Spectacular event at the Puu Oo eruption in Hawaii

New fissure opening at Puu Oo. Note lava just breaking through
to the surface in the foreground.  USGS photo

(I was going to continue yesterday's blog on Mount St. Helens here, but events in Hawaii are too interesting to ignore!)

Last Saturday at 1:42 p.m. HST, the USGS monitoring network detected the onset of deflation at Puu Oo crater, and increased harmonic tremor (which usually indicates movement of magma).  Puu Oo lies ~15 miles from the summit of Kilauea.  Twenty minutes after Pu u Oo began deflating, the summit of Kilauea also began to deflate.  At 2:16 p.m. the floor of the crater began to collapse with the development of incandescent ring fractures developing a few tens of meters away from the crater wall.  The floor continued to drop, a spatter cone within the crater collapsed, and a large scarp developed on the west side.  Lava cascaded over the scarp toward the center of the crater.  The floor continued to collapse at least through 4:26 p.m. (See previous post on activity in February.)

Map of Kilauea showing Puu Oo, Napau,
and Halemaumau. Photo: USGS

Coincident with the collapse in Pu u Oo, an earthquake swarm began along Kilauea's middle east rift zone, and tiltmeters showed deflation of this area as well.  A pre-existing crater, Napau, then started to erupt, and the eruption continues between Pu u Oo and Napau. Simultaneously, the lava lake level within Halemaumau at the summit of Kilauea is also dropping. The floor of Pu u Oo has dropped at least 115 meters (377 feet).

The volcanoes of Hawaii are rarely dangerous in the style of Mount St. Helens, but they can be. One sequence of events that can cause an explosive eruption is exactly what is going on now:  magma disappears from the summit or craters down into the underground reservoirs, or it moves out of one crater to another, leaving exposed rocks. USGS reference here. If there is groundwater nearby, it can drain into the emptied crater/conduit creating a violent steam-driven eruption. On February 21, 1924 lava in Halemaumau drained, and the floor sank to 115 m below the rim (ironically, the same number as above; this is not a typo!).  Two months later, on April 29, the floor began to sink again, and by May 7, the floor was about 210 m below the rim.  As is being observed today, this withdrawal of magma allows heavy rockfalls from the walls of Halemaumau.  Groundwater flowed into the still-hot conduit, the steam pressure built up, and on the night of May 10-11, 1924, rocks and dust were blown out of the crater.  This activity peaked on May 18, and occurred episodically until May 27. While not exactly similar (because Puu Oo is out on the east rift zone), the withdrawal of magma from both the summit and Puu Oo suggests that there is a potential for an explosive eruption over the next few months.

Footprints in muddy ash from the 1790 eruption.
Photo: USGS

Legend has it that footprints on the southwestern side of Kilauea are those of warriors caught in one of these violent eruptions.  Don Swanson and colleagues have pieced together a picture of the event (2008 AGU abstract).  A group of people, deduced to be women and children from the size of the footprints, were apparently out in the southwest area searching for obsidian for toolmaking.  An initial eruption occurred, during which this group may have taken shelter.  About 1" of muddy ash was deposited. The survivors of that phase of the eruption walked around on wet ash, leaving the footprints.  Meanwhile, warriors and their families were camped out at the summit waiting for the eruption to end, perhaps waiting for 3 days.  When they saw the sky clear the started walking southwestward along the west side of the summit area.  Unfortunately, the most powerful stage of the eruption then began, sending base surges of hot, wet ash-laden steam at them. Many were killed.

Status reports on this eruption can be found at the USGS Hawaii Volcano Observatory website. A webcam here shows the Halemaumau situation, and here shows Puu Oo.