On Sept. 24, 1971, Kilauea’s southwest rift zone erupted for the first time in 52 years.
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 scientists noted in July that changes in the volcano’s behavior might be afoot.
HVO’s speculation was confirmed on Aug. 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 a Friday, Sept. 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 Halemaumau 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 Halemaumau and Kilauea’s southwestern caldera wall, erupting lava fountains as high as 165 feet. Lava flows spread south and east, spilling into Halemaumau in a dramatic cascade that soon covered the crater floor.
Within half an hour, lava fountains were erupting from the floor of Halemaumau and up the east wall of the crater, where a 270-yard-long eruptive fissure opened on the adjacent caldera floor. At 8 p.m., ground cracks opened west of Halemaumau 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 — the first since the 1919-1920 Mauna Iki eruption. Migrating at an average rate of 33 feet per minute, erupting fissures extended nearly a kilometer 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, lava fountains erupted 1.6 miles 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 0.9 miles downrift, but once again, the SWRZ activity stopped. Late that night, active lava in Halemaumau 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 7.5 miles southwest of Kilauea Caldera. These lava fountains, between 50 and 65 feet high, were the farthest southwest of the entire eruption and 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 reappeared 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 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 eruption, on Sept. 29, parts of the SWRZ were extensively cracked and dilated 3 to 10 feet, newly erupted lava covered more than 1.5 square miles, and the floor of Halemaumau Crater had dropped 148 feet 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.
Kilauea activity update
A lava lake within the Halemaumau Overlook vent produced nighttime glow visible from the Jaggar Museum overlook and by HVO’s webcam during the past week. The lava level rose and fell slightly due to a string of deflation-inflation cycles at the summit and several brief gas-driven rise-fall cycles.
On Kilauea’s east rift zone, surface lava flows remain active at the top of the pali, within the upper part of the abandoned Royal Gardens subdivision, about 4 miles southeast of Puu Oo. The lava pond in the northeastern pit in Puu Oo crater was visible in the webcam over the past week, with the level fluctuating slightly in response to the DI events.
There was one earthquake reported felt on the Island of Hawaii in the past week. At 9:52 a.m. Sept. 15, a magnitude 3.0 earthquake occurred 5 miles northwest of Kailua-Kona at a depth of 21 miles.
Visit hvo.wr.usgs.gov for detailed Kilauea, Mauna Loa and Hualalai activity updates, recent volcano photos, recent earthquakes and more; call 967-8862 for a Kilauea 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.