This report covers activity during June through 12 September 1999; however, data for the interval 5 July to 11 September were not available when this report was compiled in early 2000.

In early June very little changed in the eruptive activity at the Pu`u `O`o vent. Lava continued to travel through the lava-tube system to the coast where it has built a lava bench, extending into the ocean. Frequent collapses of the bench prevent it from extending seaward more than ~ 200 m from the former shoreline. Slow-moving pahoehoe flows continued to occasionally break out from the tube system and spread out over the broad coastal plain. In mid-June the 24th pause in the current eruptive episode (55) occurred.

During 19-20 June a tiltmeter at the summit showed an outward tilt (inflation) taken as indicating that magma moved into the summit reservoir. Just after the 24th pause began, the tiltmeter near Pu`u `O`o recorded a slight increase in tilt away from the E rift zone. This tilt suggested that some magma entered the rift zone at the same time that magma was accumulating in the summit reservoir. The pause may have resulted from a blockage beneath or immediately uprift of Pu`u `O`o. This tilt reversal pattern was also seen during the eruptive pause in May 1999. During parts of June no lava traveled underground to the coast. The tube drained soon after the intrusion. As the tube's roof began to cool and shrink, it fell in places, forming holes and partial dams along the tube. Later, when lava reentered the tube, these created points where lava escaped to the surface. Apparently the dam at about 530 m elevation remained intact and sealed the tube from draining significant lava below that point.

During the night of 2-3 July the seaward margin of the bench, a segment up to 75 m long, slid into the ocean removing ~1.7 hectares of new land. Strong explosions occurred intermittently during the next few days, climaxing on 4 July. Below is a description from a scientist who witnessed these explosions on 2-3 July.

"Major explosions and jetting started on the bench at 1814. Bubble bursts were occurring from three locations and lava fountaining was occurring from a vent on the western part of the bench. Lava was thrown up to 150 m into the air by the fountaining. The fountain produced a nested pair of cones, approximately 30-45 m in diameter and 5-7 m tall. The bubble bursts took place about 20-40 m inland of where the waves could reach . . . [The bubbles were tens of meters in diameter.] The entire cone and surrounding bench undulated like a liquid during the most vigorous bubble-burst activity. Between explosive episodes, a series of surface flows broke out, covering most of the visible bench. One flow that poured into the ocean built a tube through the surf, which led to a series of underwater explosions about 25 m offshore."

A press release noted that early on the morning of 12 September, a swarm of small earthquakes and volcanic tremor occurred on the E rift zone. A sharp deflation of both the summit area and parts of the E rift zone was associated with a new intrusion of magma. Magma moved from both the summit area and from near Pu`u `O`o into the upper rift zone, forming a dike in the area between Pauahi Crater and Mauna Ulu. Preliminary analysis of the data suggested that 3-5 million cubic meters of magma intruded into the rift zone. About 8 hours after the start of the intrusion, the active lava bench on the south coast of Kīlauea began collapsing into the sea. Several small collapses were observed on 12 September, and by the evening of 13 September, about 2 hectares had been removed. The discharge of lava into the sea stopped completely in the afternoon of 13 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: Hawaiian Volcano Observatory (HVO), U.S. Geological Survey, PO Box 51, Hawaii National Park, HI 96718, USA (URL: https://volcanoes.usgs.gov/observatories/hvo/).