Ocean trenches are linear troughs in the ocean that form where one lithospheric plate is pushed under another. They are referred to as convergent plate margins because in the subduction process of plate tectonics theory two plates converge. Since only oceanic plates can be subducted (continental plates are too low in density and float),trenches only occur in the oceans. Thus trenches outline subduction zones.

Trenches are very deep, and the Marianas Trench off the Philipines is the deepest parts of the ocean at 35,810 ft. This is more than 7,000 ft deeper than Mt. Everest is tall.

There is typically a lot of andesitic volcanism near the trench and volcanic arcs are formed.

Things to Consider-- The trench is a place where materials from both sedimentary processes and igneous processes are recycled by the SUBDUCTION PROCESS. The sediment forced down the trench has been chemically altered by weathering processes and is saturated with water. The water acts as a flux, reducing the temperature at which the sediments melt (or are metamorphosed). The sediment has a more felsic composition (increased silica content including quartz) than does the material (protolith) from which it was derived; hence, the magma created is typically intermediate in composition (andesite) and as you know this type of andesitic volcanism is explosively dangerous.

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