Volcano Watch — Mauna Ulu—an opportunity to study a long-lived rift zone eruption

The eruption began from a series of fissures, as do most other Hawaiian eruptions, on May 24, 1969. And, like the Pu‘u ‘Ō‘ō eruption (described earlier this month), the first stage of activity at Mauna Ulu included 12 episodic lava fountains, some reaching as high as 540 m (1,770 ft), and short-lived lava flows. After December 1969, fountains gave way to almost continuous quiet effusion of lava. The eruption paused in October 1971 but restarted in February 1972, and lava continued to flow until a series of eruptions near Kīlauea's summit spelled the end for Mauna Ulu on July 24, 1974.

The Mauna Ulu eruption was, at that time, the longest-lived rift zone eruption at Kīlauea since at least the 15th century, and it occurred near Chain of Craters Road, so access for scientists was relatively easy. As a result, detailed, on-the-spot observations of eruptive activity could be made on an almost daily basis.

As was true for eruptions described in this column over the past two weeks, some of the most important new insights concerned lava flows. For example, observations from Mauna Ulu resulted in a better understanding of the development of pāhoehoe and ‘a‘ā lava flows, how lava can flow long distances through tubes, and how broad pāhoehoe fields form. The lava flow field buried several pit craters, including ‘Ālo‘i and ‘Alae—an excellent demonstration of the fleeting existence of Kīlauea's topography, and how Kīlauea grows over time.

In addition, scientists watched as a lava shield (Mauna Ulu) was constructed before their eyes, providing a sense of how other lava shields on Kīlauea and Mauna Loa were built. Interestingly, the crater at the summit of Mauna Ulu formed as the shield around it grew higher, and not due to collapse of the shield's summit.

Although ‘a‘ā lava had entered the ocean during numerous other eruptions from Kīlauea and Mauna Loa, the Mauna Ulu eruption was the first time that scientists were able to observe pāhoehoe forming lava deltas and undersea flows. In fact, the observations and video recordings of pillow lavas forming as Mauna Ulu lava flows entered the sea were the first recorded anywhere in the world.

The Mauna Ulu shield hosted a lava lake in its summit crater for much of the eruption, and HVO volcanologists observed that the lava level rose slowly and fell abruptly as gas accumulated and escaped from beneath a surface crust, a process called gas pistoning. Interestingly, the same process is occurring right now within Kīlauea's summit eruptive vent. Our understanding of the current activity is based in large part on what was observed during the Mauna Ulu eruption over 40 years ago.

The Mauna Ulu eruption also provided an opportunity to document the interactions between eruptive activity on the east rift zone and at the summit. When the rate of lava eruption at Mauna Ulu began to slow, the summit inflated and earthquake activity increased as magma backed up in summit reservoirs. Deflation at the summit accompanied surges in lava effusion. These observations allowed HVO scientists to forecast when changes in the eruption were likely to occur; they are the basis for our current understanding of the relation between Pu‘u ‘Ō‘ō and Kīlauea's summit magma reservoir.

Next week, in the final part of our series on scientifically noteworthy eruptions, we will discuss the 1959 Kīlauea Iki eruption, renowned for its high lava fountains.
Until then, you may want to attend some of this week's Volcano Awareness Month activities, described on our Web site or by calling 808-967-8844.

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Volcano Activity Update

A lava lake within the Halema‘uma‘u Overlook vent produced nighttime glow that was visible from the Jaggar Museum overlook and via HVO's Webcam during the past week. The lake level fluctuated slightly in response to summit DI events but was generally 35 to 40 m (115 to 130 ft) below the floor of Halema‘uma‘u. There was a small collapse of the northwestern wall and rim of the Overlook crater on Tuesday, January 22.

On Kīlauea's east rift zone, surface lava flows remain active near the base of the pali and near the coast and are feeding weak ocean entries scattered along the sea cliff on both sides of the Hawai‘i Volcanoes National Park boundary. Within Pu‘u ‘Ō‘ō, small flows continue to occasionally erupt from openings on the floor of the crater. This includes the small lava lake on the northeast side of the crater floor—flows from there spilled onto the northeastern flank of the Pu‘u ‘Ō‘ō cone for a few days early in the week..

There were no felt earthquakes in the past week on the Island of Hawai‘i.