DEFINITION OF CONVERGENT PLATE BOUNDARIES

Oceanic lithosphere is being destroyed by sinking back into

the mantle at the deep ocean trenches in a process called subduction.

As the oceanic slabs sink downward, they experience

higher temperatures that cause water and other volatiles to be

released, causing melts to be generated in the mantle wedge

overlying the subducting slab. These melts then move upward

to intrude the overlying plate, where the magma may become

contaminated by melting through and incorporating minerals

and elements from the overlying crust. Since subduction zones

are long narrow zones where large plates are being subducted

into the mantle, the melting produces a long line of volcanoes

above the down-going plate. These volcanoes form a volcanic

arc, either on a continent or over an oceanic plate, dependent

on which type of crust composes the overlying plate.

Island arcs are extremely important for understanding

the origin of the continental crust because the magmas and

sediments produced here have the same composition as the

average continental crust. A simple model for the origin of

the continental crust is that it represents a bunch of island

arcs which formed at different times and which collided during

plate collisions.

Since the plates are in constant motion, island arcs, continents,

and other terranes often collide with each other.

Mountain belts or orogens typically mark the places where

lithospheric plates have collided, and the zone that they collided

along is referred to as a suture. Suture zones are very

complex and include folded and faulted sequences of rocks

that form on the two colliding terranes and in any intervening

ocean basin. Often, slices of the old ocean floor are

caught in these collision zones (we call these ophiolites), and

the process by which they are emplaced over the continents is

called obduction (opposite of subduction).

In some cases, subduction brings two continental plates

together, which collide forming huge mountain belts like the

Himalaya Mountain chain. In continent-continent collisions,

deformation may be very diffuse and extend beyond the normal

limit of plate boundary deformation that characterizes

other types of plate interactions. For instance, the India-Asia

collision has formed the huge uplifted Tibetan Plateau, a

series of mountain ranges to the north including the Tien

Shan and Karakoram, and deformation of the continents

extends as far into Asia as Lake Baikal.

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