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The largest earthquakes occur near the surface where the subducting plate is still cold and strong. At an ocean-continent convergent boundary, the oceanic plate is pushed under the continental plate in the same manner as at an ocean-ocean boundary. Sediment that has accumulated on the continental slope is thrust up into an accretionary wedge, and compression leads to thrusting within the continental plate Figure The mafic magma produced adjacent to the subduction zone rises to the base of the continental crust and leads to partial melting of the crustal rock.
The resulting magma ascends through the crust, producing a mountain chain with many volcanoes. A continent-continent collision occurs when a continent or large island that has been moved along with subducting oceanic crust collides with another continent Figure The colliding continental material will not be subducted because it is too light i.
There is tremendous deformation of the pre-existing continental rocks, and creation of mountains from that rock, from any sediments that had accumulated along the shores i. Examples of continent-continent convergent boundaries are the collision of the India Plate with the Eurasian Plate, creating the Himalaya Mountains, and the collision of the African Plate with the Eurasian Plate, creating the series of ranges extending from the Alps in Europe to the Zagros Mountains in Iran.
The Rocky Mountains in B. Transform boundaries exist where one plate slides past another without production or destruction of crustal material. As explained above, most transform faults connect segments of mid-ocean ridges and are thus ocean-ocean plate boundaries Figure Some transform faults connect continental parts of plates.
Transform faults do not just connect divergent boundaries. For example, the Queen Charlotte Fault connects the north end of the Juan de Fuca Ridge, starting at the north end of Vancouver Island, to the Aleutian subduction zone. The boundary between the two plates is the Nootka Fault, which is the location of frequent small-to-medium earthquakes roughly up to magnitude 5 , as depicted by the red stars.
Explain why the Nootka Fault is a transform fault, and show the relative sense of motion along the fault with two small arrows. See Appendix 3 for Exercise As originally described by Wegener in , the present continents were once all part of a supercontinent, which he termed Pangea meaning all land. More recent studies of continental matchups and the magnetic ages of ocean-floor rocks have enabled us to reconstruct the history of the break-up of Pangea.
During the same period, the Atlantic Ocean began to open up between northern Africa and North America, and India broke away from Antarctica. To see the timing of these processes for yourself, go to time lapse of Continental Movements. Within the past few million years, rifting has taken place in the Gulf of Aden and the Red Sea, and also within the Gulf of California. Over the next 50 million years, it is likely that there will be full development of the east African rift and creation of new ocean floor.
Eventually Africa will split apart. There will also be continued northerly movement of Australia and Indonesia. The western part of California including Los Angeles and part of San Francisco will split away from the rest of North America, and eventually sail right by the west coast of Vancouver Island, en route to Alaska.
Because the oceanic crust formed by spreading on the mid-Atlantic ridge is not currently being subducted except in the Caribbean , the Atlantic Ocean is slowly getting bigger, and the Pacific Ocean is getting smaller. If this continues without changing for another couple hundred million years, we will be back to where we started, with one supercontinent. Pangea, which existed from about to Ma, was not the first supercontinent. It was preceded by Pannotia to Ma , by Rodinia 1, to Ma , and by others before that.
In , Tuzo Wilson proposed that there has been a continuous series of cycles of continental rifting and collision; that is, break-up of supercontinents, drifting, collision, and formation of other supercontinents. The eastern margins of North and South America and the western margins of Europe and Africa are called passive margins because there is no subduction taking place along them. Image: From www. The effect of the collision of the two plates deforms the leading edge of the South American Plate by folding the rocks.
This crustal shortening increases the vertical thickness whilst reducing the width of the lithosphere in the collision zone imagine a car hitting a solid wall and so produces the fold mountains of the Andes. Continued subduction of the Nazca Plate brings sea water, locked in the ocean crust, deep into the mantle. As the plate heats up the water is liberated, lowering the melting point of the mantle and causing partial melting.
This produces magma, which rises and may be erupted explosively as andesite at the surface. Andesitic magma is less dense than the surrounding material, and can have a temperature of o C.
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