When this happens, the opposite edge of these plates push against other tectonic plates. Subduction occurs when two tectonic plates meet and one moves underneath the other Fig.
Oceanic crust is primarily composed of basalt, which makes it slightly denser than continental crust, which is composed primarily of granite. Because it is denser, when oceanic crust and continental crust meet, the oceanic crust slides below the continental crust.
This collision of oceanic crust on one plate with the continental crust of a second plate can result in the formation of volcanoes Fig. As the oceanic crust enters the mantle, pressure breaks the crustal rock, heat from friction melts it, and a pool of magma develops.
This thick magma, called andesite lava, consists of a mixture of basalt from the oceanic crust and granite from the continental crust. Forced by tremendous pressure, it eventually flows along weaker crustal channels toward the surface. The magma periodically breaks through the crust to form great, violently explosive composite volcanoes —steep-sided, cone-shaped mountains like those in the Andes at the margin of the South American Plate Fig.
Continental collision occurs when two plates carrying continents collide. Because continental crusts are composed of the same low-density material, one does not sink under the other. During collision, the crust moves upward, and the crustal material folds, buckles, and breaks Fig. The Himalayan Mountains were formed by the collision between Indian and Eurasian tectonic plates. Ocean trenches are steep depressions in the seafloor formed at subduction zones where one plate moves downward beneath another Fig.
These trenches are deep up to The deepest ocean trench is the Mariana Trench just east of Guam. It is located at the subduction zone where the Pacific plate plunges underneath the edge of the Filipino plate. Subduction zones are also sites of deepwater earthquakes. Transform faults are found where two tectonic plates move past each other. As the plates slide past one another, there is friction, and great tension can build up before slippage occurs, eventually causing shallow earthquakes.
People living near the San Andreas Fault, a transfom fault in California, regularly experience such quakes. Recall that some volcanoes form near plate boundaries, particularly near subduction zones where oceanic crust moves underneath continental crust Fig. However, some volcanoes form over hot spots in the middle of tectonic plates far away from subduction zones Fig. When magma erupts and flows at the surface, it is called lava. The basalt lava commonly found at hot spots flows like hot, thick syrup and gradually forms shield volcanoes.
A shield volcano is shaped like a dome with gently sloping sides. These volcanoes are much less explosive than the composite volcanoes formed at subduction zones. Some shield volcanoes, such as the islands in the Hawaiian archipelago, began forming on the ocean floor over a hot spot.
Each shield volcano grows slowly with repeated eruptions until it reaches the surface of the water to form an island Fig. Almost all of the mid-Pacific and mid-Atlantic ocean basin islands formed in a similar fashion over volcanic hot spots. Over millions of years as the tectonic plate moves, a volcano that was over the hot spot moves away, ceases to erupt, and becomes extinct Fig.
Islands can erode through natural processes such as wind and water flow. Eventually all that remains of the island is a ring of coral reef. An atoll is a ring-shaped coral reef or group of coral islets that has grown around the rim of an extinct submerged volcano forming a central lagoon Fig. Atoll formation is dependent on erosion of land and growth of coral reefs around the island.
Coral reef atolls can only occur in tropical regions that are optimal for coral growth. The main Hawaiian Islands will all likely become coral atolls millions of years into the future. The older Northwestern Hawaiian Islands, many of which are now atolls, were formed by the same volcanic hot spot as the younger main Hawaiian Islands.
This document may be freely reproduced and distributed for non-profit educational purposes. New Zealand is located on the edge of two tectonic plates, the Indo-Australian and the Pacific plates. This position makes New Zealand geologically active with frequent earthquakes, geothermal areas and volcanoes. This plate boundary has shaped New Zealand:. Watch the GNS Science animation k , showing the future shape and deformation of New Zealand if the movement measured between were to continue unchanged.
Iceland sits on top of the Atlantic ridge, the divergent boundary between the Eurasian and North American tectonic plates: As the two plates drift in opposite directions Iceland is in effect slowly being split apart.
This plate movement makes Iceland geologically active, just like New Zealand. Ready for a quiz? Try 'The Moving Crust' interactive activity. Try this Pangaea to Present activity. Convection currents caused by heating in the Earth's mantle explain how the continents move. Who came up with this theory and what is this theory called? Image: Public Domain. How do you think this affects New Zealand?
Image: USGS. Two plates push past each other along the Alpine Fault. What land feature can be seen alongside the Alpine Fault? Image: GNS Science. Skip to main content. Stress builds up and is released as an earthquake. Divergent boundaries are where the plates slide apart from each other, and the space that this creates is filled with magma and forms new crust.
This often happens below the sea, for example, the Pacific Ocean is growing wider by about 18 cm per year. Convergent boundaries are where the plates slide towards each other. Sometimes this creates mountains, for example, the collision between the Australian plate and the Pacific plate formed the Southern Alps.
When two plates under the ocean collide, they usually create an island as one plate moves beneath the other. The Solomon Islands were created this way. Scientists now think the tectonic plates have been on the move for around 3 billion years, but only 50 or 60 years ago, people thought that the continents were set in the same position forever.
Scientists try to find out how and why things work. By studying rocks, fossils and earthquakes, they came up with the new theory of plate tectonics.
Science knowledge changes when new evidence is discovered. Scientists first used fossils and other geological evidence to show that the continents are on the move. Today, they use GPS to track tectonic plate movement.
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