EPISODE 7

"The seventh major episode of extrusive activity in Kīlauea's continuing middle E rift zone eruption occurred during the 3rd week of August. Lava flows extended NE and SE from the eruptive vent, but did not threaten any developed areas (figure 19).

"After slightly more than 2 weeks of summit inflation, low-level extrusive activity was first observed on 10 August on the floor of the spatter cone (750 km NE of Pu'u Kamoamoa) that marks the vent for episodes 4-6. Sporadic production of very small lava flows through 14 August, accompanied by weak spattering, produced enough new basalt to cover the 30-m-diameter floor of the crater with thin pahoehoe. During this period, the summit continued to inflate, and harmonic tremor, which had been low but steady in the eruptive zone since the end of episode 6, was marked by intermittent bursts of higher amplitude.

"Starting at about 0709 on 15 August, tremor intensity increased rapidly. Within an hour, the seismograph at Pu'u Kamoamoa registered a 10-fold increase in tremor amplitude. By the time the first observers reached the eruptive zone at 0850, fountains 50 m high were playing from the surface of a 20-m-deep pond within the crater. A rapidly moving lava flow had advanced 1 km NE on top of the voluminous episode 6 flow, and a smaller flow was moving slowly S. Steady lava production continued for about 57 hours, ending abruptly at about 1600 on 17 August. An aa flow that had been fed by a vigorous pahoehoe river extended 6.4 km NE and buried a large portion of the episode 6 flow. A second aa flow that advanced less rapidly extended 2.9 km SE. Basalt samples contain scattered small olivine phenocrysts. Lava temperatures measured by thermocouple ranged from 1,132 to 1,141°C.

"Strong harmonic tremor persisted throughout the period of vigorous effusion. At about 1602 on 17 August, tremor decayed rapidly and within an hour assumed a fluctuating low level. Tremor amplitude slowly decreased even further over the next few days, then remained constant and low for the rest of the month.

"Summit deflation recorded by the Uwekahuna tiltmeter began at 1000 on 15 August, several hours after the onset of intense lava production. Nearly 20 µrad of summit collapse, which ended at 1900 on 17 August, suggests that a volume decrease of at least 8 x 10⁶ m³ occurred at the summit during the eruption. For the remainder of the month, the summit reinflated at an average rate of about 1 µrad/day. At least 110 x 10⁶ m³ of magma is estimated to have been lost from the summit reservoir since the E rift eruptive activity began in January 1983.

"On 2 September, field observations of the vent area indicated that small amounts of lava had been intermittently erupted within the main vent crater since the last field check on 29 August. There was some increase in the amplitude of low-level tremor during the night of 1-2 September, probably associated with the new lava emission.

"On the morning of 6 September the eighth major episode of the eruption began. At 0503, an increase in volcanic tremor was recorded on seismographs at HVO. By 0514 the tremor had increased to about 20 times its background level of the previous day. At 0530 a major fume cloud was visible from HVO and the roar of fountains was audible. At 0730 a field crew reported a large single fountain reaching approximately 50 m above the top of the spatter cone built in earlier episodes. A major flow was advancing NE on top of flows from the last 2 major eruptive episodes.

"The eruption was steady and vigorous throughout 6 September. Fountains reached heights of 140 m above the cone. Abruptly at 0525 on 7 September, fountaining ceased and tremor amplitude dropped to slightly above inter-eruptive background levels. Significant lava production during episode 8 lasted for approximately 24 hours. Ten µrad of summit collapse were recorded."

Addendum: On 9 September at 0631 a magnitude 5.5 earthquake occurred at 9 km depth in the S flank, between the eruption vent and the sea coast. Low-level activity resumed at the episode 6-8 vent around midday on 15 September [episode 9]. Vigorous fountaining to approximately 100 m above the spatter cone began between 1700 and 1730 on 15 September, and produced a lava flow that extended 5 km NE on the N and W sides of the episode 6-8 flows. Fountaining ceased between 1900 and 1930 on 17 September.

Geological Summary. Kilauea overlaps the E flank of the massive Mauna Loa shield volcano in the island of Hawaii. Eruptions are prominent in Polynesian legends; written documentation since 1820 records frequent summit and flank lava flow eruptions interspersed with periods of long-term lava lake activity at Halemaumau crater in the summit caldera until 1924. The 3 x 5 km caldera was formed in several stages about 1,500 years ago and during the 18th century; eruptions have also originated from the lengthy East and Southwest rift zones, which extend to the ocean in both directions. About 90% of the surface of the basaltic shield volcano is formed of lava flows less than about 1,100 years old; 70% of the surface is younger than 600 years. The long-term eruption from the East rift zone between 1983 and 2018 produced lava flows covering more than 100 km2, destroyed hundreds of houses, and added new coastline.

Information Contacts: E. Wolfe, A. Okamura, R. Koyanagi, and T. Neal, HVO.