January 30, 2003
A weekly feature provided by scientists at the Hawaiian VolcanoObservatory.
A Break From the Routine on Kilauea
For more than twenty years, Kilauea has been erupting almost continuously. In the early years, the eruption alternated between dramatic fountains and periods of repose. For the last 16 years, however, Kilauea has settled into a stable routine-lava quietly flows out of flank vents on Pu`u `O`o cone into lava tubes, which wind sinuously toward the ocean. Nowadays, drama occurs mainly at the shore, where giant laze plumes tower over the ocean entries and bench collapses pepper the coast with debris.
The routine, stable as it is, occasionally breaks. Almost exactly six years ago today, early in the morning of January 30, 1997, fissures opened in the floor of Napau Crater. From those fissures, lava soon began to fountain, much to the delight of the backpackers perched at the brink of Napau Crater that morning. These lucky few witnessed a spectacle seldom seen, even here on one of the world's most active volcanoes.
Early last week, late in the afternoon of Monday, January 20, 2003, the routine broke again. Tiltmeters at Kilauea's summit and at Pu`u `O`o cone began to register prominent deflation. HVO scientists mulled over these tilt readings and concluded that the magma supply to Kilauea had stopped. Normally, Kilauea receives a relatively constant supply of magma from a source deep beneath the summit. Last Monday, that supply was interrupted-no more magma was coming into Kilauea.
The shallow magmatic system of Kilauea and Pu`u `O`o acts as a reservoir, so the eruption did not end abruptly Monday afternoon, when the deep supply of magma was interrupted. Instead, eruptive rates slowly diminished through early Tuesday morning. By sunrise Tuesday, activity at the coast had almost stopped, with only occasional flare-ups at the West Highcastle ocean entry. By afternoon, the eruption had almost died completely.
At around 4:00 P.M. Tuesday afternoon, seismometers at Kilauea's summit began to record intense harmonic tremor-a kind of seismic signal that results from magma movements at depth. The interruption in magma supply appeared to be coming to an end. HVO scientists huddled around the tiltmeter displays, awaiting the rapid inflation that would herald the return of magma into Kilauea's shallow summit system. The wait was short; less than 15 minutes after the onset of harmonic tremor, Kilauea began to inflate.
What caused the interruption of magma to Kilauea remains a mystery. One reasonable possibility is that a blockage formed in the conduit that connects Kilauea's summit reservoir to the area of deep magma supply. According to this "blocked-pipe" theory, magma accumulates-and pressure builds up-beneath the point of blockage. When the pipe finally clears, nearly a day's worth of magma rushes up into Kilauea's shallow summit reservoir and ultimately out to Pu`u `O`o.
By Tuesday night, Pu`u `O`o had gone from famine to feast. Between 5:00 P.M. and midnight, magma re-filled the reservoir within the cone, and lava returned to the upper part of the main tube. Complicating matters was that the lower parts of the tube had become partially blocked during the previous day's eruptive slow-down. Excess lava spilled from the upper tube to form large surface flows, destroying many familiar hornitos and skylights. Some of these new flows persist as of this writing. Several days were required to ream the lower parts of the lava tube fully open again, so the ocean entry at West Highcastle did not return to its previous level of activity until the weekend.
Events like these have occurred repeatedly in Kilauea's recent past. Last week's event was the fifth of its kind recorded in the last three years. Each event begins with the characteristic deflation-probably resulting from an interruption in magma supply to Kilauea's summit. The deflation persists for a little less than a day as eruption rates slowly diminish. Finally, sudden inflation at the summit marks the end of the interruption and, more often than not, the beginning of an eruptive surge at Pu`u `O`o.
Eruptive activity at the Pu`u `O`o vent of Kilauea Volcano continued unabated during the past week. Small surface flows from breakouts of the tube system persist in burning the forest above Pulama pali. Below the breakouts, lava remains confined in the tubes down to the coast at the West Highcastle entry. A brief breakout occurred just above the old sea cliff and coated nearly the entire delta surface with a new layer of lava. The public should be aware that the ocean entry areas could collapse at any time, potentially generating large explosions in the process. The steam clouds rising from the entry areas are highly acidic and laced with glass particles. The National Park Service has erected a rope barricade to delineate the edge of the restricted area. Do not venture beyond this rope boundary onto the lava deltas and benches. Even the intervening beaches are susceptible to large waves suddenly generated during delta collapse; these beaches should be avoided.
Three earthquakes were reported felt during the past week. A resident of Glenwood felt an earthquake at 4:56 a.m. on January 24. The magnitude-2.2 earthquake was located 4 km (2.4 mi) south of Volcano at a depth of 4 km (2.4 mi). On Saturday morning January 25 at 6:46, a magnitude-3.9 earthquake shook residents islandwide. The epicenter was 11 km (6.6 mi) southwest of Volcano at a depth of 29.3 km (18 mi). Three minutes earlier, a magnitude-3.3 foreshock was reported felt in Papa`aloa and Volcano.
Mauna Loa is not erupting. The summit region continues to inflate, though the rate of inflation has slowed gradually during the past month or two. The earthquake activity is low, with only 8 earthquakes located in the summit area during the last seven days.
Updated: February 20, 2003 (pnf)