Plate Tectonic Boundaries: Formation and Destruction of Oceanic Crust

Divergent Plate Boundaries

Mid-Ocean Ridges

Mid-ocean ridges are high elevations located in the middle of all oceans, with a total length of about 65,000 km. Some peaks of the ridges protrude above sea level, forming volcanic islands. The central part of the ridge consists of a system of fractures that form a large groove called the rift valley. The ridges are crossed by numerous transform faults.

The Genesis of an Ocean

The formation of a ridge (dorsal), and therefore a new ocean, under a continent begins when a hot spot appears. Hot magma from the mantle rises, slightly lifting the lithosphere and forming a dome. The rigid lithosphere then breaks, forming a system of three cracks called a triple point. These cracks, called rifts, continue to expand.

Ocean Formation Process

The formation of an ocean starts when a system of parallel fractures breaks a continent, and the plates begin to separate. The lava that emerges from these fractures creates new oceanic lithosphere, laterally pushing the existing crust, which moves away on both sides. In the early stages of ocean opening, a large crack is formed. Continued plate separation leads to the formation of narrow seas, such as the Red Sea.

Convergent Plate Boundaries

Subduction Zones

Subduction zones are areas where tectonic plates are destroyed. In these regions, a denser oceanic plate is introduced beneath another plate. Numerous earthquakes occur in subduction zones. As the plate fragment descends, it sinks into the mantle. It is believed that below 700 km, the plate is completely melted, assimilated, and incorporated fully into the mantle.

When the plate sinks downward, it forms magma that ascends toward the surface, being less dense than the adjacent material. If these magmas solidify within the crust, they form plutonic rocks. If, however, they rise to the surface, they build volcanoes. When an oceanic plate subducts beneath a continental plate, a mountain range is formed.

Ocean Trenches

The area where an oceanic plate is subducting is marked by deep, oblong depressions. Trenches can reach 100 km wide, more than 1,500 km long, and thousands of meters deep.

Island Arcs

Island arcs form when one oceanic plate subducts beneath another oceanic plate. They consist of a chain of volcanic islands arranged in a circular arc, often surrounded by a deep trench. The islands of these archipelagos are formed from magma generated by the melting of the sinking plate. Frequent earthquakes also occur in these areas as the plate is fractured and deformed as it descends. Between the island arc and the nearby mainland, there is a marine basin called the back-arc basin.