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Episode 55, Stable but Dynamic
This update current as of August 19, 1998. Eruption updates are posted every 4 to 6 weeks; more frequent updates will be made when there are drastic changes in activity or when residential areas are threatened by lava flows. Update archive.
Aerial view to the south shows a fuming Pu`u `O`o vent with the East Kamokuna ocean-entry steam plume visible in the background, 12 kilometers distant. All of the lava that reaches the ocean enters the water at the East Kamokuna entry.
Lava continues to erupt from vents on the southwest flank of Pu`u `O`o and travels 12 km to the coast through lava tubes. The Waha`ula ocean entry was officially pronounced dead on July 11, leaving a single entry at East Kamokuna.
The eruption was interrupted by a 53-hour pause on July 16-19 and a 42-hour pause on August 12-14. Both pauses ended with substantial surface flows breaking out of the lava tubes.
Episode 55 started in February 1997, and its flows reached the sea in August 1997. Since then the eruption has settled into a routine. Lava erupts quietly from Pu`u `O`o and flows through tubes to the sea. It may seem that the eruption is in a state of stasis. However, even during such periods of apparent stability, the eruption is a dynamic process, and activity can change dramatically from day to day.
The most dramatic changes in the eruption in the last few months arose when the flow of lava from Pu`u `O`o ceased for a period of time. These intervals are termed 'pauses.' They occur at random, separated by weeks to months of continuous eruption. The pauses last from a few hours to a few days. What causes the flow of lava to cease is poorly understood, but the sequence of events during any given pause is fairly consistent.
The first sign that the eruption is entering a pause is generally a reduction in the size of the plumes at the coastal entries. This indicates that the supply of lava entering the tubes has stopped. The plumes usually dwindle over a period of a few hours, the time it takes the lava to drain from the tubes into the ocean.
During a pause, magma continues to rise from the mantle into a magma reservoir located a few kilometers below Kilauea's summit. Since the eruption is no longer providing an outlet for this reservoir, the summit starts to inflate. This inflation is measured as a tiny change in the slope of the ground, which is recorded by HVO tiltmeters at Kilauea's summit.
Kilauea summit tiltmeter record (radial component of the Uwekahuna borehole) for the last seven months. Pauses #15-17 and #19-20 clearly correlate with the inflation/deflation peaks on the plot. Pause #18 lasted less than 12 hours and does not have a clear tilt signature.
During a pause, blockages develop in the drained tubes as pieces of the roof and walls collapse. When the eruption restarts, lava reenters the tube and, when it encounters a blockage, breaks out onto the surface, usually through skylights. The breakouts migrate downslope as the lava reoccupies the tube. Surface flows fed by these breakouts last from hours to days. If a surface flow continues for many days, it may form a new tube.
The 19th pause of episode 55 started late on July 16 and ended 53 hours later. The summit inflated 6 microradians (one microradian is approximately equivalent to the amount of tilt produced by placing a dime under one end of a kilometer-long beam), as meaured by the radial component of the Uwekahuna borehole tiltmeter (for those of you familiar with HVO tilt data, this is a new way of looking at the summit tilt that accentuates the short-term changes associated with the eruption; our old standard, the Ideal-Aerosmith tiltmeter, recorded about 3 microradians of inflation for the same event).
When the pause ended, lava broke out of the tube in several places and a total of 135 hectares of land was covered with fresh lava flows.
The most recent pause (#20) began on August 12 and lasted approximately 42 hours, during which time the summit inflated about 2 microradians. When the erupted restarted, flows broke out on Pulama pali and at its base. The largest flow originated from the same breakout point that was active following the July pause.
Surface flows after pause #19
Collapse pitsSeveral collapse pits on the southwest flank of Pu`u `O`o are in the process of coalescing into a single pit that is now 155 m by 185 m in size.
A similar pit began to form on the west flank of the cone in 1993. Ongoing collapse culminated in the failure of the west wall of the cone in January 1997, removing the summit and leaving a deep notch in the cone.
Kilauea is hit by dozens of earthquakes a month and, although only a few may be large enough to be felt, all of them are monitored by HVO through a network of 61 seismometers across the island. Seismometers near the eruption site also record continuous tremor caused by movement of magma through conduits beneath the eruption site.
With the death of the Waha`ula coastal entry in early July, all of the lava entering the sea does so at the East Kamokuna site. Much has been written about the hazards presented by the coastal entries in previous eruption updates, with good reason. New land, in the form of a lava bench, is constantly being created at East Kamokuna. The bench is unstable and collapses, without warning, every few weeks. Several hectares of land may be lost each time this occurs, and violent explosions can result as large volumes of incandescent rock and lava hit the sea.
Flow-field map showing lava emplaced during the Pu`u `O`o - Kupaianaha eruption since 1983. The flows emplaced following the July 1998 pause are shown in purple and cover a total area of 135 hectares. The breakouts following the August pause are not shown but occurred in the same areas. There is currently only one ocean entry, situated at East Kamokuna.
Eruption-viewing opportunities change constantly, so those readers planning a visit to the volcano should contact Hawai`i Volcanoes National Park for the most current eruption information (ph. 808-985-6000). Additional photographs and descriptions of east rift eruptive activity may be found on the University of Hawai`i's web site.
Updated: 3 September 1998