Oceanic Plate Boundaries: Constructive, Transform, and Destructive

Oceanic Plate Boundaries: A Comprehensive Look

Constructive Boundaries (Mid-Ocean Ridges)

Constructive boundaries are dorsal oceanic ridges. These are fracture zones where hot mantle material rises to the surface, causing intense volcanic activity. This volcanism produces large volumes of basalt, creating new oceanic crust. Convection currents produce divergent forces (rifting) that tend to separate the two sides of the fracture. Magma rises from within, forming the ridge's relief. The depression between the edges, occupied by the fracture, is called a Rift. In the Rift area, the crust is thin and heavily fractured.

Transform Fault Boundaries

Transform faults are discontinuous, winding ridges. Often, a ridge is interrupted and continues a few miles further to the left or right. In this case, the extension of the ocean floor creates a shear zone called a transform fault. The main feature of transform faults is active shear motion, leading to strong seismicity.

Destructive Boundaries (Subduction Zones)

In subduction zones, the oceanic lithosphere bends and sinks into the mantle. These are areas where oceanic lithosphere is destroyed. The subducting oceanic plate can be beneath either oceanic (Alpine) or continental (Andean) lithosphere. Subduction destroys lithosphere and causes intense seismicity. Magmatism is produced by melting of the basalt plate at the subducting plate's thickness. This creates a volcanic mountain range (Alpine orogenic belt) or a volcanic island arc (Andean). Metamorphism occurs due to increasing pressure.

Collision Boundaries (Orogens)

Collision occurs between two continental plates, and one of them overrides the other, eventually stopping the motion. Geological processes include the thickening of the continental lithosphere. Sediments accumulated between the two plates before the collision are deformed, broken, and piled onto the suture zone, forming a collision orogen. There is an isostatic rise of the orogen due to the push from the mantle on the thickened lithosphere. Compression due to shock and friction produces magmatism. Metamorphism and collision cause seismicity.