Continental — Continental collision boundaries. Does not always form a subduction zone. Can produce mountains. Transform boundaries. Plates slide past one another. No new crust is created. No crust is destroyed. Transform faults - San Andreas. Most are in oceanic crust. Breakup began about million years ago. America and Africa separated between and million years ago.
Africa and S. This is known as a transform plate boundary. As the plates rub against each other, huge stresses can cause portions of the rock to break, resulting in earthquakes. Places where these breaks occur are called faults. A well-known example of a transform plate boundary is the San Andreas Fault in California.
Virgin Islands. The fact that the plates include both crustal material and lithospheric mantle material makes it possible for a single plate to be made up of both oceanic and continental crust. Similarly the South American Plate extends across the western part of the southern Atlantic Ocean, while the European and African plates each include part of the eastern Atlantic Ocean.
The Pacific Plate is almost entirely oceanic, but it does include the part of California west of the San Andreas Fault. Divergent boundaries are spreading boundaries, where new oceanic crust is created from magma derived from partial melting of the mantle caused by decompression as hot mantle rock from depth is moved toward the surface Figure Most divergent boundaries are located at the oceanic ridges although some are on land , and the crustal material created at a spreading boundary is always oceanic in character; in other words, it is mafic igneous rock e.
Some of the processes taking place in this setting include:. Spreading is hypothesized to start within a continental area with up-warping or doming related to an underlying mantle plume or series of mantle plumes.
When a series of mantle plumes exists beneath a large continent, the resulting rifts may align and lead to the formation of a rift valley such as the present-day Great Rift Valley in eastern Africa. It is suggested that this type of valley eventually develops into a linear sea such as the present-day Red Sea , and finally into an ocean such as the Atlantic. It is likely that as many as 20 mantle plumes, many of which still exist, were responsible for the initiation of the rifting of Pangea along what is now the mid-Atlantic ridge see Figure Convergent 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.
The types are ocean-ocean, ocean-continent, and continent-continent. At an ocean-ocean convergent boundary, one of the plates oceanic crust and lithospheric mantle is pushed, or subducted , under the other. Often it is the older and colder plate that is denser and subducts beneath the younger and hotter plate.
There is commonly an ocean trench along the boundary. As discussed in the context of subduction-related volcanism in Chapter 4, the significant volume of water within the subducting material is released as the subducting crust is heated. 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.
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. As described above in the context of Benioff zones Figure The largest earthquakes occur near the surface where the subducting plate is still cold and strong.
To watch a simulated fly-by along New Zealand's plate boundary check out this video. Subduction zones occur when one or both of the tectonic plates are composed of oceanic crust. The denser plate is subducted underneath the less dense plate. The plate being forced under is eventually melted and destroyed. Where oceanic crust meets ocean crust Island arcs and oceanic trenches occur when both of the plates are made of oceanic crust.
Zones of active seafloor spreading can also occur behind the island arc, known as back-arc basins. These are often associated with submarine volcanoes. Where oceanic crust meets continental crust The denser oceanic plate is subducted, often forming a mountain range on the continent.
The Andes is an example of this type of collision.
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