The island of Maui is the second-largest of the Hawaiian Islands at 727.2 square miles (1,883 km2) and is the 17th largest island in the United States. Maui is part of the State of Hawaiʻi and is the largest of Maui County's four islands, bigger than Molokaʻi, Lānaʻi, and unpopulated Kahoʻolawe. In 2010, Maui had a population of 144,444, third-highest of the Hawaiian Islands, behind that of Oʻahu and Hawaiʻi Island. Kahului is the largest census-designated place (CDP) on the island with a population of 26,337 as of 2010 and is the commercial and financial hub of the island. Wailuku is the seat of Maui County and is the third-largest CDP as of 2010. Other significant places include Kīhei (including Wailea and Makena in the Kihei Town CDP, which is the second-most-populated CDP in Maui); Lahaina (including Kāʻanapali and Kapalua in the Lahaina Town CDP); Makawao; Pāʻia; Kula; Haʻikū; and Hāna.
Maui's diverse landscapes are the result of a unique combination of geology, topography, and climate. 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.
The older, western volcano has been eroded considerably and is cut by numerous drainages, forming the peaks of the West Maui Mountains (in Hawaiian, Mauna Kahalawai). Puʻu Kukui is the highest of the peaks at 5,788 feet (1,764 m). The larger, younger volcano to the east, Haleakalā, rises to more than 10,000 feet (3,000 m) above sea level, and measures 5 miles (8.0 km) from seafloor to summit, making it one of the world's tallest mountains.
The eastern flanks of both volcanoes are cut by deeply incised valleys and steep-sided ravines that run downslope to the rocky, windswept shoreline. The valley-like Isthmus of Maui that separates the two volcanic masses was formed by sandy erosional deposits.
Maui's last eruption (originating in Haleakalā's Southwest Rift Zone) occurred around 1790; two of the resulting lava flows are located (1) at Cape Kīnaʻu between ʻĀhihi Bay and La Perouse Bay on the southwest shore of East Maui, and (2) at Makaluapuna Point on Honokahua Bay on the northwest shore of West Maui. Although considered to be dormant by volcanologists, Haleakalā is certainly capable of further eruptions.
Maui is part of a much larger unit, Maui Nui, that includes the islands of Lānaʻi, Kahoʻolawe, Molokaʻi, and the now submerged Penguin Bank. During periods of reduced sea level, including as recently as 20,000 years ago, they are joined together as a single island due to the shallowness of the channels between them.