Lithospheric Plates, Benioff Zones, and Volcanoes
Earthquake focus depth contours delineate undulating surfaces which may represent the tops and bottoms of lithospheric plates 50 to 75 km thick, dipping beneath island arcs. The same data can be contoured as a single surface only with difficulty and with anomalous points. Active volcanoes lie above axes of undulations which plunge down the slope of the surface.