The 12th major episode ended on 1 December after 35 hours of sustained lava production. Approximately 8.1 x 10⁶ m³ of lava was erupted, making episode 12 the second smallest of 1983, and bringing the total volume of lava erupted to about 140 x 10⁶ m³.

"Residents of Hilo (about 35 km NNE of the vent) first sighted visible fountains at about 0450 on 30 November, suggesting that fountains may have been higher during the early morning hours of 30 November than during the remainder of the eruption. When an HVO crew reached the vent at 0830, multiple low (10-60 m-high) fountains were playing within the crater of Pu`u O as well as on its N and S rims and in the breach in its NE rim. These changed little throughout the rest of episode 12. However, beginning shortly after midnight on 1 December, a vent just inside the N rim of Pu'u O produced a 50-m-high steam jet that distributed tephra about 1 km W and SW. The steam jet persisted until the end of episode 12 and was the first of its kind observed on Kīlauea in 1983.

"A vigorous flow was fed by a lava river that issued steadily through the eruption from a major breach of the cone's NE flank, the dominant exit from the crater since the beginning of episode 6. Like the major flows of episodes 6-11, the main episode-12 flow advanced 2.5 km NE, then turned ENE along the S edge of episode 6-11 lavas, veered directly E, and eventually halted on top of 1977 lava 8.2 km from the vent. Smaller flows comprising about 1/3 of the erupted volume extended N, S, and ESE of the cone (figure 22).

"As the eruption stopped abruptly at 1545 on 1 December, harmonic tremor amplitude decreased (figure 23). For the remainder of December, low harmonic tremor and shallow microearthquake activity persisted in the vicinity of Pu'u O. In early January, the interior of Pu'u O was a complex of craters separated by low septa. The cone was gradually cooling; incandescence that was conspicuous in numerous cracks and holes immediately after episode 12 had almost completely disappeared. A small amount of oxidized fume issued from the cone.

"As in previous recent episodes, hand samples of episode 12 are slightly porphyritic with scattered small olivine phenocrysts. Temperatures measured in actively advancing pahoehoe sheets near the vent ranged from 1,135° to at least 1,141°C.

"Minor summit subsidence as recorded by the Uwekahuna tiltmeter (figure 23) began at 0200 on 30 November; vigorous subsidence began at about 0700, approximately 2 hours after the onset of vigorous effusion at the vent. Summit deflation continued until about 1900 on 1 December. Total subsidence, measured E-W, was 13.9 µrad, suggesting a summit volume loss of about 6 x 10⁶ m³. By the end of December the E-W tiltmeter at Uwekahuna showed a net recovery of approximately 12 µrad, including a small deflationary episode, late in the month.

"A 2-km-long zone of new steaming cracks parallel to the axis of the E rift was first observed on 17 December in the rain forest 7 km NE of Pu'u O. Ground observation suggested that at least 1 m of very recent extension perpendicular to the rift had occurred in the zone. The new cracks disappeared at their uprift end under episode 12 lava 1 km NE of Pu'u Kiai, and the evidence suggests that the lava flow may have been broken locally by propagation of the cracks. However, the lava was also emplaced against fresh fault scarps that apparently had not moved subsequently. The cracks were within a narrow (approximately 25-m-wide) graben that has apparently been the locus of such extensional movements since prehistoric time. The zone of cracking was approximately on strike with the January 1983 eruptive fissure, but it was entirely downrift of the shallow earthquake swarm that recorded emplacement of the January 1983 eruptive dike."

Tom Casadevall and Barry Stokes made airborne measurements of SO₂ and CO₂ with a COSPEC and a Moran Infrared Spectrometer. On 2 December, about 16 hours after the end of episode 12 lava production, SO₂ was being emitted at about 260 t/d from Pu`u O. On 9 December, the rate of SO₂ emission had dropped to 20 t/d. On both days, CO₂ production was below the 800-1,000 t/d detection limit. Successive traverses through the plume in the south half of the summit crater yielded values of 3,600 t/d of CO₂ and 300 ± 30 t/d SO₂ on 9 December.

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, T. Neal, and B. Stokes, HVO; T. Casadevall, USGS CVO, Vancouver WA.