Some of these rifting events were accompanied by volcanic activity; the ground would gradually rise m before abruptly dropping, signalling an impending eruption. Between and , the displacements caused by rifting totalled about 7 m. When the continental crust stretches beyond its limits, tension cracks begin to appear on the Earth's surface. Magma rises and squeezes through the widening cracks, sometimes to erupt and form volcanoes. The rising magma, whether or not it erupts, puts more pressure on the crust to produce additional fractures and, ultimately, the rift zone.
East Africa may be the site of the Earth's next major ocean. Plate interactions in the region provide scientists an opportunity to study first hand how the Atlantic may have begun to form about million years ago. 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. Each one is unique because of the density of the plates involved.
Earth's surface is made up of two types of lithospheric plates: continental and oceanic. The crust that makes up continental plates is thicker yet less dense than oceanic crust because of the lighter rocks and minerals that compose it. Oceanic plates are made up of heavier basalt , the result of magma flows from mid-ocean ridges. When plates converge, they do so in one of three settings: oceanic plates collide with each other forming oceanic-oceanic boundaries , oceanic plates collide with continental plates forming oceanic-continental boundaries , or continental plates collide with each other forming continental-continental boundaries.
Earthquakes are common any time large slabs of Earth come into contact with each other, and convergent boundaries are no exception. In fact, most of the Earth's most powerful quakes have occurred at or near these boundaries. The surface of the Earth is made up of nine major tectonic plates, 10 minor plates, and a much larger number of microplates.
These plates float on top of the viscous asthenosphere, the upper layer of Earth's mantle. Because of thermal changes in the mantle, tectonic plates are always moving—through the fastest-moving plate, the Nazca, only travels about millimeters per year. Where plates meet, they form a variety of different boundaries depending on the direction of their motion. Transform boundaries, for example, are formed where two plates grind against each other as they move in opposite directions.
Divergent boundaries are formed where two plates pull apart from each other the most famous example is the Mid-Atlantic Ridge, where the North American and Eurasian plates diverge. Convergent boundaries are formed wherever two plates move toward each other.
In the collision, the denser plate is typically subducted, meaning it slides below the other. When two oceanic plates collide, the denser plate sinks below the lighter plate and eventually forms dark, heavy, basaltic volcanic islands. The Caribbean and South Sandwich island arcs are found in the Atlantic, while the Indonesian archipelago is a collection of volcanic arcs in the Indian Ocean. When oceanic plates are subducted, they often bend, resulting in the formation of oceanic trenches.
These often run parallel to volcanic arcs and extend deep beneath the surrounding terrain. The deepest oceanic trench, the Mariana Trench , is more than 35, feet below sea level. It is the result of the Pacific Plate moving beneath the Mariana Plate.
When oceanic and continental plates collide, the oceanic plate undergoes subduction and volcanic arcs arise on land. At transform boundaries, plates move past each other. This is one of the most common causes of earthquakes. At convergent boundaries, plates move toward each other.
They can push together and cause mountain ranges to form. At other times, one plate gets pushed down beneath the other plate. This can cause volcanoes. At divergent boundaries, plates move apart from each other. When this happens, new plate material forms.
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