Plate Tectonics: Divergent, Convergent & Transform Boundaries

Divergent Plate Boundaries

Most divergent boundaries, where plate expansion occurs, are located along mid-ocean ridges. Here, as plates move away from the ridge axis, fractures are created and immediately filled with molten rock rising from the hot asthenosphere below. Through the continuous expansion of the plates and the ascent of magma, new oceanic crust (lithosphere) is added between the diverging plates. This crustal extension is accompanied by alternating episodes of fault formation and volcanism. Adjacent to the expansion axis, crustal blocks are bounded by faults, forming elongated valleys known as rift valleys. As rifting continues, the rift valley lengthens and deepens, eventually expanding into an ocean. At this point, the valley will become a narrow, linear sea, similar to the current Red Sea.

Convergent Plate Boundaries

Since the Earth's surface area remains constant, lithosphere must also be consumed. Convergent boundaries are zones where plates come together, and the lithosphere is subducted and absorbed into the mantle. When two plates converge, the leading edge of one bends downward, allowing it to descend into the asthenosphere. The region where an oceanic plate descends is called a subduction zone. As the oceanic plate slides beneath the other, it bends, producing a deep-sea trench.

Ocean-Continent Convergence

When an oceanic plate descends beneath a continental plate, part of the sediment it carries, as well as fragments of oceanic crust, separate and attach to the edge of the continental plate. This chaotic accumulation of deformed sediments and pieces of oceanic crust is called an accretionary wedge. When the descending plate reaches a depth of about 100 to 150 kilometers, heat drives water and other volatile components from the subducted sediments into the overlying mantle. The partial melting of mantle rock generates magmas less dense than the surrounding mantle rocks, which ascend due to buoyancy. This magma accumulates beneath the overlying continental crust. A portion of this silica-rich magma can migrate to the surface, giving rise to volcanic eruptions. Mountains formed partly by the volcanic activity associated with the subduction of oceanic lithosphere are called continental volcanic arcs.

Ocean-Ocean Convergence

When two oceanic plates converge, one descends beneath the other, initiating volcanic activity similar to what occurs at an ocean-continent convergent boundary. In this case, volcanoes form on the ocean floor rather than on a continent. If this activity persists, it will eventually build structures that emerge as islands. This newly formed volcanic land is called a volcanic island arc.

Continent-Continent Convergence

When two converging plates carry continental crust, neither plate will subduct beneath the other due to the low density and buoyant nature of continental rocks. The result is a collision between the two continental blocks. This process folds and deforms the sediments accumulated along the continental margins as if placed in a giant vise. The result is the formation of a new mountain range composed of deformed and metamorphosed sedimentary rocks, fragments of island arcs, and possibly fragments of oceanic crust.

Transform Plate Boundaries (Faults)

These boundaries are characterized by horizontal displacement faults (transform faults), where plates move alongside one another without producing or destroying lithosphere. Most transform faults connect two segments of a mid-ocean ridge. They are part of prominent break lines in the oceanic crust known as fracture zones, which include both the active transform faults and their inactive extensions within the plates. These fracture zones are located approximately every 100 km along the direction of the ridge axis.