On the basis of the changing crustal thickness of the trench-island arc-backarc basin system and the distinctive extension mode of the island arc in plan and from the viewpoint of structural mechanics, the island arc appears to be in essence an arch with two articulations that span the convergent plate boundary; its axis along vhich there is the maximum crustal thickness is the arch axis.Tectonic, seismic and volcanic belts in an island arc are usuallg, all parallel to the arch axis. As the supporting points at both ends of an island arc correspond to two articulations of the arch, the blocks at the supporting paints should have enough load-bearing capacity? therefore, the blocks generally have larger crustal thickness and are often situated on the continent or big islands. The stress on the subdticting plate boundary may mainly act on the supporting points at both ends through transmission by the arch axis. Consequently there would occur severe structural deformation, seismism and volcanism at the supporting points.The optimum arch axis required by an island arc is possible a parabola, for the applied stress on the subducting plate boundary appears to be vertical uniform load acting on the arch with two articulations. In order to keep the balance of the plate boundary, the island arc would extend outward like a parabola, leading to spreading of the back-arc basin toward the island arc. This is believed to be, in essence, different from mid-ocean ridge spreading.