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Subscriber Exclusive Content. Why are people so dang obsessed with Mars? Measles Cases and Deaths Surge Worldwide. Written By: Tiffany Dazet. JAN 08, The existence of dark matter has been considered a 'given' for decades as a way to understand some of the less e Written By: Annie Lennon. FEB 20, NASA scientists have confirmed that their car-sized Perseverance Mars rover is 'healthy' following its arrival o FEB 24, In addition to the millions of humans who suffered through Texas' historic freeze last week, local wildlife also exp JUL 09, The Kepler Space Telescope has captured evidence of mysterious free-floating planets, or planets that are alone in deep 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.
Most of this growth occurred during the past 10 million years. The Himalayas, towering as high as 8, m above sea level, form the highest continental mountains in the world. Moreover, the neighboring Tibetan Plateau, at an average elevation of about 4, m, is higher than all the peaks in the Alps except for Mont Blanc and Monte Rosa, and is well above the summits of most mountains in the United States.
Below: Cartoon cross sections showing the meeting of these two plates before and after their collision. The reference points small squares show the amount of uplift of an imaginary point in the Earth's crust during this mountain-building process.
The zone between two plates sliding horizontally past one another is called a transform-fault boundary, or simply a transform boundary. The concept of transform faults originated with Canadian geophysicist J. Tuzo Wilson, who proposed that these large faults or fracture zones connect two spreading centers divergent plate boundaries or, less commonly, trenches convergent plate boundaries.
Most transform faults are found on the ocean floor. They commonly offset the active spreading ridges, producing zig-zag plate margins, and are generally defined by shallow earthquakes. However, a few occur on land, for example the San Andreas fault zone in California. This transform fault connects the East Pacific Rise, a divergent boundary to the south, with the South Gorda -- Juan de Fuca -- Explorer Ridge, another divergent boundary to the north.
The Blanco, Mendocino, Murray, and Molokai fracture zones are some of the many fracture zones transform faults that scar the ocean floor and offset ridges see text. The San Andreas is one of the few transform faults exposed on land. The San Andreas fault zone, which is about 1, km long and in places tens of kilometers wide, slices through two thirds of the length of California.
Land on the west side of the fault zone on the Pacific Plate is moving in a northwesterly direction relative to the land on the east side of the fault zone on the North American Plate.
Oceanic fracture zones are ocean-floor valleys that horizontally offset spreading ridges; some of these zones are hundreds to thousands of kilometers long and as much as 8 km deep. Examples of these large scars include the Clarion, Molokai, and Pioneer fracture zones in the Northeast Pacific off the coast of California and Mexico.
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