|Activity 3: Subduction Zones and the Arc-Trench Gap|
|Subduction zones include (Fig. 1):|
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
activity. Indeed, many
of the largest recorded earthquakes have occurred along subduction zones. When locations of
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
dip, the depth to
plate beneath the volcanic
arc is rather uniform
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.