Shimmering hot crust on lava lake covers most of the crater floor on September 25. One small fountain plays along the southwest margin of the lake. The flat terrace surrounding the lake is about 50 m below the steaming cracks on the north rim of the crater (right), where most ground observations of the lake are made. The depth to the lake is measured from the near crater rim.

Overview

Lava never completely left the crater following the intrusion farther uprift on September 12 (see previous update). Most of the lava drained underground, but a small pad remained near the center of the crater. It slowly rose, and by September 16 the lake was about 92 m below the east rim of the crater and 50-60 m in diameter. In the next week the lava rose to bout 63 m below the east rim and covered most of the crater floor. The lake has remained at this approximate level for the past month, episodically rising 10-15 m and occasionally overflowing onto the top of the adjacent terrace and resurfacing it.

New collapse pit forms. A new collapse pit formed in the west gap of the crater during the intrusion and withdrawal of the lava lake. This pit, termed the West Gap Pit, is 30-50 m wide and 12-15 m deep. It was inactive until October 16, when spattering began from a vent on its floor. Some time between October 17 and 21 (probably on the 18th or 19th), the pit filled with lava and overflowed, producing a shelly pahoehoe flow 160 m long.

Lava reappears outside Pu`u `O`o. On September 23 lava reappeared on the ground surface south of Pu`u `O`o, the first time since the intrusion on the 12th. Small vents supplied lava in Puka Nui, from a new spatter cone termed the "mini-vent" just west of the minishield southeast of Pu`u `O`o, and from several skylights in the old lava tube farther south. The largest breakout was at about the 670-m (2200-feet) elevation. The next day another breakout was occurring just downslope at about 625-m (2050-feet) elevation. Over the next several days most surface activity was concentrated at these two breakouts and another at about 530 m (1750 feet), which sent a flow to the top of Pulama pali at about 460 m (1500 feet).

On October 1 a channelized `a`a flow from the 530-m breakout made it down the pali to about the 300-m (1000-feet) elevation. Most of the flow crossed earlier episode 55 flows, but in places the flow entered forest, starting small fires and triggering methane explosions. On October 3 a pahoehoe and `a`a flow made it to the base of the pali at about the 90-m (300-feet) elevation. This flow had stopped moving by the next day.

On October 5, flows resumed from the 655-m (2150-feet) breakout site, which on September 23-30 had gradually built a perched pond (a low, shield-like structure with a small pond on its top) about 15 m (50 feet) high. Surface flows from this perched pond continued for the next several days, never moving more than a few hundred meters before stagnating. This continued activity built the perched-pond structure to a height of 36 m above its downslope base; the flat top of the structure is 175 m in diameter.

A new breakout site, at 640-m (2100-feet) elevation, was first recognized on October 12, between the old 655-m and 625-m breakouts. The flows moved short distances downslope and burned brush and small trees in bordering kipuka. Finally, on October 17, a 100-m-wide `a`a flow from the 530-m breakout point made it about halfway down Pulama pali before stagnating at the 365-m (1200-feet) elevation.

The next flow to descend the pali began in late afternoon of October 22. By 0530 the next morning, the flow had reached about the 335-m (1100-feet) elevation on the pali and was burning bordering vegetation. The pahoehoe flow was continuing in late morning, the time that is being written.

Lava tube is blocked. All during this period the lava tube active before September 12 has been empty below the lowest breakout point. That means that no lava was traveling underground to the coast. The tube drained just after the intrusion, and its roof began to cool. Rocks shrink and break when they cool, so the roof fell apart in places, forming dams along the tube. When lava reentered the tube, the dams blocked its flow in places--the points of surface breakouts. Apparently the dam at about 530-m elevation persists, so that the tube cannot carry lava beyond that point.

Lava discharge has decreased. Measurements of lava flux during this period have been difficult. However, estimates are consistently about 150,000-200,000 cubic meters per day, only about half that before the September 12 intrusion. This decline probably results from some change within Kilauea itself, not in the magma supply rate from the mantle. The summit of Kilauea is now as inflated as before the intrusion, but the surface output is down. This implies that more underground storage has been created in the east rift zone, possibly owing to widening of the zone during the intrusion.

The level of lava in the Pu`u `O`o lake has varied 10-15 m since late September. The terrace around the edge of the lake marks the high stand of the lake. The level can change within minutes, hours, and days.

Lava erupts from Puka Nui for first time

Puka Nui ("large hole" in Hawaiian) is a collapse pit on the south side of Pu`u `O`o that formed in December 1997. It has been growing ever since, by collapse of tephra from the north wall of the pit and by engulfing other cones and smaller pits. However, no lava had erupted from Puka Nui until September 23, 1999. Since then, several episodes of spattering have taken place from growing cones on its floor. A short-lived pond of lava spilled over the southeast rim of Puka Nui during the night of September 24-25. Another small flow covered the floor of the pit between October 5 and 12.

Lava breakouts from tube create perched ponds

Lava breakouts are flows that reach the surface through a skylight or crack in a lava tube. This informal term is useful at Kilauea, where such flows are common. Breakouts can form when the underlying tube becomes "over-full", so that the capacity of the tube is exceeded because of increased upstream supply (as a river in flood). This happened on February 1, 1996, when a torrent of lava filled the tube to overflowing.

Breakouts more often form when the tube is actually blocked by a dam. Breakouts since September 23 have resulted from dams within the tube. The roof and walls of the tube cooled when the tube emptied following the intrusion on September 12. Rocks shrink and crack when they cool, spalling off the sides of the tube. Consequently piles of rocks formed at a number of places in the tube. When lava again entered the tube, it had to spill onto the surface upstream of a strong dam, forming breakouts.

Long-lived vents for lava breakouts sometimes form low shield-shaped structures with a pond of lava on top. Small flows spill out from the pond, building the banks of the pond higher. As a result, the surface of the pond can rise, spill out again, and rise once more. This process, repeated many times, results in a "perched pond" standing high about its surroundings. Perched ponds formed at several places where breakouts took place since September 23. They form seemingly anomalous high areas on the otherwise gently sloping flow field. The highest new perched pond, at 655-m (2150-feet) elevation, stands 36 m above its downslope base and is 175 m wide. An easily accessible perched pond can be seen by hikers in Hawai`i Volcanoes National Park on the north flank of Mauna Ulu.

Surface flows move into kipukas

Aerial views of lava moving through rainforest along the edge of the flow field above Pulama pali (see map below). The forest in this area is now found only in kipuka, areas surrounded by lava emplaced earlier in the Pu`u `O`o - Kupaianaha eruption.

Views of lava limited for visitors

All the eruptive activity since the September 12 intrusion has been in areas that are off limits to visitors to Hawai`i Volcanoes National Park. Such visitors can charter aircraft to get excellent aerial views of lava flows and the Pu`u `O`o area. On a few nights, those lucky enough to be at the end of the Chain of Craters Road have been treated to long-distance views of glowing lava flowing down Pulama pali. A tripod and long-lensed camera are necessary to capture these scenes adequately. Unfortunately there is no way to predict when flows will be visible or when they will once again pour into the sea.

Flow-field Map

Map showing area covered by lava flows emplaced during the Pu`u `O`o - Kupaianaha eruption between 1983 and July 1, 1999. Flows active between March 28 and July 1, 1999, are shown in pink. A tube delivered lava to the ocean a few hundred meters west of a prominent littoral cone (star) at Kamokuna (click map for a larger view of the map). This stopped with the intrusion of September 12. Since then, surface flows have covered a small area above Pulama pali, mostly confined to the orange area of episode-55 flows near the trace of the lava tube. Several flows in October 1999 advanced part way down the pali before stagnating.

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