VOLCANO WATCH: A Geologic Tour of the Hawaiian Islands (Maui)

In this shaded relief and bathymetric map of Maui County, colors indicate water depth, from shallow (orange and yellow) to deep (blue and purple), with shades of gray indicating island areas above sea level. From: U.S. Geological Survey Geologic Investigations Series Map I-2809, “Hawaiʻi’s Volcanoes Revealed.”

As part of Volcano Awareness Month, our January “Volcano Watch” articles are taking us on a geologic tour of the Hawaiian Islands. Today’s stop: Maui, as well as the islands of Lānaʻi, Molokaʻi, and Kahoʻolawe, all of which form Maui County.

To imagine the landscape of Maui County as it would have appeared about 1 million years ago, think of the Island of Hawaiʻi today, with several large, coalesced volcanoes that form a single large island. Such was the heyday of “Maui Nui,” when at least seven volcanoes built an island that was about 50 percent bigger than the Island of Hawaiʻi is today.

The oldest of Maui Nui’s volcanoes, Penguin Bank, is now submerged off the west coast of Molokaʻi. From there, successively younger volcanoes are West Molokaʻi and East Molokaʻi. When these three volcanoes began to grow on the seafloor is poorly known, but they probably range from slightly over 2 million years old (Penguin Bank) to slightly less than 2 million years old (East Molokaʻi).

The sequence of volcanoes then progressed with Lānaʻi, West Maui, Kahoʻolawe, and finally, Haleakalā on East Maui.  The formation of these four volcanoes probably occurred between 1.5 and 2 million years ago.

Why so many volcanoes in such a small area? Studies of the entire chain of Hawaiian volcanoes and seamounts suggest that magma supply to the surface began increasing a few million years ago. More magma means more eruptions, which might explain why the Hawaiian hot spot went from forming individual island volcanoes (Niʻihau and Kauaʻi), to an island with two volcanoes (Oʻahu), to islands made up of several volcanoes (Maui Nui and the Island of Hawaiʻi).

Despite their close proximity, the volcanoes of Maui Nui have quite different eruptive histories. For example, Lānaʻi was short-lived, going extinct after its vigorous shield stage, with no eruptions since about 1.35 million years ago. West Molokaʻi and Kahoʻolawe were also short-lived, but they experienced minor postshield volcanism before going extinct about 1 million years ago. East Molokaʻi and West Maui persisted longer and were the sites of rejuvenated eruptions just 300,000 years ago (the most recent such eruption on Molokaʻi formed Kalaupapa Peninsula).

Haleakalā, the longest-lived of the Maui Nui volcanoes, is currently waning from a long postshield sequence of volcanism. Eruptions there occur about as frequently as they do on Hualālai volcano on the Island of Hawaiʻi. The most recent eruption on Haleakalā took place about 400 years ago, well within the time that Polynesians settled on the Hawaiian Islands. Future eruptions at Haleakalā are likely, which is why the USGS Hawaiian Volcano Observatory (HVO) maintains a monitoring network there.

Because all of the Maui Nui volcanoes are beyond their vigorous shield-building stages, erosion has dominated for the past 1 million years or so. Water and landslides have helped create dramatic valleys, including the summit “crater” of Haleakalā.

The most spectacular of the Maui Nui landslides occurred from East Molokaʻi, where the massive Wailau slide sliced off the volcano’s summit, creating spectacular sea cliffs on the island’s north side. This landslide deposited rocky debris over 160 km (100 mi) across the ocean floor.

The islands of Maui Nui have also subsided over time—a normal consequence of the volcanoes’ weight on the sea floor in addition to their motion away from the buoyant hot spot. It was this subsidence, plus rising sea levels, that flooded the land between Maui Nui’s volcanoes, creating the separate islands we see today, probably within the last few hundred thousand years. With continued subsidence at present rates, Haleakalā (East Maui) itself could become isolated from West Maui by a seaway within another 10,000–20,000 years.

As Volcano Awareness Month winds down next week, we’ll make the final stop in our geologic tour of the islands on the Island of Hawaiʻi.

Before then, HVO scientists are offering Volcano Awareness Month talks in the Konawaena High School cafeteria on January 25, Hawai‘i  Volcanoes National Park on January 26, Ocean View Community Center on January 27, and University of Hawaiʻi at Hilo on January 28.

Volcano Activity Updates

Kilauea continues to erupt at its summit and East Rift Zone. During the past week, the summit lava lake level varied between about 34 and 43 m (110–140 ft) below the vent rim within Halema‘uma‘u Crater. On the East Rift Zone, scattered lava flow activity remained within about 6 km (4 mi) of Puʻu ʻŌʻō, and is not currently threatening nearby communities.

Mauna Loa is not erupting. Seismicity remains elevated above long term background levels. GPS measurements continue to show deformation related to inflation of magma reservoirs beneath the summit and upper Southwest Rift Zone of Mauna Loa.

One earthquake was reported felt on the Island of Hawai‘i during the past week. On Monday, Jan. 18 at 1:52 a.m., a magnitude-3.9 earthquake occurred 6.7 km (4.1 mi) north of the Mauna Loa summit at a depth of 12.0 km (7.5 mi).

Volcano Watch is a weekly article and activity update written by scientists at the U.S. Geological Survey`s Hawaiian Volcano Observatory.