September 29, 2005

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

Pu`u `O`o crater quietly fumes--for now

Fume rises from spatter cones and pits on the floor of Pu`u `O`o crater. The most active of these is the East Pond Vent, the crucible-shaped cone at the left end of the crater. The East Pond Vent is surrounded by a rumpled skirt of pahoehoe, which poured from the vent in February 2005. The crater is 400 m (1,300 feet) long. View is to the south. USGS photograph by Tim Orr.

By day, the six vents arrayed across the crater floor of Pu`u `O`o emit copious clouds of steam and magmatic gas. Darkness reveals that many of them glow brightly, owing to the presence of lava not far below the surface. Yet these vents can go for months without producing a single blob of spatter, let alone an actual lava flow.

Since 2000, sustained activity within the crater has been limited to two periods: January-April 2002, and August 2003-February 2004. Both of these intervals repaved the entire crater floor with fresh pahoehoe. The level of the crater floor rose a total of about 50 m (165 feet) during these two periods.

During such intervals, new crater vents appear, and existing vents can take on a whole new appearance. Typically, a crater vent starts out as a spatter cone, then, as activity wanes, collapses to form a pit, which may fill with a small lava pond. Sometimes the pond crusts over, and new spatter cones build on top of the crust. Vents may disappear altogether, drowned beneath the rising level of lava when flows pond on the crater floor.

Compared with these periods, 2005 has been tame but not entirely quiet. During a few weeks in February, intense spattering built several new cones at existing vents but produced only a few short flows at the east end of the crater. Since then, the East Pond vent has hosted a small lava pond, which occasionally spits out a little spatter.

If the crater vents aren't erupting flows, where's all that lava at the ocean coming from? The lava tubes feeding the flows to the ocean originate at vents on the southwest flank of the cone (behind the fume in the photograph). The crater vents serve as relief valves for the shallow magmatic plumbing beneath Pu`u `O`o. During periods when the flank vents are clogged, or when there's a surge in the magma supply to Pu`u `O`o, magma backs up into the crater.

For the moment, flank vents are efficiently delivering lava to the ocean via the Kuhio (PKK) tube system, and the crater vents are quietly fuming and biding their time.

Activity update

Eruptive activity at Pu`u `O`o continues. On clear nights, glow is visible from several vents within the crater and on the southwest side of the cone. Lava continues to flow through the PKK lava tube from its source near Pu`u `O`o to the ocean, with few surface flows breaking out of the tube. Flows are visible intermittently on the steep slope of Pulama pali and on the coastal plain. As of September 29, lava is entering the ocean at East Lae`apuki, in Hawai`i Volcanoes National Park.

Access to the ocean entry and the surrounding area remains closed, due to significant hazards. If you visit the eruption site, check with the rangers for current updates, and remember to carry lots of water when venturing out onto the flow field.

One microearthquake was felt on Hawai`i Island during the week ending September 28. At 2:09 a.m. on September 26, a magnitude-1.6 earthquake located 15 km (9 miles) east of Mauna Loa's summit, at a depth of 7 km (4 mi), was felt in Ocean View.

Mauna Loa is not erupting. During the week ending September 28, ten earthquakes were recorded beneath the summit area. Half were short-period events at shallow-to-intermediate depths (less than 15 km). The other half were deep (greater than 40 km), long-period events southeast of the summit. Inflation of Mauna Loa continues.

Visit our web site (hvo.wr.usgs.gov) for daily volcano updates and nearly real-time Hawai`i earthquake information.