what areaewhere an oceanic plate goes down into the mantle?
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Subduction zones occur where oceanic crust collides with continental crust.  Since oceanic crust has more density, it is always subducted underneath continental crust.
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It's widely believed in the scientific community that the earth has a liquid mantle (that's where magma/lava comes from) and a solid super-hot iron core.  Good luck digging through that.
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The average depth of the oceans is about five times the average elevation of the land. In general, the continents stand about three miles above the ocean floor. According to the National Geographic Atlas, the deepest-known part of the ocean measures [10,924] meters (35,839 feet), in the Marianas Trench near Guam. If the world's highest mountain, Mount Everest (29,141 feet), were to be placed into this trench, it would be covered by over 1.25 miles of water.
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Depends on who you read.  Ranges are from 700-1300C or 820-1020C.\n\nOne site says, \n"temperature between 1300 and 700 degrees C"\nhttp://www.cobweb.net/~bug2/rock7.htm\n\nWhile another says,\n"We show that the mantle in this region is at a minimum temperature of 820 C and a maximum temperature of 1020 C; the latter is some 200 C colder than that suggested in a recent interpretation of an observed collocated low velocity zone from a teleseismic survey"\nhttp://www.dias.ie/~ajones/papers/100.pdf
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mantle\nThe Mantle. It is divided into two parts. The upper portion, which is called the Asthenosphere, is a plastic like fluid. This is where the mantle convection currents occur. The rest of the mantle is made up of dense rock. The mantle is the thickest layer. Approximately 2900 km thick
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lithosphere is the solid outermost shell of a rocky planet. On the Earth, the lithosphere includes the crust and the uppermost layer of the mantle (the upper mantle or lower lithosphere) which is joined to the crust. for more see http://en.wikipedia.org/wiki/Lithosphere..\n\nAsthenosphere is the region of the Earth between 100-200 km below the surface — but perhaps extending as deep as 400 km — that is the weak or "soft" zone in the upper mantle. It lies just below the lithosphere, which is involved in plate movements and isostatic adjustments. In spite of its heat, pressures keep it plastic, and it has a relatively low density. Seismic waves, the speed of which decrease with the softness of a medium, pass relatively slowly though the asthenosphere, the cue that originally alerted seismologists to its presence; thus it has been given the name low-velocity zone. for more see http://en.wikipedia.org/wiki/Asthenosphere ..
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Oceanic crust is formed by volcanism at the mid-ocean ridges, also called spreading centers. Magma upwells from within the asthenosphere or mantle. It often does not erupt onto the sea floor but rises through complex cracks and fissures where it encounters cooler rock and sea water. Other areas actively erupt and lava flows in sheets typically less than 10 meters thick. For example, at the Juan de Fuca Ridge, the process creates about 6 meters of new crust in 100 years. Oceanic crust consists of basalt, often referred to as MORB (mid-ocean ridge basalt).
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I think its called 'subduction', where one tectonic plate goes under another. It occurs at 'subduction zones', such as on the west coast of the United States. The reason it occurs is that one plate is expanding and the edges need to go somewhere (under another plate). For example, the Atlantic plate is expanding from the center because there is a north-to-south line of underwater volcanoes, spewing out magma and forcing the 2 halves of the plate apart to the west and east.
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10,916 meters (35,814 feet), to Challenger Deep in the Marianas trench, which is the deepest known part of the Earth's oceans.  The Marianas trench is in the western north Pacific Ocean.  \n\nSee sources.
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Basaltic oceanic crust is more dense.\n\n^^^^^^^^^^^^^^^^^^^^^^^^^^^^^\n\nCONTINENTAL CRUST\n# Less Dense (lighter weight): average density = 2.7 gm per cubic centimeter\n# Thicker: 0 to 40 km, average 35 km under high mountains, the crust can be thickened, up to 65 km. Under the craton, the average thickness is 30 km.\n# Mineral Composition: granitic (felsic) rocks such as granodiorite and granite enriched in Potassium (K), Sodium (Na), Aluminum (Al), and Silica (Si)\n# "Floats" on top of the Mantle\n\nOCEANIC CRUST\n# More Dense (heavier): average density = 3.0 gm per cubic centimeter\n# Thinner: 0 to 10 km, average 5 km, thinnest at Mid-Ocean Ridges\n# Mineral Composition: mafic rocks such as basalt and gabbro enriched in Magnesium (Mg) and Iron (Fe)\n# Underlies Ocean Basins.
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mariana trench in the pacific ocean
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The plate which is being subducted under the other plate carries down volatiles with it (carbon dioxide, water etc from in the clay and sediments and rocks and stuff) - and these volatiles allow melting to occur in the mantle at a lower temperature than it would ordinarily do. Some of this melt (magma) may then rise up through the overlying crust (so on the overriding plate, not the one which is being pushed under) and erupt out at the surface as lava... so forming volcanoes.
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The deepest "hole" drilled into the Earths Crust was 4,644 feet deep. Scientist were trying to reach the Moho, a boundary formally known as the Mohorovicic discontinuity. It marks the division between Earth's brittle outer crust and the hotter, softer mantle
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Mariana Trench in Pacific.
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Water.\n\nWater is one of the key parts of the formation of magma in subduction zones. The water that is in the subducting plate lowers the melting point of the rock by a few hundred degrees C. The water in the subduction plate will boil and migrate upwards into the overlying wedge of mantle rock where it will reduce the melting point of the overlying mantle. This magma will migrate upwards and form a volcano. \n\nAnother part of this process is that within rock there are many different minerals, all with different melting points, so the ones that melt at the lowest temperatures tend to form magma first, so the resulting magma comes from a partial melt of the source rock.\n\nHere's another diagram (I'd draw you my own but I can't post anything but text here):\nhttp://earth.leeds.ac.uk/dynamicearth/subduction/cordillera_islandarc.htm
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the upper mantle.\n\nSee all layers at http://en.wikipedia.org/wiki/Mantle_(geology)
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plate boundries?  \n\nMountains, volcanos, earthquakes, vents, hot springs.