**************************************************************************************** Glow From The Halema'uma'u Overlook Vent Snuffed Out by Collapse From: James Kauahikaua <jimk@xxxxxxxx> **************************************************************************************** A sequence of rockfalls, some quite large, within the Halema'uma'u vent at the summit of Kīlauea Volcano began at 1:38 p.m. H.s.t on June 30, 2009. The initial rockfall produced a seismic signal equivalent to a magnitude-2.4 earthquake and was felt at the Hawaiian Volcano Observatory (HVO) and the adjacent Jaggar Museum. A nearby scientist heard the beginning of this rockfall followed by a loud explosion. The normally white gas plume turned a thick brown for several minutes; later collection confirmed an increase in tephra entrained in the plume with at least one later dusting. The initial earthquake was followed by several more rockfall seismic events, culminating in two more locally-felt earthquakes at 1:38 and 2:29 p.m. In addition to feeling the collapse events, National Park Rangers working at Jaggar Museum reported hearing booming sounds during several of them. No changes in vent shape or size were observed during the first 50 minutes of the biggest seismic events, but afterward and continuing for the next two hours, chunks of the vent rim fell into the cavity in the floor of Halema'uma'u Crater. Precise vent dimensions are not yet available. By 4:00 p.m., more than 30 such rim-collapse events had been recorded by seismometers with a few more occurring through today, July 1. Seismic tremor amplitudes decreased by more than 50 percent. By 6 p.m., the levels were at their lowest values since August 30, 2007. Episodic tremor has now returned with 3 minute bursts of tremor occurring every 15 minutes. Last night, for the first time in over a month, there was no glow from the summit vent. HVO geologists who flew over the summit today reported seeing rocky rubble within the vent which is obviously the result of yesterday's collapse. Vent collapses like this are often the result of a drop in the level of lava in the vent which destabilize the vent walls initiating a cascade of collapses. The summit gas plume was visibly thinner by this morning. Initial reports suggest that sulfur dioxide emissions from the summit vent may have decreased, but around noon today as the plume was blown toward HVO, the air quality monitoring station at HVO recorded concentrations of sulfur dioxide suggesting emissions are still elevated above pre-2008 levels. More precise emission rate measurements will be possible when prevailing trade winds return. ============================================================== To unsubscribe from the volcano list, send the message: signoff volcano to: listserv@xxxxxxx, or write to: volcano-request@xxxxxxxx To contribute to the volcano list, send your message to: volcano@xxxxxxxx Please do not send attachments. ==============================================================
