As erupting fissures migrated down Kilauea’s southwest rift zone in September 1971 for the first time in 52 years, many spectators hiked into the Ka`u Desert for a closer view of the rare event. Hawai`i Volcanoes National Park rangers patrolled the area on horseback to keep people a safe distance from the lava fountains. Photo courtesy of NPS.

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Time-lapse movie of Pu‘u ‘O‘o Crater

(Volcano Watch is a weekly article written by scientists at the U.S. Geological Survey’s Hawaiian Volcano Observatory.)

Kilauea’s east rift zone had been erupting almost nonstop since May 1969, but activity at the Mauna Ulu vent was declining by mid-1971. In fact, USGS Hawaiian Volcano Observatory (HVO) scientists noted in July that changes in the volcano’s behavior might be afoot.

HVO’s speculation was confirmed on August 14, when lava erupted at the summit of Kilauea for about 10 hours. The east rift zone eruption continued at a low level, but during the next six weeks, inflation and seismicity at Kilauea’s summit increased.

Then, on Friday, September 24, at 7:12 p.m., a swarm of small, shallow earthquakes, accompanied by continuous volcanic tremor beneath Kilauea’s summit caldera, triggered HVO’s monitoring alarms. Less than 10 minutes later, observers saw a red glow from the caldera floor just west of Halema`uma`u Crater. By 7:30 p.m., they could also hear the roar of lava fountains. Kilauea’s summit was erupting for the second time in six weeks!

The initial fissures broke out between Halema`uma`u and Kilauea’s southwestern caldera wall, erupting lava fountains as high as 50 m (165 ft). Lava flows spread south and east, spilling into Halema`uma`u in a dramatic cascade that soon covered the crater floor.

Within half an hour, lava fountains were erupting from the floor of Halema`uma`u and up the east wall of the crater, where a 250-m- (270-yard-) long eruptive fissure opened on the adjacent caldera floor. At 8:00 p.m., ground cracks opened west of Halema`uma`u and across Crater Rim Drive, rendering the road impassable.

Minutes later, a fissure opened on the southwest caldera rim, erupting lava fountains that soon moved into Kilauea’s southwest rift zone (SWRZ)—the first since the 1919-1920 Mauna Iki eruption. Migrating at an average rate of 10 m (33 ft) per minute, erupting fissures extended nearly a kilometer (0.6 mi) down the SWRZ within a couple of hours.

Fed by the fountains, fast-moving lava flows quickly spread across Crater Rim Drive, eventually flooding one of HVO’s instrument vaults—destroying a tiltmeter, two seismometers, and other volcano-monitoring tools. The flows also poured into gaping cracks along the SWRZ, sending a large quantity of lava back underground.

By 10:22 p.m., fountains from the SWRZ fissures had ceased and activity within the caldera had greatly diminished. It appeared that the eruption was ending.

But on Saturday morning, September 25, lava fountains erupted 2.5 km (1.6 mi) farther down the SWRZ, with vents continuing to migrate down the rift. As new fissures opened, uprift vents died. In less than two hours, lava fountains had moved nearly 1.5 km (0.9 mi) downrift, but once again, the SWRZ activity stopped. Late that night, active lava in Halema`uma`u was no longer visible. It seemed that the eruption was over.

Early Sunday morning, however, lava erupted southwest of Mauna Iki. Fissures continued to open downrift, and, by mid-morning, a line of fountains was erupting about 12 km (7.5 mi) southwest of Kilauea Caldera. These 15-20-m (50-65-ft) high lava fountains—the farthest southwest of the entire eruption—were readily visible from Highway 11. The views resulted in traffic congestion near milepost 36 as thousands of people flocked to see the activity.

On Sunday afternoon, lava fountains re-appeared uprift of Mauna Iki. Over the next three days, lines of erupting fissures “hopscotched” up and down the SWRZ in a repeatedly observed sequence of events. At the “head” of a migrating fissure, fumes issued from the ground as cracks opened and widened. Then, in quick succession, molten clots of lava were ejected a few meters (yards) into the air, dense clouds of dark fume billowed skyward, and then fountains of lava erupted. At the “tail” of a fissure, activity diminished in reverse, from lava fountains to gas venting.

By the end of this 5-day-long eruption on September 29, parts of the SWRZ were extensively cracked and dilated 1-3 m (3-10 ft), newly erupted lava covered more than 4 sq km (1.5 sq mi), and the floor of Halema`uma`u Crater had dropped 45 m (148 ft) as liquid lava drained into new cracks.

Kilauea’s 1971 SWRZ eruption was the coup de grace of the 1969-71 Mauna Ulu activity, which barely dragged on until mid-October.