Panorama of Halema`uma`u pit crater (3 images spliced together); views are from the northeast crater rim. The crater is about 1 km in diameter and 85 m deep.
The lowest discernable ledge above the crater floor marks the high point of lava (like a bathtub ring) reached during the September 1974 eruption; some lava subsequently drained away, leaving behind a "ring" or ledge around most of the crater wall. The dark area in the central crater floor is part of the 1974 ponded flow.
The prominent ledge nearly half-way up the crater wall represents the high point of a lava lake that was active for 251 days in 1967-68. During a 5-day eruption in in September 1971, both at Haleama`uma`u and along the upper southwest rift zone, the crater floor subsided by as much as 45 m when magma moved from the summit reservoir beneath the caldera into the rift zone. The subsidence left the highest bathtub ring as the only reminder of the 1967-68 lava lake.
The dark area on the right crater floor is a small lava flow erupted on April 30 - May 1, 1982. The eruptive fissure extended from the northeast crater rim northeastward across the caldera floor for about 1 km (see image #16).
Halema`uma`u is the principal source of volcanic gas emissions in the summit caldera. Between 1979 and 1982, the average daily emission rate of sulfur dioxide gas from the caldera was 170 ± 50 tonnes. The emission rate increased to 260 ± 90 tonnes per day in 1983-84 during the early stages of the Pu`u `O`o - Kupaianaha eruption on Kilauea's east rift zone. Between 1997 and mid-1998, the emission rate was less than about 150 tonnes per day.