The Hawaiian Islands comprise more than just the islands inhabited and visited by humans. A total of 19 islands and atolls constitute the State of Hawaii, with a total landmass of 6,423.4 square miles.
The Islands are actually exposed peaks of a submersed mountain range called the Hawaiian Ridge-Emperor Seamounts chain. The range was formed as the Pacific plate moves very slowly (around 32 miles every million years—or about as much as your fingernails grow in one year) over a hot spot in the Earth's mantle. Because the plate moves northwestwardly, the Islands in the northwest portion of the archipelago (chain) are older, which is also why they're smaller—they have been eroding longer and have actually sunk back into the sea floor.
The Big Island is the youngest, and thus the largest, island in the chain. It is built from five different volcanoes, including Mauna Loa, which is the largest mountain on the planet (when measured from the bottom of the sea floor). Mauna Loa and Kilauea are the only Hawaiian volcanoes still erupting with any sort of frequency. Mauna Loa last erupted in 1984. Kilauea has been continuously erupting since 1983.
Mauna Kea (Big Island), Hualalai (Big Island), and Haleakala (Maui) are all in what's called the post-shield-building stage of volcanic development—eruptions decrease steadily for up to a million years before ceasing entirely. Kohala (Big Island), Lanai (Lanai), and Waianae (Oahu) are considered extinct volcanoes, in the erosional stage of development; Koolau (Oahu) and West Maui (Maui) volcanoes are extinct volcanoes in the rejuvenation stage—after lying dormant for hundreds of thousands of years, they began erupting again, but only once every several thousand years.
There is currently an active undersea volcano to the south and east of the Big Island called Kamaehu that has been erupting regularly. If it continues its current pattern, it should breach the ocean's surface in tens of thousands of years.
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