Volcano Watch — A geologic tour of the Hawaiian Islands: Maui

To imagine the landscape of Maui County as it would have appeared about 1 million years ago, think of the Island of Hawai‘i 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 Hawai‘i is today.

The oldest of Maui Nui's volcanoes, Penguin Bank, is now submerged off the west coast of Moloka‘i. From there, successively younger volcanoes are West Moloka‘i and East Moloka‘i. 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 Moloka‘i).

The sequence of volcanoes then progressed with Lāna‘i, West Maui, Kaho‘olawe, 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 (Ni‘ihau and Kaua‘i), to an island with two volcanoes (O‘ahu), to islands made up of several volcanoes (Maui Nui and the Island of Hawai‘i).

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

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 Hawai‘i at Hilo on January 28. Details are posted on HVO's website (http://hvo.wr.usgs.gov/) or you can email askHVO@usg.gov or call 808-967-8844 for more information.

Volcano Activity Update

Kīlauea 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 Pu‘u ‘Ō‘ō, 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, January 18, at 1:52 a.m., HST, 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).