EPISODES 8 and 9

"Episodes 8 and 9 of Kīlauea's long-lived middle E rift zone eruption occurred during September. All lava issued from the same vent, 750 m NE of Pu'u Kamoamoa, that has been the dominant locus of eruptive activity starting with episode 4 in mid-June. The major episode 8 and 9 flows extended NE along the NW edge of the episode 6 and 7 flows.

Low-level activity, 2-5 September. "In the pre-dawn hours of 2 September, following about 2 weeks of low but constant harmonic tremor in the eruptive zone and steady inflation of the shallow summit reservoir, approximately 4 days of low-level, intra-crater eruptive activity began at the vent. Harmonic tremor during this 4-day period, although still low, was characterized by intermittent episodes of slightly higher intensity. Prior to the renewal of activity, the crater floor was dominated by a steep-walled deep hole; an incandescent crack issued fume at the W base of the interior crater wall. By 0900 on 2 September, the deep central depression had been filled, and a 4 m-high mound of spatter surrounded a fuming and sporadically spattering conduit near the center of what had become a nearly flat, 30 m-diameter crater floor. At the W edge of the floor, several very small lava flows had been extruded from the area of the incandescent crack. Intermittent extrusion from this area continued through 4 September, producing about 300 m³ of small solidified flows. On 5 September, a lava pond 5 m deep covered the crater floor, and a lava stream carrying an estimated 1,000-2,000 m³/hour issued from the vent at the pond's W edge, disappearing downward near the pond's center. Although a 'bathtub ring' showed that the pond had earlier been as deep as 15 m, no significant change in the pond's volume was seen during 6 hours of observation on 5 September. Minimal spatter production implied that the lava of this intra-crater activity was relatively degassed.

EPISODE 8, 6-7 September 1983

"Beginning at 0503 on 6 September, harmonic tremor rapidly increased. Within 15 minutes, tremor amplitude was an order of magnitude higher, and remained intense throughout the eruption. By about 0530 the roar from the fountaining vent could be heard and the eruption plume seen from the summit region.

"Although brief, the eruption was vigorous. Fountains rose 100-200 m from the surface of the pond within the crater. As in episodes 6 and 7, lava issued from a breach in the NE rim of the crater, producing an aa flow that extended NE for more than 4 km. A subordinate aa flow extended about 2 km SE.

"Harmonic tremor decreased rapidly from about 0520 on 7 September, and fountaining stopped at about 0525, approximately 24 hours after it had begun. Steady low tremor continued until 14 September. Two open conduits were left extending steeply downward from the W edge and the center of the crater floor.

EPISODE 9, 14-17 September 1983

"After a week of quiescence, sporadic spattering began in the crater shortly after midnight on 14 September, and harmonic tremor gradually increased to mark the onset of episode 9. Lava began to cover the crater floor at about 1030 on 15 September. After an hour, when the pond was about 5 m deep, a few hundred m³ of lava spilled through the deep breach in the NE crater rim. The remainder, about 3,000 m³, drained back into the central conduit and disappeared at about 1300, 2.5 hours after the pond began to form. Two hours later, a second pond, not deep enough to overflow the breach, formed and drained within 30 minutes. Refilling of the pond began again about 5 minutes later, and within 15 minutes (by about 1540) lava had overtopped the spillway from the crater. A flow supplied at a rate of about 10,000 m³/hour began advancing NE along the evacuated episode-8 channel. A small dome fountain 3-8 m high played on the pond surface above the central conduit, and there was almost no spatter. Beginning about 1700, harmonic tremor increased gradually. At about 1711, the first spatter was visible above the approximately 50 m-high W rim of the crater, and by 1730 the tremor had increased to the large amplitude typical of vigorous lava production. By 2130 the fountain was visible from the Wahaula Visitor Center near the coast (approximately 11 km SE of the active vent), which suggests a fountain height on the order of 300 m.

"Vigorous eruption continued for just over 2 days, ending at about 1915 on 17 September. The major flow of this episode, aa supplied during vigorous eruption at about 1-2 x10⁵ m³/hour, advanced more than 5 km NE through the rain forest along the NW edge of the episode-8 flow. Fountains were initially very high, but became erratic in height and direction, ranging up to about 200 m high. Rapidly changing in inclination and azimuth of trajectory, they heavily armored the growing spatter cone (about 60 m high) with spatter-fed flows.

"At the eruption's end, 2 apparent conduits much like those seen before the eruption were visible in the crater floor. Near the center of the floor was a nearly cylindrical, vertical, fuming hole approximately 4 m in diameter. At the W edge of the floor was a fuming, glowing, elongated hole 1 m wide and 2-3 m long.

"Harmonic tremor in the eruption zone diminished rapidly at the end of the eruption. By 1930 on 17 September it had dropped to the low level characteristic of repose periods. Since then tremor has remained continuous and low.

"The volume of new basalt, mostly aa, produced during episodes 8 and 9 is approximately 16 x 10⁶ m³, and the total volume of basalt produced since the beginning of episode 1 on 3 January is about 106 x 10⁶ m³. The episode 8 and 9 basalts are slightly porphyritic, with scattered small olivine phenocrysts.

"Summit reservoir deflation and inflation associated with episodes 8 and 9 extended the strongly cyclic pattern that has been established in recent months. Deflation approximately coincident with the eruptions was recorded at Uwekahuna vault by rapid inward tilt changes of 14.8 µrad for episode 8 and 16.2 µrad for episode 9. These correspond approximately to summit volume losses of 6 and 6.5 x 10⁶ m³, respectively. The total deflationary volume loss at Kīlauea's summit since the beginning of January is about 122 x 10⁶ m³."

Addendum: The 10th major episode of eruptive activity began early 5 October following 3 days of minor and intermittent lava emission and drainback in the episode 4-9 vent. Tremor amplitude increased at 0327, and lava fountaining started by about 0430. Fountains rose to maximum heights of about 300 m, producing a substantial volume of airfall pumice, and fed two lava flows that extended 4 km NE and SE of the vent. Fountaining ceased at about 1700 on 7 October, and inflation of the summit region resumed. Extremely low-level harmonic tremor continued as of 11 October.

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.