Complex sentence: It also lies above a hotspot, the Iceland plume, which is believed to have caused the formation of Iceland itself, the island first appearing over the ocean surface about 16 to 18 million years ago.
Split into simpler sentences: It also lies above a hotspot, the Iceland plume. The plume is believed to have caused the formation of Iceland itself, the island first appearing over the ocean surface about 16 to 18 million years ago.