Activity 3: Subduction Zones and the Arc-Trench Gap
Subduction zones include (Fig. 1):
  • A deep-sea trench
  • A forearc region
  • A volcanic arc
  • A back-arc basin

The deep-sea trench contains sediment scraped off the subducting plate. In contrast, the forearc region collects sediment eroded from the volcanic arc. The volcanic arc is a 10 to 20 km wide zone of volcanoes that are evenly spaced along the length of the arc. The distance between the trench and the volcanic arc, the arc-trench gap, varies considerably worldwide.


Fig. 1: Cross section through a subduction zone.
Subduction zones are the sites of considerable earthquake activity. Indeed, many of the largest recorded earthquakes have occurred along subduction zones. When locations of earthquakes are plotted in a crustal cross section, they define a plane dipping beneath the volcanic arc. This plane, known as the Benioff-Wadati zone, is probably the top of the cold subducting plate, which is diving into the mantle. This dip of this plane varies considerably. Despite the variation in Benioff-Wadati zone dip, the depth to the subducting plate beneath the volcanic arc is rather uniform worldwide, varying from 90 to 110 km.
Earthquake depths vary significantly from one subduction zone to another. In the Mariana arc, earthquakes have been recorded to depths of nearly 700 km.
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