February 14, 1997

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

Pu`u `O`o: extinct or waiting?

Pu`u `O`o was alive and well on Wednesday, January 29, with acrid fumes drifting across the landscape and an active pond glowing red at night. The next morning, it seemed to be a dead, gutted hulk. So goes the cinder-and-spatter cone whose name has become synonymous with 14 years of eruption at Kilauea Volcano. What happened?

On Wednesday, January 29, at 6:41 p.m., the east rift zone eruption changed its style. Magma from Kilauea Volcano's main conduit was cut off from Pu`u `O`o. As a consequence, over the next 24 hours the flow of lava through shallow tubes dwindled to a dribble and the coastal steam plume dissipated.

These changes resulted from the opening of new cracks underground along the east zift zone near Napau Crater, 6 km (4 mi) closer to Kilauea's summit. As the cracks opened, magma rapidly drained from the magma reservoir that lies deep beneath the summit caldera and upper east rift zone. The lava pond at Pu`u `O`o also drained, apparently into subterranean cracks nearby. The force that drove the drainback was gravity, but gravity needed an opportunity. Its chance came when a part of the Earth's crust became so weakened that it could fracture and open with only the slightest change in magmatic pressure.

With its lava pond and plumbing system drained, Pu`u `O`o lost much of its underlying support. First the crater floor collapsed. Then large slices of crater wall slid downward, filling the empty pipe with debris. The collapses generated clouds of red rock dust that roiled upward 300 m (1,000 ft) or so and drifted downwind to be deposited as a thin blanket across an area reaching 5 km (3 mi) southwestward from the cone.

In the aftermath, the cone gained a new shape. Its crater walls are nearly vertical, and the crater floor, once only 60 m (200 ft) below the rim, is now about 250 m deep (820 ft). Collapse wasn't limited to the crater, however. The "Great Pit," a once-circular crater on the cone's northwest flank, has widened into a great gash. The gash enlarged abruptly as magma drained away from Pu`u `O`o. Before dawn on Thursday, January 30, the gash had consumed blocks from the summit and lowered the cone by as much as 45 m (150 ft).

Today, Pu`u `O`o steams slightly as rain and fog dissipate the heat that remains. The crater is deeply notched where it connects to the gash. The cone is notably lower when seen from many viewpoints in the Puna district. The glow is extinguished.

Is Pu`u `O`o extinct or merely waiting? Only time will tell. Magma supplied from deep in the Earth is slowly refilling the summit magma chamber that lies beneath Kilauea caldera, 16 km distant (10 mi). Once that space has been reoccupied, magma is likely to seek other pathways to the surface. Scientists at HVO are uncertain what to expect. But given the lengthy eruptive history from the Pu`u `O`o vent area, we wouldn't be surprised by a reappearance of lava in the crater pond or from a new vent nearby. Pu`u `O`o is one site where money might be made by those who bet on future eruptions.