Types of islands. Cutaway artwork looking below the sea to show types of volcanic islands. At upper right a large volcanic island towers out of the sea, formed by an underwater volcano that grew in height with each eruption until it broke through the surface of the sea. Plants (green) have started to colonise that island. At lower right, smaller islands have been eroded. Eventually, the islands may end up as coral atolls (lower centre), as the central portion of the island collapses, leaving corals to form the bedrock for the ring of land seen here. The trench at left marks the edge of the continental shelf, and sediment deposits from a river are seen at centre left. See E680/207-209 for a set of images on this topic.