Monday, September 25, 2006 3:35 PM AKDT (2335 UTC)

5846'12" N15340'19" W, Summit Elevation 6903 ft (2104 m)
Current Level of Concern Color Code: YELLOW


On September 17, an explosion of ash, gas, and steam from Fourpeaked Mountain 320 km (200 mi) SW of Anchorage on the Alaska Peninsula marked the onset of unrest at this long-dormant, ice-clad volcano. AVO elevated the level of concern color code to YELLOW on September 20. Since then, AVO has gathered aerial and ground-based information and also initiated installation of geophysical instrumentation to better track activity at Fourpeaked. Based on our observations and limited geologic understanding of the volcano, it is possible that significant eruptive activity could occur in the coming days to weeks.

Over the weekend and continuing today, AVO flew a variety of missions to Fourpeaked volcano. The main findings are as follows: (1) Observers saw a linear series of vents running north from the summit for about 1 km (0.6 mi). Most of these vents were vigorously emitting steam and other volcanic gasses. In the immediate vicinity of the vents, the glacier had been disrupted and showed signs of subsidence. (2) Gas measurements reveal that emission rates of sulfur dioxide, hydrogen sulfide, and carbon dioxide were all high. The sulfur dioxide emission rate was roughly equivalent to that measured at Augustine before its January 2006 eruptions. A distinct sulfur smell was evident up to 50 km (30 mi) from Fourpeaked's summit. (3) Marked scouring of a glacier flowing west from the summit indicates flooding, probably from the September 17th event. (4) Thermal measurements of 60 degrees C (140 degrees F) where recorded at the vents, though the abundant steam was likely obscuring hotter areas.

All evidence, including the ash emission of September 17th, the abundant volcanic gases, the presence of new vents at the summit, and the disruption and floods occurring at and below the summit glaciers, suggest the presence of new magma at shallow levels beneath the volcano. Because AVO has had no instrumentation network on Fourpeaked, we do not have the geophysical measurements necessary to provide independent evidence for the presence of new magma.

Fourpeaked volcano is not known to have erupted in the last 10,000 years, though geological investigations have been limited and ice covers much of the area. Because of this, the range of sizes and styles of past eruptions is not well-constrained. However, the composition of the volcano indicates that eruptions of Fourpeaked can be explosive, possibly producing plumes that reach in excess of 10 km (33,000 ft) above sea level and local ash fall.

Based on all currently available evidence, AVO believes that an eruption from Fourpeaked in the next days to weeks is possible. Given below are some possible future scenarios for the current unrest at Fourpeaked, listed in order, with the most likely scenario listed first:

(1) A small to moderate eruption will occur, which may produce ash plumes exceeding 10 km (33,000 ft) above sea level. Lava flows may also occur.
(2) No eruption occurs. Unrest gradually subsides to background levels.
(3) A large eruption will occur, which would produce ash plumes exceeding 10 km (33,000 ft) above sea level, and possible widespread ash fall.

Even minor volcanic activity can result in floods, debris flows, and lahars (volcanic mud flows) into the nearby drainages. These can occur without obvious signs of volcanic activity like ash plumes or loud explosions. Thus the local area is considered especially hazardous.

AVO has begun constructing a network of geophysical monitoring instruments at Fourpeaked. Over the last two days, two seismometers were installed between 10 and 20 km (6 - 12 mi) from the summit. These instruments give AVO volcanic earthquake detection capability, but do not provide enough information for short-term forecasts of volcanic eruptions. Additional seismometers will be installed in the coming days. If conditions permit, these instruments will be closer to the summit and should provide better forecasting capability. AVO also plans the installation of web cameras and instruments for measuring ground deformation.

AVO will continue to monitor the volcano with satellite data, observation flights, and new geophysical data streams as they become available.


Tom Murray, Scientist-in-Charge, USGS (907) 786-7497

Steve McNutt, Acting Coordinating Scientist, UAF (907) 474-7131

The Alaska Volcano Observatory is a cooperative program of the U.S. Geological Survey, the University of Alaska Fairbanks Geophysical Institute, and the Alaska Division of Geological and Geophysical Surveys.