Volcano Watch — Maui Nui, the Bigger Island

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Last week, we presented a brief history of the island of Hawaii, the largest island in the Hawaiian chain. However, before the island of Hawaii formed, Maui Nui was an even larger island. 

Last week, we presented a brief history of the island of Hawaii, the largest island in the Hawaiian chain. However, before the island of Hawaii formed, Maui Nui was an even larger island. It consisted of six or seven coalesced volcanoes, including Haleakalā, West Maui, Kaho'olawe, Lāna‘i, East Moloka'i, West Moloka'i, and Penguin Bank, which was apparently a volcano separate from West Moloka'i.

To review material covered two weeks ago, each Hawaiian volcano progresses through a series of eruptive stages that start with the preshield stage, followed, in sequence, by the shield stage, postshield stage, and, following a period of erosion, the rejuvenated stage. Not all volcanoes go through the postshield or rejuvenated stages, and preshield stage lavas are presumably buried beneath later lavas on all the volcanoes except Lō‘ihi Seamount, which has not yet progressed past the preshield stage. The lavas erupted during each stage are chemically distinct, thereby allowing identification of the different stages.

The youngest of the volcanoes that made up Maui Nui is Haleakalā. An active volcano in its rejuvenated stage, it last erupted about 1790 and has a poorly-established eruptive recurrence interval of several hundred years. Three fissure, or rift, zones extend to the northwest, east, and southwest. A large summit depression, originally interpreted as a caldera and later as an erosional feature, may have been formed from the coalescence of the headwalls of two landslides to the north and south of the summit.

The volcano consists of shield-stage lava (1.1 million to 900,000 years old), postshield-stage lava (860,000 to 410,000 years old), and rejuvenated stage lava (younger than 400,000 years old). Haleakalā is unique in that rejuvenated-stage vents are aligned along the rift zones. Haleakalā is a potentially dangerous volcano that is likely to erupt again within the next several hundred years.

Kaho'olawe, whose eastern half slid away during a catastrophic landslide, is an extinct volcano that has undergone the shield and postshield stages. Only a few vents and flows represent the postshield stage, and a west-southwest rift zone is identified by aligned vents.

West Maui Volcano is an extinct volcano whose evolution includes shield (1.6 to 2.0 million years old), postshield (1.5-1.2 million years old), and rejuvenated stages. The rejuvenated stage is represented by only a handful of vents and flows, located mainly near the town of Lahaina. Numerous cones, domes, dikes, flows, and pyroclastic deposits delineate the postshield stage. Deep erosion has exposed nearly 4,900 vertical feet of the volcanic stratigraphy on West Maui.

Lāna‘i is an extinct volcano with only the shield stage represented. Its lavas have been dated at 1.28 million years—too young, compared to the ages of nearby volcanoes. A large landslide deposit on the sea floor south-southwest of Lāna‘i, named the Clark landslide, was derived from Lāna‘i or Penguin Bank.

East Moloka'i is an extinct volcano with lavas erupted during the shield (1.5 million years old) postshield (1.5-1.35 million years old), and rejuvenated stages (570,000 to 350,000 years old). Its summit caldera was bisected by an enormous landslide, named Wailau, that slid to the north and deposited half-mile-sized blocks on the sea floor as far as 100 miles north of the island. The landslide apparently occurred during the late part of the shield stage; the steep northern cliff marks its headwall. The sea cliff on the north side of the volcano exposes about 4,000 feet of stratigraphic section, including lavas erupted during the shield and postshield stages. The rejuvenated stage is represented only by the lavas that formed the Kalaupapa Peninsula.

West Moloka'i, an extinct volcano with lavas erupted during the shield (1.9 million years old) and postshield (1.8-1.75 million years old) stages, has no exposed caldera complex. A series of normal faults that step down to the east is located on the east flank of the volcano. These faults probably mark the headwall of a landslide that down-dropped the summit and eastern half of the volcano towards the east. Such a landslide presumably occurred before East Moloka'i Volcano had grown and buttressed the eastern flank of West Moloka'i Volcano.

To the west of West Moloka'i Volcano, a broad shoal called Penguin Bank appears to be a separate volcano that has now subsided below sea level and is covered with a coral deposit of unknown thickness. Lavas recovered from the southern flank of Penguin Bank are all from the shield stage and are distinct in composition from those of adjacent West Moloka'i. Much of the original caldera complex may be strewn across the sea floor as blocks in the Clark landslide.

About 300,000 to 400,000 years ago, Maui Nui, which grew from west to east, subsided to form two islands, one consisting of Penguin Bank, Moloka'i, and Lāna‘i, and the other consisting of Maui and Kaho'olawe. Kaho'olawe then separated from Maui, and finally Lāna‘i separated from Moloka'i, both within the last 100,000 to 200,000 years. Penguin Bank probably submerged within the last several hundred thousand years.

With continued subsidence at the present-day rates, Haleakalā and West Maui will become separate islands in about 15,000 years. At its largest, Maui Nui stretched from about 42 miles west-southwest of the present west shoreline of Moloka'i to roughly 47 miles east of the eastern tip of Maui. Although some of the oldest western land may have subsided below sea level somewhat before the easternmost part of the island formed, Maui Nui probably had a maximum size of about 6,200 square miles, some 2,150 square miles larger than present-day Hawaii.