1 ) 

Where oceanic plates collide, subduction leads to increased friction and melting. The less dense material begins to rise through the crust, creating a series of volcanic islands.

2 ) 

Radioactive elements beneath the crust cause intense heating, often away from plate boundaries. As the volcano breaks through the crust and crust moves, an arc is formed.

3 ) 

As plates diverge, magma is forced to the surface. After subsequent underwater eruptions, eventually island arcs are formed along ridges, such as in the Mid Atlantic.

4 ) 

As continental plates collide, enormous pressure is exerted and folding occurs. Where folding occurs in coastal locations, some land can be cut off by the sea leading to arc formation.