Volcano Watch — The Mauna Ulu eruption 1969-1974

The Mauna Ulu eruption on Kīlauea's east rift zone began 28 years ago this month, on May 24, 1969. For the next 2.5 years eruption was almost continuous and often spectacular. The eruption was the longest and largest on the rift zone in post-contact time until surpassed by the Pu`u `O`o eruption. After a 3.5-month pause (October 15, 1971-February 3, 1972), Mauna Ulu resumed erupting and continued to July 1974.

Mauna Ulu formed between `Alo`i and `Alae Craters, popular stops along the old Chain of Craters Road. These craters are now only memories, for lava filled both of them during the eruption. The present road, rerouted around the Mauna Ulu area, bends away from the rift zone 1.3 km (0.8 mi) west of `Alo`i and intersects the old road at the top of Holei Pali.

Twelve fountaining episodes highlighted the first six months of the Mauna Ulu eruption. Six produced fountains 300 m (1,000 ft) or more high. One, on September 6, towered to 540 m (1,770 ft), a height surpassed only once in this century (580 m (1,900 ft) during the 1959 Kīlauea Iki eruption). The fountains were readily seen and heard from Volcano, and the Uwekahuna overlook at the U.S. Geological Survey's Hawaiian Volcano Observatory (now the Jaggar Museum) was often crowded with spectators enjoying the show. A privileged few watched some of the fountains from the top of nearby Pu`u Huluhulu, where the heat was nearly unbearable.

The fountains fed fast-moving lava flows that traveled as far as the coast, 12 km (7 mi) away. In the wee hours of June 26, hundreds of visitors along the coastal road in the Kealakomo ahupua`a were entranced by the majestic spectacle of lava pouring in broad surges down 220-m (700-ft) high Holei Pali.

Between periods of fountaining, lava splashed and circulated in the vent of Mauna Ulu, sometimes forming low fountains, sometimes quietly upwelling and overflowing the vent. Similar activity continued for months after the last high fountain on December 30, 1969. Overflows built the Mauna Ulu shield until mid-1970 and again added to it in 1972-74. By the time it stopped growing, Mauna Ulu stood 121 m (400 ft) above its pre-eruption base. Its summit crater, at times as deep as 200 m, had shallowed to about 30 m (100 ft) when the eruption ended.

Lava tubes gradually developed as flows moved away from Mauna Ulu during periods of continuous weak activity. The tubes frequently carried lava into the ocean in 1970-74, from west of `Apua Point to east of Ka`ena Point, burying the road and archeological sites at Kealakomo.

`Alae was a large crater southeast of Mauna Ulu, 440 m (1,440 ft) in diameter and 165 m (540 ft) deep. It filled grudgingly. By midnight on August 3-4, 1969, flows from Mauna Ulu had almost filled `Alae to within 10 m (30 ft) of its rim. Four hours later, hundreds of earthquakes accompanied the opening of a wide crack across the crater floor, and most of the liquid lava in the lake drained away. In 30 extraordinary minutes, lake level dropped 80 m (260 ft) as 10 million cubic meters (13 million cubic yards) poured underground. The crack, locally 10 m (30 ft) wide at the surface, transported the lava 7.5 km (4.5 mi) downrift, where some eventually erupted later in the day.

Fountain-fed flows in September and October cascaded as red-hot Niagaras into `Alae, completely filling it on October 20, 1969. By 1974, more flows had built a shield nearly 90 m (295 ft) high over the buried crater; perched ponds on the shield can be seen makai of the Napau trail.

The writer witnessed many of these remarkable events. The memories are indelible.

Volcano Activity Update

Kīlauea's east rift zone eruptive activity is limited to a sustained lava pond within the Pu`u `O`o Crater and to two vents on the west and south sides of Pu`u `O`o. The western vent has built a 15-meter-high spatter cone and occasionally sends flows to the west and southwest. Lava from the southern vent flows into a perched pond which feeds `a`a flows to the southeast. The recent media blitz on the eruption was due to the availability of excellent video footage and not because of any change in the eruption.

There were no earthquakes reported felt during the past week.