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As we have already mentioned, volcanoes are concentrated in specific areas. Eighty percent of approximately 600 activevolcanoes (including volcanoes on the ocean floor) occur on the converging boundaries of the plates, 15 percent on the diverging edges, while the rest occurs within the plates. Volcanic crests situated in parallel to the boundary of two converging plates are quite prominent.
When a continental plate collides with an oceanic plate and overlaps its edges, a mountainous volcanic range will form in the vicinity. An example of this occurrence are the Cascades (part of the Cordillera, USA). Mount St. Helens is one of the Cascade volcanoes. In the beginning of 1980, after 123 years of inactivity, the mountain violently erupted. Following a series ofearthquakes, a new crater formed on the summit of the mountain and the volcano continued to erupt.
This eruption culminated in a terrific explosion, that blew off the northern side of the mountain. An area of 500 square kilometres was devastated by hot ashes reaching a temperature of 500 degrees Celsius, gas, and steam. The energy released during this explosion was 1300 times greater than the energy of the atomic bomb that fell on Hiroshima. If the present activity continues, Mount St. Helens volcano could remain active for the next 20 years.
When two oceanic plates collide, the volcanic activity on theocean floor results in the formation of a volcanic island arc. The Japanese island arc with Mount Fuji, a volcanic cone which gradually grew, is one example. Another example of a volcano resulting from the collision of two oceanic plates is Mont Pelée on Martinique.
For a long time, the scientists were baffled by the occurrence of the volcanic activity within the plates. Hawaiian islands in the middle of the Pacific plate are a good example. In addition to active volcanoes, there is a series of older, extinct volcanic mountains and crests. This phenomenon did not fit the plate tectonics theory. Then the formation of the so-called hot spots was discovered. This is the molten flow from the interior of the earth's mantle, or even from the transitional region leading to theearth's core.
Apparently, this phenomenon is linked to a given place and does not move with the lithospheric plates. When only one plate slides slowly over a hot spot, a new volcano grows next to a volcano, which formed over a hot spot. This results in a gradual formation of a volcanic crest. This theory was confirmed by ocean floor drilling, which shows that the age of the lavas increases with the distance of the volcanoes and volcanic crests relative to the hot spots.
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