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1969-1974 Mauna Ulu Eruption

The Mauna Ulu eruption, on Kīlauea's east rift zone, started on May 24, 1969 and lasted until July 22, 1974. At the time, it was the longest-lasting and most voluminous eruption on Kīlauea's flank in at least 2200 years. The eruption lasted 1,774 days, not counting a break of 3 1/2 months in late 1971—early 1972, and produced 350 million cubic meters (about 460 million cubic yards) of lava. The ongoing Pu‘u ‘Ō‘ō eruption has far surpassed these figures, but the Mauna Ulu eruption was more accessible to the public, with a viewing platform established at one point to observe a lava lake in the crater.

Prelude—Three brief eruptions preceded the big one

Only a few hours of increased seismicity heralded the start of the eruption, but it was not unexpected. In the previous 9 months, three other short-lived eruptions broke out along the rift zone—a lava flow that erupted in February 1969 blocked the Chain of Craters Road near where the Mauna Ulu eruption began. Following the February eruption, the magma reservoir beneath the summit of Kīlauea began to swell, and by May 10 it had reached the level that preceded the February outbreak. On May 21, the HVO staff met to plan its course of study when the new eruption started, but there was no thought that the eruption would surpass its predecessors many times over.

The first 7 months—High fountains, big lava flows, and lava falls higher than Niagara

The eruption started along a 4-km- (2.5-mi-) long fissure system but quickly centered between ‘Ālo‘i and ‘Alae pit craters (both later filled by lava), where Mauna Ulu ("growing mountain") was built. This area was flat, partly covered by recent lava flows and hosting hot fumaroles. The large spatter cone of Pu‘u Huluhulu towered above, serving as an observation point and fountain-measuring station during the eruption.

Twelve fountaining events took place in 1969, the final eleven solely from the site of Mauna Ulu. Fountains soared as high as 540 m (1,770 ft), the highest at Kīlauea except for one during the 1959 eruption. The fountains generally played for several hours, building slowly to a maximum height and then dying within a few minutes. Fallout from the fountains built a tephra mound south (downwind) of the vent.

Lava flows spread outward during the fountaining episodes, entering ‘Alae Crater several times and once spilled into the ocean 12 km (7.5 mi) away. The lava flows covered and bulldozed the deposits from the fountains, creating a chaotic landscape south of the vent.

‘Alae was nearly full on August 4 when one or more cracks suddenly opened across it; liquid lava in the crater drained away, dropping 80 m (260 ft) in 30 minutes. Later that day, some of this lava eventually reached the surface far down the rift zone. Then, on August 5–6, Mauna Ulu sent lava pouring into ‘Alae in cascades higher and wider than American Falls at Niagara.

Between fountaining episodes, lava was usually visible in the vent, where it rose and fell almost rhythmically in a process called gas pistoning. Lava spilled from the elongate vent many times and created a lava shield that eventually matured into the Mauna Ulu edifice we see today.

December 31, 1969 to June 14, 1971—Mauna Ulu gets its name as a shield grows high and a trench forms

No high fountains occurred during this period, but lava flows often left the vent to build up the shield, which prompted the name Mauna Ulu for the growing structure. By July 5, the edifice reached a height of 80 m (260 ft), not to be surpassed until 1972. A new fissure opened across ‘Ālo‘i Crater on April 9, eventually filling the crater with lava.

Starting in July, the walls of the summit fissure began to collapse, the fissure gradually widening into a crater containing a lava lake. Lava from this lake moved underground and fed lava flows erupting from an extension of the original fissure on the east flank of Mauna Ulu. Flows moved into and through ‘Alae Crater, now filled, and advanced 12 km to the ocean on two occasions. A subsidence trench formed along the east-flank fissure but never connected with the elongate summit crater.

June 15 to October 15—Mauna Ulu goes into hibernation

The eruption waned after about June 15, 1971. Fissure eruptions at Kīlauea summit took place in August and September as the lava lake slowly dropped. Mauna Ulu's activity ended on about October 15, when lava disappeared from view, and observers surmised that the eruption was finished.

February 3, 1972 to December 9, 1973—Eruption resumes and ‘Alae shield forms

Kīlauea's summit inflated as Mauna Ulu hibernated, indicating pressurization of the volcano's plumbing system. On February 3 (possibly the 2nd), lava began to enter the summit crater of Mauna Ulu, and by February 5 the crater was half full. In the next 2 days, lava spilled from the crater into the trench, moved to the east end of the trench, and entered the lava tube into ‘Alae Crater, which had remained open since 1971. Clearly the Mauna Ulu eruption had resumed.

During the next 15 months, lava frequently spilled on the surface from the overflowing crater lake, building the shield higher for the first time since July 1970. Of great interest was the growth of a new low shield around ‘Alae, where the filled crater had frequent outflows that moved only short distances, stacking up to form a rootless shield. This shield was ultimately fed through a lava tube from the crater lake in Mauna Ulu itself.

Many lava flows also reached far from ‘Alae, cascading into, and filling the deep pit in Makaopuhi Crater, previously untouched by the eruption. Other flows from ‘Alae covered long stretches of the Chain of Craters Road and frequently entered the sea.

New fissures opened uprift twice during this time, in and near Hi‘iaka and Pauahi Craters on May 5 and in and near Pauahi on November 10, 1973. These were the first eruptions to take place far from the Mauna Ulu edifice. However, activity eventually returned to Mauna Ulu after each of these small, isolated eruptions.

December 10, 1973 to July 22, 1974—Low fountains, shield grows high, but eruption dies

During this time, all eruptive activity was at Mauna Ulu itself, where short-lived fountains 40–80 m (130-260 ft) high alternated with longer periods of quiet overflows from the crater. The overflows added to the shield, which eventually reached its maximum height of 121 m (397 ft) above the pre-eruption base.

After June 2, the lava lake became increasingly sluggish. Though apparently unfazed by an eruption at Kīlauea's summit on July 19–22, the lava lake disappeared from view, and, in hindsight, the end of the eruption was declared on July 22.

Additional Resources for Mauna Ulu

Trail Guide

Hawai‘i Volcanoes National Park, Mauna Ulu Eruption Guide

Volcano Watch articles about the eruption of Mauna Ulu