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Volcano Watch — A look back at Mauna Loa

December 1, 2011

Hikers reaching the rim of Moku‘āweoweo, Mauna Loa's summit caldera, can gaze across the expanse of lava—erupted in 1984—that covers most of the caldera floor. If you have not had this experience, you can witness it vicariously through Mauna Loa Webcam images posted on the Hawaiian Volcano Observatory's Website (

Aerial view by the Naval Air Service of the 1933 Mauna Loa eruption...
Aerial view by the Naval Air Service of the 1933 Mauna Loa eruption from a fissure across the rim and floor of Moku‘āweoweo Crater.

If we could remove the 1984 lava from the floor of Mauna Loa's caldera, you would see flows erupted in 1975. Peeling away that flow, you would encounter lava erupted in 1949, and below that, lava erupted in 1940. Although these earlier flows are now buried, we can still see evidence of those eruptions today—a cinder cone from the 1949 eruption stands on the southwestern caldera rim, and a cinder cone from the 1940 eruption, now surrounded by 1984 lava, stands on the caldera floor near the southern end of the caldera.

Digging deeper, you would eventually find yourself standing on lava from Mauna Loa's 1933 summit eruption. Little, if any, of that lava remains visible at the surface today, but lack of exposure does not negate this short-lived eruption.

The 1933 eruption was heralded by a cluster of earthquakes on Mauna Loa in October of that year. Then, at dawn on December 2, the onset of continuous tremor marked the beginning the eruption.

The fissure that fed the eruption cut across the southwest end of the caldera, through the caldera's rim, and down Mauna Loa's southwest rift zone for a distance of about 1 km (2/3 mile). Lava fountains at least 60 m (200 ft) high sent flows across the caldera floor, and lava cascaded into South Pit and Lua Hohonu, two pit craters on Mauna Loa's upper southwest rift.

The fissure eruption, outside the caldera, ended the first day, but within the caldera the eruption continued unabated. A dense, white column of fume towered above the volcano, and Pele's hair fell on villages on Hawai‘i's west coast. The eruption was soon focused at two principle fountains near the southwest end of the caldera, and horseshoe-shaped cones 30–60 m (100–200 ft) high grew around the erupting vents.

Fountaining continued over the next several days, spreading lava northeast across Moku‘āweoweo. But, by December 12, the fountains had begun to dwindle as the eruption declined. On December 18, the eruption ended. Half of the caldera floor, the floors of South Pit and Lua Hohonu, and a small area of Mauna Loa's summit just outside the caldera were buried beneath 23–30 m (77–100 ft) of lava.

In January 1934, R. F. Lueck, a National Park ranger, and T. Generozza, a soldier with the U.S. Army, hiked to the summit of Mauna Loa, where they set up camp near the newly erupted cones. Crawling into their sleeping bags, they were prepared for a cozy night atop the still-warm lava. Their coziness, however, was soon disturbed by discontinuous rumbling noises, which seemed to emanate from directly beneath them.

Lueck reported, "After about two hours of this, we began to think of a lot of places we would rather be than just here." He went on to say, "At 8 p.m. we felt a shock more severe than any we had felt before, so we proceeded to leave that particular spot, as fast as our legs would carry us in the night, across the pumice beds and rough lava."

The two intrepid hikers did spend the night in the caldera, but at a safer distance, far from their original camp at the base of the 1933 cones. As it turns out, what they experienced was the volcano's response to the cooling and contraction of the magma column beneath the cone, not a sign of impending new activity.

Mauna Loa's current dearth in activity belies its much more active past, when eruptions occurred, on average, every 4–5 years. Mauna Loa will someday return to its old habits, and hikers will have something new to see when they visit the summit of the world's most massive volcano.


Volcano Activity Update

A lava lake was present within the Halema‘uma‘u Overlook vent over the past week, resulting in night-time glow that was visible from the Jaggar Museum overlook. The lake, which is about 100 m (330 ft) below the floor of Halema‘uma‘u and visible by Webcam, rose and fell slightly during the week in response to back-to-back deflation-inflation cycles.

Eruptive activity on Kīlauea's east rift zone was restricted to surface flows that had reached to about 6.5 km (4 miles) southeast of Pu‘u ‘Ō‘ō, into the top of the Royal Gardens subdivision. The flows traveled through a lava tube that is fed by the September 21 fissure on the upper east flank of the Pu‘u ‘Ō‘ō cone. There were also a few tiny, intermittent flows erupted within Pu‘u ‘Ō‘ō crater.

One earthquake beneath Hawai‘i Island was reported felt this past week. A magnitude-3.0 earthquake occurred at 2:45 p.m., HST, on Monday, November 28, 2011, and was located 3 km (2 mi) southwest of Kalapana at a depth of 8 km (5 mi).

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