A mature island arc develops into a chain of relatively large islands (such as Japan or Indonesia) as more and more volcanic material is extruded and sedimentary rocks accumulate around the islands. The magma, which is lighter than the surrounding mantle material, rises through the mantle and the overlying oceanic crust to the ocean floor where it creates a chain of volcanic islands known as an island arc. It mixes with the overlying mantle, and the addition of water to the hot mantle lowers the crust’s melting point and leads to the formation of magma (flux melting). The significant volume of water within the subducting material is released as the subducting crust is heated. The subducted lithosphere descends into the hot mantle at a relatively shallow angle close to the subduction zone, but at steeper angles farther down (up to about 45°). There is commonly an ocean trench along the boundary as the crust bends downwards. Often it is the older and colder plate that is denser and subducts beneath the younger and warmer plate. The types are ocean-ocean, ocean-continent, and continent-continent.Īt an ocean-ocean convergent boundary, one of the plates (oceanic crust and lithospheric mantle) is pushed, or subducted, under the other (Figure 4.6.1). The San Andreas Fault is one of the best examples of lateral plate motion.Modified from "Physical Geology" by Steven Earle*Ĭonvergent boundaries, where two plates are moving toward each other, are of three types, depending on the type of crust present on either side of the boundary - oceanic or continental. When two tectonic plates slide past each other, the place where they meet is a transform or lateral fault.
This can lead to the formation of huge, high mountain ranges such as the Himalayas. Since neither plate is stronger than the other, they crumple and are pushed up. About 80% of earthquakes occur where plates are pushed together, called convergent boundaries.Īnother form of convergent boundary is a collision where two continental plates meet head-on. Sometimes the molten rock rises to the surface, through the continent, forming a line of volcanoes. The rocks pulled down under the continent begin to melt. Subduction causes deep ocean trenches to form, such as the one along the west coast of South America. When a continental plate meets an oceanic plate, the thinner, denser, and more flexible oceanic plate sinks beneath the thicker, more rigid continental plate. The Great Rift Valley in Africa, the Red Sea and the Gulf of Aden all formed as a result of divergent plate motion.Ĭonvergent (Colliding): This occurs when plates move towards each other and collide.
The earthquakes that occur along these zones, called spreading centers, are relatively small. Molten rock from the mantle erupts along the opening, forming new crust.
The three main types of plate movements include:ĭivergent (Spreading):This is where two plates move away from each other. The movements of the plates help shape the geological features of our planet.
Other plates include continents, and some plates include both continents and ocean. Some of the plates have ocean water above them. When the plates finally give and slip due to the increased pressure, energy is released as seismic waves, causing the ground to shake. Most seismic activity occurs at three types of plate boundaries-divergent, convergent, and transform.Īs the plates move past each other, they sometimes get caught and pressure builds up. Movement in narrow zones along plate boundaries causes most earthquakes.
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