Minutes later, a fissure opened on the southwest caldera rim, erupting lava fountains that soon moved into Kīlauea’s southwest rift zone (SWRZ)-the first since the 1919-1920 Mauna Iki eruption. At 8:00 p.m., ground cracks opened west of Halema‘uma‘u and across Crater Rim Drive, rendering the road impassable. 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. The initial fissures broke out between Halema‘uma‘u and Kīlauea’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. Kīlauea’s summit was erupting for the second time in six weeks! By 7:30 p.m., they could also hear the roar of lava fountains. Less than 10 minutes later, observers saw a red glow from the caldera floor just west of Halema‘uma‘u Crater. Then, on Friday, September 24, at 7:12 p.m., a swarm of small, shallow earthquakes, accompanied by continuous volcanic tremor beneath Kīlauea’s summit caldera, triggered HVO’s monitoring alarms. The east rift zone eruption continued at a low level, but during the next six weeks, inflation and seismicity at Kīlauea’s summit increased. HVO’s speculation was confirmed on August 14, when lava erupted at the summit of Kīlauea for about 10 hours. In fact, USGS Hawaiian Volcano Observatory (HVO) scientists noted in July that changes in the volcano’s behavior might be afoot. Kīlauea’s east rift zone had been erupting almost nonstop since May 1969, but activity at the Mauna Ulu vent was declining by mid-1971. Hawai‘i Volcanoes National Park rangers patrolled the area on horseback to keep people a safe distance from the lava fountains. As erupting fissures migrated down Kīlauea's southwest rift zone in September 1971 for the first time in 52 years, many spectators hiked into the Ka‘ū Desert for a closer view of the rare event.
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