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Volcanowatch

May 21, 2009

A weekly feature provided by scientists at the Hawaiian Volcano Observatory.


Today is the 40th birthday of the wondrous Mauna Ulu eruption

Lava cascades more than 300 feet into `Alae Crater, August 6, 1969 (USGS photo).
Lava cascades more than 300 feet into `Alae Crater, August 6, 1969 (USGS photo).

Just before daybreak on May 24, 1969, a new eruptive fissure split open Kīlauea's east rift zone at its bend near `Alo`i Crater. During the previous 9 months, three small eruptions in the bend had lasted only a few days. When spattering at the main vent just northeast of `Alo`i stopped on May 29, we thought this would be just another pea in the pod.

Sloshing sounds continued to be heard below the vent, however. To our astonishment, a double lava fountain 100 m (330 feet) high rapidly blossomed on June 12. Clearly this eruption was something different!

Over the next 5 years, the largest outpouring of lava at Kīlauea since the 15th century filled two pit craters (`Alo`i and `Alae) and parts of three others (Makaopuhi, Pauahi, Hi`iaka), covered 12 km (7 miles) of highway, and produced the second highest lava fountain (540 m, nearly 1,800 feet) recorded in Hawai`i. It also built a broad lava shield over the main vent and a large rootless shield above buried `Alae, and sent lava flows into the sea, covering the village site of Kealakomo.

The main vent area was the site of 8 episodes of lava fountaining higher than 150 m (500 feet), the last on December 30, 1969. Each fountain played for several hours, and HVO scientists watched and measured several from the overlook on Pu`u Huluhulu. The intense radiant heat of the fountains, 600 m (2,000 feet) away, forced us to crouch behind the overlook's stone wall except for brief periods of observation and measurement. Several times, dry grass and small shrubs burst into flame around us. Fallout from two fountains landed just below the overlook; one prompted hurried evacuation.

Between fountains, lava was almost always near the surface, and we could observe processes, such as gas pistoning, at closer range than ever before. During gas pistoning, expanding gas bubbles trapped under the crust slowly lift lava up and out of the vent. Eventually the crust on the lava breaks, gas rapidly escapes, and the lava drains back underground.

Such "breathing" cycles, typically involving 30-60 minutes of rise and a few minutes of drainback, were so common that we learned to anticipate when the crust would break and drainback begin. We could then trigger drainback by throwing rocks through the crust, initiating sudden gas release.

Lava flowed into `Alae several times, and the crater, once 165 m (540 feet) deep, was nearly full by midnight, August 3. Only 4 hours later, a crack cut the crater floor, and, within 30 minutes, much of the lava drained underground. Some eventually erupted 7.5 km (4.5 miles) down the east rift zone. The spectacular, and unprecedented, emptying of `Alae left a crater some 100 m (330 feet) deep?but only for a day.

Incredibly, a high fountain started feeding lava toward `Alae at 9 p.m., August 5. Anticipating a marvelous spectacle, two of us ran to the crater's south rim, hurrying to beat lava flows headed our way. We made it in time to see broad rivers cascading down the walls of `Alae in a display higher and wider than American Falls at Niagara. The heat was excruciating and thermal updrafts so strong that we could barely stand, but the wondrous scene was worth twice the discomfort.

`Alae was partly refilled by the unforgettable lava falls, which lasted a few hours, and became completely full a month later. The smaller `Alo`i Crater suffered a similar indignity, partly filling on December 30 before the coup de grace was administered on April 24, 1970.

Mauna Ulu grew higher after each high fountain and vent overflow. By the end of June 1970, the shield stood 80 m (260 feet) high. About then, the edifice was christened Mauna Ulu ("growing mountain" in Hawaiian). No further growth occurred until early 1972. Another spurt in early 1974 brought it to its final height of 121 m (about 400 feet).

Today, the Mauna Ulu eruption has been largely forgotten, hidden in the shadow of its upstart cousin from Pu`u `Ō `ō. But Mauna Ulu deserves its place as one of the great Kīlauea eruptions. Use the national park's new "Mauna Ulu Eruption Guide" and self-guided trail to recapture some of the excitement that started 40 years ago today.

Kīlauea Activity Update

The Waikupanaha and Kupapa`u ocean entries remain active and are sending up robust laze plumes. Frequent small collapses have prevented either entry from building a large delta. There have been no lava breakouts from anywhere along the tube system reported in the last week. At Kīlauea's summit, the vent within Halema`uma`u Crater continues to emit elevated amounts of sulfur dioxide gas, resulting in high concentrations of sulfur dioxide downwind. Glow, gas-rushing noises, and the emission of juvenile ash during the past week suggest that a small lava lake is still present at shallow levels below the floor of Halema`uma`u Crater.

Felt Earthquakes

Two earthquakes beneath Hawai`i Island were reported felt this past week, both on Sunday, May 17, 2009. A magnitude-3.1 earthquake occurred at 3:15 p.m., H.s.t., and was located 14 km (9 miles) northwest of Kailua at a depth of 10 km (6 miles). On the opposite side of the island, a magnitude-2.5 earthquake occurred at 5:43 p.m. and was located 3 km (2 miles) southwest of Hakalau at a depth of 33 km (21 miles).

Visit our Web site (http://hvo.wr.usgs.gov) for detailed Kīlauea and Mauna Loa activity updates, recent volcano photos, recent earthquakes, and more; call (808) 967-8862 for a Kīlauea activity summary; email questions to askHVO@usgs.gov. Volcano Watch is a weekly article and activity update written by scientists at the U.S. Geological Survey's Hawaiian Volcano Observatory.

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Updated: May 26, 2009 (pnf)