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Volcano Watch — Pu‘u ‘Ō‘ō changes steadily and abruptly

July 12, 2001

The ground surface subsided abruptly about six weeks ago at Pu`u `O`o, Kīlauea Volcano's active cinder-cone vent. Gaping cracks opened around the edges of the subsidence zone, centered on the southwest edge of the cone. A collapse pit about 30 m (100 ft) in diameter and 15 m (50 ft) deep nibbled into the cone's margin. 

A second collapse pit continues to deepen within the floor of Pu`u `O`o's crater. One large rock slide and several smaller ones ensued.

Many of these structures likely formed within a short period of time, perhaps a few hours or a day. The timing of events is uncertain because our view of the site was frequently obscured by the fume cloud that emanates from Pu`u `O`o. Once formed, however, the pits continued to enlarge for a few weeks, because the cinders don't maintain their integrity on oversteepened slopes.

These episodic collapses are slowly engulfing the cinder cone. Although not as dramatic as the collapse of January 1997, when the entire west flank disappeared overnight, these latest changes will ultimately carve a notch that breaches the southwest flank of the cinder cone, exposing the inner crater walls.

In contrast to episodic collapse at the surface is the relentless subsidence measured by tiltmeters. A tiltmeter on the north flank has recorded constant tilting inward toward Pu`u `O`o's crater since it was installed in January 2000. Indeed, the cumulative tilt has dragged the tiltmeter off scale once already, requiring a tedious repositioning of the instrument within its shallow borehole.

Total tilt so far is about 850 microradians, which corresponds to about 25 cm (1 ft) of subsidence of the crater floor relative to the tiltmeter. We lack sufficient instruments to precisely determine the center of collapse, but the source is approximated by combining the tilt data with our ground observations.

These collapse events typically occur without discharge of lava onto the ground at Pu`u `O`o. Instead the molten stuff slips away in lava tubes, remaining invisible to our observations until seen at a skylight 1.6 km (1 mi) distant. With it, each day, goes a small part of the cone.

If rocks had no strength, the cracks and pits would change daily. But the strength of the lava layers beneath the cinders allows the ground to resist cave-ins and subsidence even as the foundation is plucked and piped away by magma. The abrupt changes that we describe every few months may be precipitated by a drop or surge in the magma supply or by earthquakes, which jostle an already weakened Pu`u `O`o. The constant subterranean erosion finally exceeds the endurance of rocks at the surface.

Continuous and episodic events characterize many geologic processes. The Pacific plate drifts northwestward continuously, but the islands grow in spurts. The building of an active volcanic island continuously loads the oceanic crust, but earthquakes occur sporadically as the forces overcome the strength of the rocks. A sea arch may be a tourist attraction for decades, then, within a week, merely a pile of rubble in the surf zone. Lavaentering the ocean slowly builds a superficially stable bench, but one night the whole thing slides down the submarine slope, leaving no trace at the shoreline.

Memorize the skyline presented by Pu`u `O`o today, because it may be startlingly different in the future.

Volcano Activity Update

Eruptive activity of Kīlauea Volcano continued unabated at the Pu`u `O`o vent during the past week. Lavamoves away from the vent toward the ocean in a network of tubes and descends Pulama pali in three separate areas. A breakout from the tube system above Pulama pali on July 9 fed a flow streaming down the pali and pooling in the coastal flats. This activity lasted for two days until the molten lava surface crusted over. Small surface flows, primarily ooze-outs from inflated areas, are occasionally observed in the coastal flats. Lava continued to enter the ocean in the area east of Kupapa`u throughout the week.

There were no earthquakes reported felt during the week ending on July 12, 2001. 

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