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