angle is steep and the subducting plate appears to "peal away" from the overriding plate causing extension in the volcanic arc region and the formation of a back-arc basin spreading center.

When two continental plates collide, partial subduction of leading oceanic crust is accompanied by intense deformation, uplift, and eventual suturing of segments of the subducted plate to the base of the overriding plate (Fig. 17.5C).

A subducted plate is comparatively cold and rigid when it starts to descend, but it warms and softens as it moves downward, but undergoes mineralogic changes.  However, subducted plates have been imaged reaching the 660 km discontinuity between the upper and lower mantle or even to the core-mantle boundary.

Earthquakes
Most of the world’s major earthquakes occur at convergent plate boundaries and the zone along which most occur is the Benioff Zone.
Faulting that generates the earthquakes is both along the boundary between the two lithospheric plates and within the descending slab itself.
The earthquake foci define the descending slab.

Volcanic Arcs
Volcanic (or magmatic) arcs are found at convergent plate boundaries and can form either on continental crust (e.g., Andes, Cacades) or oceanic crust (e.g., Japan, Aleutians).
Volcanoes usually occur 100 to 300 km away from the trench that marks the position of the subduction zone at the surface, and are located on the same side of the trench as the earthquake epicenters. The exact position of the volcanic arc relative to the trench depends upon subduction angle.
Fluids generated by the dehydration of the down-going slab percolate into the overlying (and hotter) mantle wedge and promote melting.  The slab itself may undergo some melting. These melts migrate to the surface and are erupted along the volcanic arc or formed intrusions in the crust.