Volcano Watch — Pu‘u ‘Ō‘ō

The eruption along Kīlauea's East Rift Zone continues with little change. Lava from two vents on the south and west sides of the Pu'u 'O'o cone is transported underground through a lava tube to the ocean about six-and-a-half miles to the southeast. No lava can be seen at either of the active vents.

Most of the activity at the ocean is along the western edge of the lava flow field and near the end of Chain of Craters Road inside Hawaii Volcanoes National Park. Some of the more dramatic changes related to the ongoing eruption have been taking place at Pu'u 'O'o.

The Pu'u 'O'o cone was built of cinder and spatter during the period of high fountains that lasted from early 1983 until July 1986. When activity migrated downrift to the Kupaianaha vent in mid-1986, the cone stood about 835 feet above the surrounding area and the vertical conduit inside the cone was a mere 65 feet in diameter. Today, after a number of collapses, the highest rim of the cone is about 65 feet below the maximum height in 1986, and the crater inside the cone has enlarged to roughly 920 by 650 feet.

Much of the bottom of the crater is occupied by flat-lying lava flows that formed as overflows of the active lava pond that still occupies the eastern part of the crater floor. Blocky talus, the angular blocks of lava that have tumbled down the steep interior walls of the crater, now covers nearly half of the flat floor. The crater has enlarged due to collapse of the crater walls, particularly during times when the lava pond drains away. The most recent collapse occurred during the first week of May.

Other changes have occurred around the cone. The small spatter cones at the episode 51 vent on the west side of the cone collapsed in January 1993. The cones at the episode 53 vent on the south side of the cone collapsed in December 1993. These collapses have continued to enlarge during episodic collapse events since they formed.

Collapses are commonly accompanied by small explosive events that eject red dust or angular blocks onto the surrounding slopes of the cone. Another collapse pit formed in March 1993 about halfway up the west flank of the cone, between the episode 51 vent location and the crater inside Pu'u 'O'o. This pit has now coalesced with the episode 51 collapse pit. It has also enlarged substantially in October and November 1994. On April 27, 1995, this pit again enlarged and produced a red dust deposit on the northeast flank of Pu'u 'O'o.

The most recent collapses inside Pu'u 'O'o Crater were fairly large. The largest measured at least 320 feet along the rim and 65 feet away from the rim. It collapsed along the eastern rim of the cone and removed a trail we used in order to get around the crater and a sampling box we deployed on its rim.

The sampling box, which we call the "tear catcher," collects Pele's tears and hair ejected from the lava pond during violent sloshing and splashing in the pond. These samples are recovered each week and are analyzed for their chemical composition, which allows us to see changes in the eruption that might indicate an upcoming change in activity. A new tear catcher is now deployed about 30 feet from the rim on the east side of the cone, but newly formed cracks in the area suggest that this site might also collapse soon.

Our only other chance to sample the lava is through skylights in the tube system or in areas where the lava forms surface flows near the coast. As it travels towards the coast, however, the lava cools and crystallizes . Such cooling and crystallization cause changes in the composition of the lava that make the subtle changes in the eruption more difficult to see.

Each week, we make a series of observations at the cone to keep track of changes in the lava pond and the rim of the cone. In general, the activity of the lava pond is the first thing we look at. Usually the lava in the pond circulates from the southwest to the northeast, but the circulation changes with the depth to the pond surface.

We also measure the depth from the rim to the pond surface, since this depth gives a measure of the magmapressure inside Kīlauea. We have found that changes in the summit region correlate with changes in the depth to the pond inside Pu'u 'O'o. The depth to the pond, as measured relative to the lowest point on the rim (the northeast spillway) varies between about 215 and 310 feet. This depth is measured by determining the angle from the north rim to the pond surface below the east spillway. Knowing the distance across the crater allows us to calculate the depth using simple geometry. In general, the pond level drops during, and probably before, pauses in the eruption.

We measure changes in the width of cracks around the rim to understand the collapse process. We also maintain a time-lapse camera on the rim of the cone that takes a picture every few minutes. This camera records events that occur during the majority of the time when we do not have personnel on site to make direct observations.

Periodically we also measure the deformation of the ground surface around Pu'u 'O'o, using laser distance-measuring devices or other instruments that accurately determine position. These techniques allow us to quantify the changes in the size and shape of the cone.

Pu'u 'O'o continues to change as the eruption progresses. We expect such changes to continue in the future. The pond inside Pu'u 'O'o, like the lava pond at the now-defunct Kupaianaha vent, may continue to get deeper and deeper and eventually it may crust over completely. We also expect the crater to continue to enlarge as more of the walls collapse into the cone. Such collapses are most likely to occur if the pond drains away, as it does during eruptive pauses or when a new outbreak occurs downrift.

The largest collapses in Pu'u 'O'o occurred in November 1991 when the episode 49 fissure opened up between Pu'u 'O'o and Kupaianaha. A similar event in the future could lead to a major collapse inside the cone.