why do oceanic plates subduct under continental plates

When two oceanic
plates collide one oceanic plate is eventually subducted under the other. Where one plate slides under the other is referred to as the 'subduction
zone'. As the subducting plate descends into the mantle where it is being
gradually heated a benioff zone is formed. This benioff zone is a zone of
shallow,intermediate and deep focused earthquakes. Some deep focused earthquakes
that occur at ocean ocean- collision boundaries can be as deep as 670 kilometres. As the subducted plate descends into the mantle it is gradually heated allowing
the formation of magma. The magma that forms is andesitic in composition
and begins to form when the subducted plate reaches a depth of 100 kilometres. This andesitic magma is formed from the partial melting of the asthenosphere
just above the subduction zone. This partial melting of the subducting plate
is due to the loss of water as it descends into the mantle. The andesitic
magma is now less dense than the surrounding material so it rises through
the crust and erupts to form an arc of volcanoes called an island arc. The
distance between the trench and the island arc depends greatly upon where
the subducting plate reaches the 100 kilometer depth.

If the subduction angle
is steep then the distance between the arc and the trench will be short. If
the suduction angle is shallow the distance is longer. The main features
are indicated in the diagram below. The swell is seen by a bulge in the in
the downgoing plate where it is subducted into the mantle. where the plate
subducts into the mantle is known as the trench. the forearc ridge contains
highly deformed sedimentary and metemorphic rock. The backarc region is located
behind the arc and can be compressed or extended.
1. Divergent boundaries There are places on earth where two plates are separating or spreading apart, such as at oceanic ridges. Rift valleys and occur when the lithosphere is under tensional stress. At spreading zones, new magma comes up from the mantle, pushing two plates apart and adding new material at their edges. Spreading zones are usually found in oceans along with mid-ocean ridges. For example, the North American and Eurasian plates are spreading apart along the mid-Atlantic ridge. As the new material flows out of the ridge, it pushes the existing ground floor out, until it eventually sinks under another plate, which leads us into a different type of boundary.

Earthquakes with low Richter along boundaries with normal fault motion tend to be shallow focus. These quakes can have focal depths of less than 20km. This indicates the brittle lithosphere must be thin along the diverging plate boundaries. 2. Transform boundaries These are found where plates slide past one another. The San Andreas Fault is an example of a transform -fault plate boundary along the north western Mexican and California coast. Earthquakes along transform boundaries show strike-slip motion on the faults, they form fairly straight linear patterns and tend to be shallow focus earthquakes with depths usually less than about 100 km. Richter magnitudes could be large. As seen in the image above, the trees (they look like small dots) in the aerial view of San Andreas fault have been offset by the slipping of the plates. The North American Plate to the right and the Pacific Plate to the left. 3. Convergent boundaries Convergent boundaries are the place where two tectonic plates converge (i. e. two plates move toward each other).

These zones tend to be where compressional stresses are active and this results in thrust or reverse faults being common. Converging plate boundaries are of two types: occur where oceanic lithosphere is pushed beneath continental or oceanic lithospheres. Where two plates converge at an oceanic trench a subduction boundary is formed as cold oceanic lithospheres are pushed back down into the mantle. This happens because the oceanic plate is denser than the continental plate so, as they move together, the oceanic plate is forced underneath the continental plate. In this case, one plate overrides, or "subducts" the other, pushing it slowly downward into the mantle where it melts to form magma. A subducted lithosphere remains cold and brittle as it descends and can fracture under compressional stress. These fractures generate earthquakes that define a zone of quakes at increasing focal depth under the overriding plate. This zone is called the. Depths of up to 700km are reached in the Benioff Zone. Examples of subduction zones are found along the northwest coast of the United States, Mexico, western Canada, southern Alaska, South America, Central America, Japan, Philippines, Caribbean Islands and the Aleutian Islands. where two plates of continental lithosphere collide result in fold-thrust mountain belts.

The continental crust is squashed together as the plates push together and is forced upwards. This is called folding. are created by this process of folding. Where two continental plates converge and push towards each other fold mountains can also be formed. This is how mountain ranges such as the Himalayas and the Alps were formed. Earthquakes occur due to the thrust faulting and range in depth from shallow to about 200 km. Examples are found along the Himalayan Belt into China, along the Northern edge of the Mediterranean Sea through Black Sea and Caspian Sea into Iraq and Iran. Convergent boundary zones are characterized by deep-ocean trenches, shallow to deep earthquakes, and mountain ranges containing active volcanoes. In general, where an oceanic and a continental plate collide, the denser oceanic plate will be forced under (subduction) the other.

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