On September 17th, 2006 it was a peaceful day in south central Alaska.
The skies were unusually clear for the time of year, so it thus came as a surprise when
several people in Kodiak, Homer, and Anchorage reported a gray plume of ash rising into the
atmosphere.
These people subsequently phoned the Alaskan Volcano Observatory, reporting that the plume
was originating just southwest of the Douglas volcano, near where a secondary small peak
was located.
This secondary peak was referred to as Fourpeaked, and was at the time thought to be either extinct
or dormant.
What had just begun was the initial phase of an unexpected phreatic eruption from a
volcano which had not erupted in at least 10,000 years, potentially longer.
This unexpected eruption served as a friendly reminder that volcanoes can go on the order
of tens of thousands of years between individual eruptions, much like seen at McDonald Island
in Australia which hadn't erupted for 75,000 years before unexpectedly erupting again in
1992.
The Fourpeaked volcano is located in the highly scenic and volcano rich Katmai National Park
and Preserve, where it is centered 98 miles southwest of the city of Homer.
Partially surrounded by snow and glacier covered peaks, the small summit of Fourpeaked is northeast
of the Kaguyak caldera and southwest of the Douglas volcano.
This volcano gets its name from one of the glaciers which covers its edifice and is known
as the Fourpeaked glacier.
Although covered by a thick ice sheet, the overall volcano measures about 9 miles wide
and stands about 3,300 feet taller than surrounding older glacially carved mountains.
This still impressive height suggests that Fourpeaked has been erupting for at least
25,000 years, but this figure assumes that it had the same eruptive rate as the nearby
Mount Augustine.
Based on the extent of glacial erosion on its edifice, it is likely far older.
Although little is known about this volcano, two potential lobes of andesite lava can be
spotted on its flanks, which traveled within a 4.25 mile radius of its summit.
It can be assumed based on what little evidence we do have that Fourpeaked produced moderately
explosive eruptions an order of magnitude less explosive than the nearby Mount Redoubt.
Fourpeaked's edifice represents a typical stratovolcano, with overlapping layers of
andesite lava and ash.
However, there are hints of some more explosive prior eruptions due to the detection of dacite
lavas.
Although Fourpeaked was not known to have erupted in the last 12,800 years until 2006,
this does not necessarily mean that it truly did not erupt during this timespan.
When a 3300 foot long fissure opened on the northwest summit area of this volcano, the
heat from a series of phreatic explosions carved out a chain of explosion craters.
This heat subsequently melted a large volume of snow and ice, triggering a large glacial
flood known as a Jökulhlaup.
This flood carried with it boulders of material up to 30 feet wide, leaving behind a brown
to gray coating where it flowed.
Luckily, this flood harmed no one as people do not live within 30 miles of the Fourpeaked
volcano.
Instead, it flowed down the Douglas River into the Cook Inlet.
After the ten hour eruption ended, the flood ceased.
In total, this unexpected eruption was assigned a volcanic explosivity index of a 2.
Scientists later noted similar looking deposits to those generated by the 2006 eruption in
other sections of the glacier, suggesting that several previous glacial outburst floods
have occurred.
This suggests that other recent volcanic eruptions may have occurred in the last 15,000 years,
although not all glacial floods are caused by volcanic eruptions.
In Iceland for example, they can also be caused by unusually warm weather or increased fumarole
related activity at a volcano.
In hindsight, the eruption during 2006 was not completely unexpected as satellites detected
an uplift signal originating from both Fourpeaked and Douglas between 2005 and 2010, where a
maximum uplift of 6 centimeters was detected.
Fourpeaked is unlikely to erupt again for some time, but we could be greeted by another
surprise eruption in the immediate future as many nearby seismometers failed, leaving
this volcano largely unmonitored by 2020.
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