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. The backarc region is located behind the arc and can be compressed or extended. Ocean-ocean collision zone. When an oceanic and a continental plate collide, eventually the oceanic plate is subducted under the continental plate due to the high density of the oceanic plate.
Once again a benioff zone forms where there are shallow intermediate and deep focus earthquakes. As the oceanic plate is subducted sediment is scraped off to form an acretionary wedge at the point of collision between the two plates. When the oceanic plate is subducted due to partial melting of the asthenosphere magma with an andesitic composition is formed. The magma formed is less dense than the surrounding material so it rises to the surface to form a magmatic arc on the edge of the continent which the oceanic plate is subducted under.
Over time the continental margin, due to compression forms into a folded mountain belt. As time goes on the hot magma rising upward from the subduction zone causes further compression of the mountain belt. Deep mountain roots form and are gradually metamorphosed and intruded with granitic plutons. Explosive volcanic activity is commonly associated with this type of collision boundary.
This is shown in the diagram below. This process repeats thousands of times, bring the magma towards the surface. A volcano will form if the magma reaches the surface.
When magma does reach the surface it is then called lava. You will learn more about volcanoes in the following lessons. As the volcano erupts it may build a mountain. The lava along with ash and other pyroclastic material will continue to build the mountain higher with each eruption. This is a cross section of the Earth in the Southern Hemisphere. The map shows a subduction zone that has created the Peru-Chile Trench at the western edge of South America.
This subduction zone has produced the Andes Mountains which run along the entire west coast of South America. It also shows you the Mid-Atlantic Ridge which is spreading the Atlantic Ocean making it wider and wider. The cross section shows two processes at work;. The pink lines on this map of the Pacific Ocean represent deep ocean trenches.
These trenches are some of the lowest points on the crust of the Earth. Marianas Trench north of New Guinea is the deepest point on the Earth's surface at 36, feet below sea level. Marianas Trench is 7, feet deeper than Mount Everest is high!!!!
Trenches surround almost all of the Pacific Ocean. There are trenches wherever continental plates and oceanic plates collide. The Java Trench in the Indian Ocean is the deepest point of that ocean at 24, feet below sea level. Write the answers to the following questions in complete sentences on a piece of paper. Use the page titles located directly under the questions to move your way through the lesson to locate the answers.
When you finish the questions click on the Earth icon to return the program to the beginning. In your own words explain what happens at a subduction zone. Geologists believe that, if spreading continues, the three plates that meet at the edge of the present-day African continent will separate completely, allowing the Indian Ocean to flood the area and making the easternmost corner of Africa the Horn of Africa a large island. The size of the Earth has not changed significantly during the past million years, and very likely not since shortly after its formation 4.
The Earth's unchanging size implies that the crust must be destroyed at about the same rate as it is being created, as Harry Hess surmised. Such destruction recycling of crust takes place along convergent boundaries where plates are moving toward each other, and sometimes one plate sinks is subducted under another.
The location where sinking of a plate occurs is called a subduction zone. The type of convergence -- called by some a very slow "collision" -- that takes place between plates depends on the kind of lithosphere involved.
Convergence can occur between an oceanic and a largely continental plate, or between two largely oceanic plates, or between two largely continental plates. If by magic we could pull a plug and drain the Pacific Ocean, we would see a most amazing sight -- a number of long narrow, curving trenches thousands of kilometers long and 8 to 10 km deep cutting into the ocean floor. Trenches are the deepest parts of the ocean floor and are created by subduction.
Off the coast of South America along the Peru-Chile trench, the oceanic Nazca Plate is pushing into and being subducted under the continental part of the South American Plate. In turn, the overriding South American Plate is being lifted up, creating the towering Andes mountains, the backbone of the continent.
Strong, destructive earthquakes and the rapid uplift of mountain ranges are common in this region. Even though the Nazca Plate as a whole is sinking smoothly and continuously into the trench, the deepest part of the subducting plate breaks into smaller pieces that become locked in place for long periods of time before suddenly moving to generate large earthquakes. Such earthquakes are often accompanied by uplift of the land by as much as a few meters. On 9 June , a magnitude This earthquake, within the subduction zone between the Nazca Plate and the South American Plate, was one of deepest and largest subduction earthquakes recorded in South America.
Fortunately, even though this powerful earthquake was felt as far away as Minnesota and Toronto, Canada, it caused no major damage because of its great depth. Oceanic-continental convergence also sustains many of the Earth's active volcanoes, such as those in the Andes and the Cascade Range in the Pacific Northwest.
The eruptive activity is clearly associated with subduction, but scientists vigorously debate the possible sources of magma: Is magma generated by the partial melting of the subducted oceanic slab, or the overlying continental lithosphere, or both? As with oceanic-continental convergence, when two oceanic plates converge, one is usually subducted under the other, and in the process a trench is formed.
The Marianas Trench paralleling the Mariana Islands , for example, marks where the fast-moving Pacific Plate converges against the slower moving Philippine Plate. The Challenger Deep, at the southern end of the Marianas Trench, plunges deeper into the Earth's interior nearly 11, m than Mount Everest, the world's tallest mountain, rises above sea level about 8, m.
Subduction processes in oceanic-oceanic plate convergence also result in the formation of volcanoes. Over millions of years, the erupted lava and volcanic debris pile up on the ocean floor until a submarine volcano rises above sea level to form an island volcano.
Such volcanoes are typically strung out in chains called island arcs. As the name implies, volcanic island arcs, which closely parallel the trenches, are generally curved. The trenches are the key to understanding how island arcs such as the Marianas and the Aleutian Islands have formed and why they experience numerous strong earthquakes. The descending plate also provides a source of stress as the two plates interact, leading to frequent moderate to strong earthquakes.
The Himalayan mountain range dramatically demonstrates one of the most visible and spectacular consequences of plate tectonics. When two continents meet head-on, neither is subducted because the continental rocks are relatively light and, like two colliding icebergs, resist downward motion.
Instead, the crust tends to buckle and be pushed upward or sideways. The collision of India into Asia 50 million years ago caused the Indian and Eurasian Plates to crumple up along the collision zone. After the collision, the slow continuous convergence of these two plates over millions of years pushed up the Himalayas and the Tibetan Plateau to their present heights.
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