 Each volcanic cone in the chain of the Hawaiian Islands is built of dark, iron-rich/quartz-poor rocks, which poured out of thousands of vents as highly fluid lava over a period of millions of years.Several of the volcanoes were close enough to each other that lava flows on their flanks overlapped one another, merging into a single island.Maui is such a "volcanic doublet," formed from two shield volcanoes that overlapped one another to form an isthmus between them.