A volcanic arc is a line of volcanoes on the overriding plate of a subduction zone, parallel to the trench, standing above the place where the slab has reached a depth of about a hundred kilometres. It is an island arc when it rises from the sea floor, as in Japan or Indonesia, and a continental arc when it stands on a continent, as in the Andes or the Cascades.
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Example 22.4 (North-east Japan)
In the band of the figure, the easternmost volcano is Iwate, at E, and the Japan Trench lies at E. One degree of longitude there is , so . The deep earthquakes dip at about , and
The earthquakes deeper than that lie beneath the front itself have a median depth of : the two agree, close to the world average.