Discovery Science: Earth – Plate tectonics – Plate Boundaries

Earth Science: Plate Boundaries

For new ocean crust to continue to form, older parts of the crust must first be destroyed. This happens when tectonic plates collide with each other.

When lithospheric plates meet each other, one plate can slide below the other in a process called subduction. This process, and the changes in the suface associated with it, happens at different rates. The East Pacific Ridge, for example, grows five inches (12 cm) each year, but the Mid-Atlantic Ridge grows only two inches (five cm). Areas where the plates drift away from each other are called divergent plate boundaries, and areas where they collide are called convergent plate boundaries.

The slower the drift of an oceanic plate, the cooler and heavier it becomes. When plates collide, the heavier plate is subducted. The rocks and sediments deposited on the plate melt in the upper mantle, forming magma that rises to the surface and flows from volcanoes as lava. Volcanoes that lie above sea level are visible as chains of islands bending in the direction of the descending plate.

An oceanic trench, the point of contact of the plates, is a part of every island arc. The less extreme the angle of descent of the subducting plate, the farther away the melting zone is, forming oceanic trenches and island arcs long distances from each other, and flattening the channel between them. The greater the angle of descent, the faster subduction takes place, creating an oceanic trench close to the island arc.

On a collision course

Ocean crusts are composed chiefly of rocks, such as basalt and gabbro, making them heavier than continental crusts, which are primarily composed of granite and gneiss. Because of this, when plate tectonic movements cause an oceanic plate to collide with a continental plate, the oceanic plate is usually subducted, causing earthquakes and volcanic eruptions in the subduction zone.

When two continental plates collide, however, subduction does not take place. Instead the plates push against each other and buckle upward, forming mountains at the point where they meet.

Shifting with consequences

Sometimes neither subduction nor collision takes place. Instead plates slide against each other in a motion called shearing. In shearing zones, the plates can interlock, causing earthquakes as they tear free from each other.

The most well-known shear zone is the San Andreas Fault, but the lines of fracture in the mid-ocean ridges are also sites of frequent earthquake activity.


HOT SPOTS are points of volcanic activity located in the center of the Pacific Ocean. They are not situated on a tectonic boundary; rather, the Pacific plate skims over the top of the hot spots.

Nonetheless, magma flows constantly rise from a depth of around 10,000 feet (3.000 m) as a result of a mantle plume—an area of unusually hot rock in the mantle. The regular lava flows continue to form islands, including the Hawaiian and Galapagos Islands.