Much of the lava starting at the SW flank of Pu`u `O`o was delivered to the ocean via a system of lava tubes. One of the ocean entry points, the Highcastle entry, was continuously active and intermittently explosive throughout July.

On 22 July the lower Highcastle bench and a stranded littoral cone on the upper bench collapsed, an event followed by littoral explosions. The collapse displaced an area ~100 x 15 m in size that dropped into the ocean. Around 21 July a small skylight opened allowing a view into the tube carrying the Highcastle flow. The flow was initially visible in the skylight, but within a week a crust formed over the flow.

Although the Highcastle entry vented lava during 1-14 August, its output seemed variable, perhaps influenced by the fluctuating discharge of flows upslope. For example, mild explosive activity was observed on 7-8 August, an interval when no lava escaped onto the coastal plain. Two pahoehoe flows were noted upslope on 3 August, originating at 660 and 650 m elevation. The upper flow was not active on 11 August; however, that day the lower flow was still active and burning forest at the 590 m elevation. On August 11 observers looked through the skylight at 735 m elevation and measured a stream of lava 15 m wide and 19 m deep.

The volume of lava escaping at the Kamoamoa entry diminished during late-June; on 4 July, lava from the Kamoamoa tube stopped entering the ocean. Lava escaping at the base of the slope called Paliuli fed a flow that intermittently entered the ocean around Lae'apuki. This flow stagnated on 13 July. On 21 July a sheet flow from 375 m elevation burned through the Thanksgiving kipuka, formerly an easily discernible island of vegetated land within the Kamoamoa flow at the base of Paliuli. Having stagnated around 28 July, this flow did not reach the ocean. Active flows were observed on the coastal plain on 23 June and 1 July. They were also sporadically active on the slope of Pulama Pali (390-175 m elevation) and, on 17 July, one flow reached within 100 m of Paliuli.

On 1 August, a flow began escaping from the Kamoamoa tube at ~490 m elevation and cascaded down Pulama Pali. By 3 August the flow had spilt into three distinct parts, including a voluminous flow on the E, a pahoehoe sheet flow with many active streams on the W, and an 1,800-m-long aa-pahoehoe flow following a channel in the middle. By mid-August, the W flow had reached the base of Pulama Pali, the E flow had crossed the flats between Pulama Pali and Paliuli, and the middle flow had stopped moving.

Continued collapse of the Pu`u `O`o cone caused a black dust plume seen on 4 July. On 6 July a new debris deposit was noticed on the crater floor. The active lava pond within the crater of Pu`u `O`o was 90-95 m below the N spillway rim in late June, but was not visible after 4 July. Later in the July, the lava pond remained small and deep, ~95 m from the N spillway rim. The pond at Pu`u `O`o continued to shrink and during the first half of August its surface was over 100 m below the N spillway rim. A sluggishly moving crust had formed over much of the pond, and its only open areas were on the W and N edges.

There were two episodes of "gas-piston" bursts. One episode took place during 3 June-30 July, consisting of intermittent bursts of 1- to 2-minutes duration. Another episode took place during 26-28 July, consisting of frequent bursts of about 1-minute duration.

Tremor on the East Rift Zone during July through mid-August was chiefly of low amplitude. In the interval from 4 to 17 July, tremor bursts and banded tremor were both occasionally seen. During 18-31 July, tremor reached 2x background; bursts on 13-15 July reached to 4-5x background. During 30 June-31 July there was a low number of microearthquakes beneath the summit area and low to average number beneath the East Rift Zone. On 1-2 August the number of microearthquakes increased to high levels, but then decreased through mid-August.

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: Tari Mattox and Paul Okubo, USGS Hawaiian Volcano Observatory (HVO), Hawaii Volcanoes National Park, HI 96718, USA.