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This chapter overviews global and local features of oceanic trenches. Most oceanic trenches are associated with plate subduction boundary systems; they form due to the downward bending of an oceanic p...late entering a subduction system. Their total length is 47,900 km, longer than Earth’s circumference. There are 27 hadal trenches with their deepest points situated in the hadal zone (water depths<−6km). Most of these are located along erosive plate subduction margins. The hadal trenches occupy 33% of the entire hadal seafloor and accommodate more than 90% of the global seafloor with water depths of<−7km. The depths of oceanic trenches are explained systematically by the present-day oceanic crustal age, sediment thickness, and isostatic correction. Recent high-resolution bathymetric surveys have revealed that the seafloors within oceanic trenches do not always show V-shaped structures with very steep landward and seaward slopes. Several trenches locally accommodate flat floors with small isolated depositional basins along the trench axis. For example, the Japan Trench has several tens of small trench-fill basins with their largest area of ∼ 30 km^2. Further detailed surveys are necessary to better understand the geomorphology of deep trench floors worldwide – the least studied places on the Earth’s surface.show more
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