Volcano Watch — Eruption's end will come slowly (Submitted title: The Mauna Ulu Eruption: 1969-1974)

Before the current activity on Kīlauea's east rift zone, the most long-lived rift eruption was that of Mauna Ulu, which began 25 years ago, on May 24, 1969. This eruption lasted until July 24, 1974. There were two main parts of the Mauna Ulu eruption, separated by a hiatus between October 1971 and February 1972. The first half of the eruption lasted roughly 2.5 years and produced an estimated 185 million cubic meters of lava (about 240 million cubic yards). The second half of the eruption lasted another 2.5 years and produced an additional 160 million cubic meters of lava. The lavas erupted between 1969 and 1974 from Mauna Ulu, and closely associated vents covered 17.6 square miles and added about 230 acres of new land to Hawaii.

The eruption at Mauna Ulu began on May 24, 1969, and, like the eruption of Pu'u 'O'o, the earliest part was characterized by episodes of high fountaining until December 1969, with fountains reaching heights of nearly 1,800 feet. The next stage of the eruption was passive effusion of lava from a new fissure just west of Mauna Ulu, then a new fissure just east of Mauna Ulu near Alae Crater. This quiet, effusive phase of the eruption was similar to that which took place at Kupaianaha from July 1986 to February 1992. The main differences are that Kupaianaha was located farther away from Pu'u 'O'o than the Alae fissure was from Mauna Ulu, and that this phase of the current eruption lasted much longer.

Starting in June 1971, the surface of the lava pond inside Mauna Ulu began to drop, and the summit area of Kīlauea simultaneously began to inflate (uplift) as additional magma entered the summit magma reservoir. A brief eruption took place inside the summit caldera just east of Halemaumau during August 1971, and another took place inside the caldera and along the southwest rift zone in late September 1971. By October 15, lava could no longer be seen inside Mauna Ulu Crater, and the first part of the Mauna Ulu eruption was over. However, magma continued to enter the volcano, as seen by rapid inflation of the summit region. No eruptions took place for 3.5 months, but by February 3, 1972, lava quietly reentered the Mauna Ulu Crater and the second half of the eruption was under way.

During the 1972-1974 part of the eruption, activity was confined to Mauna Ulu and a satellitic shield at the former site of Alae Crater, except for two fissure eruptions along the upper east rift zone—one near Pauahi Crater in May 1973 and the other near Pauahi and Hiiaka Craters in November and December 1973. An eruption in the uppermost east rift zone near Keanakakoi Crater and within the summit caldera from July 19 to 22 marked the end of the Mauna Ulu eruption, but not the end of activity at Kīlauea, since there followed a second summit eruption in September and a large southwest rift eruption that started on the last day of 1974. After each of these eruptions, the summit of Kīlauea again inflated as magma continued to arrive and swell the magma reservoir beneath the summit. The summit deflated catastrophically during a magnitude-7.2 earthquake that occurred on November 29, 1975. This earthquake moved the south flank seaward and created a large underground volume in which additional magma arriving from below could be stored. Eruptive activity did not resume on Kīlauea until 1977.

The Mauna Ulu eruption is important because it is our best (and only!) previous example of a long-lived rift eruption. The sequence of events that occurred at Mauna Ulu provides us with some insight into how the current eruption may eventually end.

The current eruption is the most long-lived rift eruption on Kīlauea in historic time; it began on January 3, 1983 and is now in its twelfth year of nearly continuous activity. Several major phases of the eruptive activity, starting with an initial period lasting from the start of the eruption until July 1986, characterized by periodic high-fountaining events built the Pu'u 'O'o cinder and spatter cone.

This phase of activity was followed by development of a new vent, Kupaianaha, located several miles downrift from Pu'u 'O'o, which passively issued tube-fed pahoehoe flows from July 1986 until February 1992. These flows covered a large area, including the village of Kalapana, and entered the sea for several years. Since late 1991, five additional vents have formed, with the last four located on the south and west flanks of the Pu'u 'O'o cone. The first was a fissure eruption between Kupaianaha and Pu'u 'O'o. The currently active vents have erupted primarily tube-fed pahoehoe flows that have entered the ocean nearly continuously since November 1992. During the nearly 11.4 years of the current eruption, approximately 1 cubic kilometer of lava have erupted, about 34 square miles of land has been covered by lava, and about 500 acres of new land have been added to Hawaii.

There are similarities between events at Mauna Ulu and those at Pu'u 'O'o-Kupaianaha. Each eruption began with a period of high fountains, and each evolved into a passive, effusive eruption. The first half of the Mauna Ulu eruption ended following a decrease in eruptive output and a decrease in the level of the lava pond in Mauna Ulu, despite the continued addition of magma into the summit reservoir. When Kupaianaha shut down in 1992, the lava volume also decreased slowly over time, and the summit of Kupaianaha collapsed (indicating a withdrawal of magma and drop in the magma level under the crusted-over vent), despite the continued addition of magma into the summit reservoir. In addition to the inflation seen at the summit, the lava pond inside Pu'u 'O'o rose. The shutdown at Kupaianaha after slightly more than nine years of eruptive activity may be similar to the end of the first half of the Mauna Ulu eruption that lasted 2.5 years.

Based on the events at Mauna Ulu and at Kupaianaha, we suspect that the end of activity at Pu'u 'O'o will be heralded by a steady decline in lava output coupled with a lowering of the lava pond. However, we also expect the summit to reinflate with magma as the activity at Pu'u 'O'o wanes. Such pressurization of the summit region could result in eruptive outbreaks further up the east rift zone from Pu'u 'O'o, at Kīlauea's summit, in the southwest rift zone, or in any combination of these locations. Eruptions occurred at all three places as Mauna Ulu activity waned in 1974. However, at the present time, we see no signs suggesting that the current eruption is slowing down.