Question

Island Arcs Form along?

Answer 1

Ok so as a lithospheric slab is being subducted, the slab melts when the edges reach a depth which is sufficiently hot. The remelted material from the subducting slab rises and leaks into the crust, forming a series of volcanoes. These volcanoes can make a chain of islands called an "island arc". Island Arcs are formed on the opposing edge of a subducted slab. In each case there is an associated subducting slab and a trench.

Answer 2

Island arcs form when one oceanic plate subducts beneith another subducting plate. This subduction allows water to get into the mantle, causing volcanism. This volcanism pops up in a chain along the boundary between the plates. Since you have two plates on a sphere, the general shape they take is an arc.

Answer 3

I think your talking about coral reef atolls? Usually a volcano raises up, builds up a little island, and the heat attracts coral to build around it. When the volcano subsides, arcs, rings etc. are formed.