"Two eruptive episodes occurred on the E rift zone in June [and one in early July]. Both produced relatively small volumes of lava (less than 6 x 10⁶ m³ each) and unusually high fountains (in excess of 300 m) seen from HVO for the first time since the eruption began in January 1983. Episode 20 (7-8 June) and episode 21 (30 June) produced lava flows as long as 4.5 km, all on or NW of the E rift zone. Following episode 21, Pu'u O, the steep-sided spatter cone that has been the locus of eruptions for more than a year, stood about 140 m above the pre-1983 surface and was mantled by an extensive blanket of tephra. Between episodes, an active column of magma was visible in an open conduit several tens of meters below the floor of Pu'u O's summit crater. It periodically emitted gas and small particles of spatter in bursts of less than a minute, separated by intervals of 5-12 minutes. This gas-pistoning was reflected in the tremor record from the seismic station near the eruption site.

EPISODE 20

"Low-level repose-period activity preceded Episode 20. The time-lapse film shows that intermittent spatter was visible above the spillway as early as 6 June at 2328 but was confined to the crater; it continued until 0448 on 7 June. For the next 11.5 hours only heavy fume was recorded.

"Episode 20 occurred largely at night, so the record consists primarily of time-lapse photography and the tiltmeter and seismic data described below. At 1911 on 7 June continuous low-level spatter activity commenced and sporadic fountaining began 65 minutes later. Lava first poured over the spillway at 2104, and from 2200 until 0624 on 8 June high fountaining was continuous. Maximum fountain heights were estimated at 300 m, because they were visible above the horizon from HVO. The last fountains recorded on film were at 0627; they were followed by intense fuming that decreased over the next several hours. During the period of vigorous eruption, 4 lava flows moved N and NE of Pu'u O, the longest reaching about 4 km from the vent (figure 13-30).

EPISODE 21

"On 30 June, observers reached the eruption site at 1045, a few minutes after episode 21 lava topped the spillway and just before vigorous fountaining began. A maximum fountain height of approximately 390 m was measured by theodolite at 1115. This high level was maintained for about 4 hours after which fountaining decreased as flow apparently increased down the spillway. Early in the eruption, a fissure vent opened on the SW flank of Pu'u O, producing a line of low fountains and a pahoehoe flow that ponded on the S side of Pu'u Kamoamoa (about 0.5 km to the W). A large lava flow that divided into several tongues extended 3-4.5 km NE of the vent, generally on top of episode 20 flows. A third flow of spatter-fed aa from the N rim of the crater overran a similar episode 20 flow for about 1 km N of Pu'u O. A thin blanket of tephra, including an early deposit of reticulite, was deposited SW of the vent. The eruption ended abruptly at 1827 with a simultaneous termination of fountaining and lava flow at the spillway. Heavy fuming and sporadic burning of hydrogen gas followed the eruption for several hours.

"Approximately 3.7 x 10⁶ m³ of new basalt was produced in episode 20 and a similar amount (mapping still in progress) was produced in episode 21.

EPISODE 22

"At the time of this writing, episode 22 has just ended, having produced 5 additional flows and another tephra blanket during activity that lasted from about 1930 on 8 July to 1017 on the 9th."

Deformation and Seismicity. "The E-W tilt measured at Uwekahuna increased irregularly after the brief episode 19 eruption of 16-17 May. On 7 June at about noon, deflation of the summit region began slowly, increasing to a rapid rate after 2100 when eruptive activity of episode 20 began. Subsidence continued through the eruption and for nearly 10 hours after fountaining activity had ceased. The post-eruptive deflation accounted for 30% of the total tilt change.

"Following episode 20, the return to summit inflation was unusually slow. For the first week, summit tilt increased by less than 2 µrad. During the next 10 days there was a 5-µrad increase in tilt followed by 5 days of little change. On 28 June, slow inflationary tilt resumed for about a day until the summit began to subside slowly. Rapid deflation commenced on 30 June at about 1100, shortly after the start of episode 21 lava production at Pu'u O. Deflation continued through the eruption and for nearly 9 hours afterwards. Post-eruption deflation accounted for nearly 40% of the total tilt change.

"Harmonic tremor at the Kamoamoa seismic station continued at low levels during the repose periods between brief eruptive stages. As in previous episodes, tremor increased to high amplitudes shortly before high fountaining began and returned to background levels when fountaining ended. High tremor of episode 20 began on 7 June at 2120 and ended on 8 June at 0627. Episode 21 tremor began on 30 June at 1028 and ended the same day at 1827. Small bursts of increased tremor during the repose periods coincided with brief intervals of gas-pistoning (09:5). This phenomenon of gas-pistoning (restricted to the vent conduit) has occurred regularly since episode 19. It became erratic several hours prior to the onset of episode 21, coincident with a slight increase in background tremor, and resumed its periodic pattern soon after episodes 20-21 ended.

Gas measurements. "COSPEC measurements of SO₂ emissions during the repose period between episodes 20 and 21 averaged 1.4 metric tons per hour, rising to nearly 6 metric tons per hour during gas bursts. Measured SO₂ emission during episode 21 was about 1,000 metric tons per hour."

Further Reference. Wolfe, E.W. (ed.), 1988, The Pu`u `O`o Eruption of Kīlauea Volcano, Hawaii, Episodes 1-20, January 3, 1983 Through June 8, 1984; USGS Professional Paper 1463, 8 papers.

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: G. Ulrich, A. Okamura, and R. Koyanagi, HVO.