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 36,201 feet below sea level. Marianas Trench is 7,173
feet deeper than Mount Everest is high!!!!
Trenches surround almost all of the Pacific Ocean. Some of the other
trenches of the Pacific are the Aleutian, Peru-Chile, Kuril, and the Japan
trench.
There are trenches wherever continental plates and oceanic plates
collide. In a typical "island-arc" environment, volcanoes lie along the
crest of an accurate, crustal ridge bounded on its convex side by a deep
oceanic trench. The granite or granite like layer of the continental
crust extends beneath the ridge to the vicinity of the trench. Basaltic
magmas, generated in the mantle beneath the ridge, rise along fractures
through the granitic layer. These magmas commonly will be modified or
changed in composition during passage through the granitic layer and
erupt on the surface to form volcanoes built largely of non-basaltic
rocks. -- Excerpt from: Tilling, 1985, Volcanoes: USGS General
Interest Publication
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