Iapetus Ocean
Southwest of the Iapetus, a volcanic island arc evolved from the early Cambrian (540 million years ago) onward. This volcanic arc was formed above a subduction zone where the oceanic lithosphere of the Iapetus Ocean subducted southward under other oceanic lithosphere. From Cambrian times (about 550 million years ago) the western Iapetus Ocean began to grow progressively narrower due to this subduction. The same happened further north and east, where Avalonia and Baltica began to move towards Laurentia from the Ordovician (488-444 million years ago) onward.