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Volcanowatch

May 6, 2010

A weekly feature provided by scientists at the Hawaiian Volcano Observatory.


Over the Edge, or the Webcam Takes a Plunge

The dramatic change in Pu`u `Ō `ō landscape from 1992 (top) to 2005 (bottom). Both photos are taken from the same location, looking toward the east.
The dramatic change in Pu`u `Ō `ō landscape from 1992 (top) to 2005 (bottom). Both photos are taken from the same location, looking toward the east.

At HVO's Monday meeting, geologist Tim Orr updated the staff on current eruption conditions at Kīlauea's summit, east rift zone vents and flow field.

He noted that a certain area of incandescence in the Pu`u `Ō `ō crater recently appeared to be shifting upward in the view of the Webcam on the crater rim. In an "a-ha!" moment, he realized that it was not the hot area that was rising, but the camera that was sinking and/or tilting downward.

Tim reminded us of the numerous cracks on the rim of the crater and pointed out some large, slowly widening cracks surrounding the camera site. The conclusion was inescapable: parts of the rim would fall in one day—probably soon—and would take the camera with them.

He didn't realize just how prophetic his words were! The very next morning, at about 3 a.m., a large seismic signal suggestive of a collapse was recorded on a nearby seismometer, and a small offset was recorded on a nearby tiltmeter. Sure enough, the last image gathered by the Webcam overnight was at 2:56 a.m.—the camera was gone.

Tour helicopter pilots confirmed that there had been a collapse of a sliver of the north rim, with no sign of the camera.

Such collapses of the rim are not uncommon, occurring at least a few times per year. Most of these collapses involve fairly small areas but, nevertheless, add up over time, resulting in loss of height and change in the shape of the Pu`u `Ō `ō cone. Catastrophic collapses have also changed the cone significantly in a matter of minutes.

Monitoring the 27-year-old east rift zone eruption has provided us the opportunity to witness and document the complex evolution of a basaltic vent structure like Pu`u `Ō `ō.

The Pu`u `Ō `ō cone formed during the early phases of the eruption. Fallout from spectacular lava fountains between 1983 and 1986 built the cinder-and-spatter cone 255 m (837 ft) high. The cone was strikingly asymmetric, because prevailing trade winds caused most of the airborne fragments to pile up on the southwest side of the conduit.

In 1986, the eruption migrated downrift and created a lava lake that fed lava through tubes to the coast for the next 5.5 years. During this time, repeated collapses at Pu`u `Ō `ō gradually formed a crater approximately 300 m (almost 1,000 ft) in diameter.

After the eruption shifted back to Pu`u `Ō `ō in 1992, flank-vent eruptions formed lava shields banked against the west and south sides of the cone, burying the lower third of the cone.

The most dramatic changes to the cone were the catastrophic collapses of 1997 and 2007. On the night of January 30, 1997, the conduit leading from the summit magma reservoir to Pu`u `Ō `ō suddenly depressurized as magma was diverted to an intrusion and subsequent eruption at Napau Crater. In rapid succession, the crater floor, then the west wall of the cone, collapsed. The height of the cone was reduced by 34 m (more than 100 ft).

A similar sequence of events was repeated in 2007 during the Father's Day intrusion, when the crater floor dropped about 100 m (300 ft) and a portion of the south wall of the cone collapsed into the resulting void.

From its formation in 1983-1986 to the present, the cone has lost more than 30 percent of its height due to collapses. The lower third of the cone has been buried by lava from multiple flank vents.

Observing the changes at Pu`u `Ō `ō gives us much insight into the evolution of other eruptive structures, not only in Hawai`i, but all around the Earth, and even on other planets.

Collapses of the cone, such as the one that took the camera this week, are also a reminder of the many hazardous conditions that necessitate the closure of the Pu`u `Ō `ō area. Great, safe viewing of Pu`u `Ō `ō is possible from the Napau Trail in the National Park and, as soon as our intrepid geologists reinstall the camera, from the Webcam on the rim through HVO's Website.

Kīlauea Activity Update

Breakouts, scattered from the top of the Pulama pali to the coast along the new Quarry flow, were active throughout the past week, and lava continues to enter the water at the Ki ocean entry. The ocean entry has become better established and the surface flows near the end of Highway 130 have slowed dramatically. As of this writing (Thursday, May 13), a few small breakouts continue near the end of the road, but most of the surface activity is found farther upslope.

At Kīlauea's summit, a ponded, circulating lava surface deep in the collapse pit within the floor of Halema`uma`u Crater was visible via Webcam throughout the past week. The lava surface remained relatively steady, showing little variation in depth below the rim of the vent. Volcanic gas emissions remain elevated, resulting in high concentrations of sulfur dioxide downwind.

Two earthquakes beneath Hawai`i Island were reported felt during the past week. A magnitude-2.3 earthquake occurred at 12:21 p.m. on Saturday, May 8, 2010, H.s.t., and was located 1 km (1 mile) southeast of Pa`auilo, at a depth of 13 km (8 miles). A magnitude-2.4 earthquake occurred at 6:00 a.m. on Wednesday, May 12, 2010, H.s.t., and was located 10 km (6 miles) southeast of Waiki`i, at a depth of 20 km (12 miles).

Visit our Web site (http://hvo.wr.usgs.gov) for detailed Kīlauea and Mauna Loa activity updates, recent volcano photos, recent earthquakes, and more; call (808) 967-8862 for a Kīlauea activity summary; email questions to askHVO@usgs.gov.

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Updated: May 28, 2010 (pnf)