A model for the tectonic evolution of this area may be applicable to island arcs in general. A continuously emerging geanticlinal platform overlies a thickening oceanic crust, plates of which have contrasting physical properties in response to horizontal stresses in the mantle. In the axis of deformation along this boundary volcanic magmas originate. Successively younger deposits show increasing movement along old high-angle faults. Island arcs lack the immense quantities of terrigenous sediments which are present in normal mountain systems of continental margins, hence their geologic structure reflects primary orogenic forces.