Posted by: magmatist | October 29, 2013

Sherman Peak ice/rock debris avalanche plugs Sherman Crater’s east breach.

Aerial view looking south. John' s airplane is approximately over Mount Baker's summit.

Aerial view looking south. John’ s airplane is approximately over Mount Baker’s summit. Click to enlarge any photo.

Just before sunset on October 28, 2013, pilot John Scurlock photographed the debris from a large snow, ice and rock debris avalanche that fell off the northeast slope of Sherman Peak. These are called ‘debris avalanches’ because they contain volcanic rock from the mountain surface below the snow pack as well as snow, rather than an ‘avalanche’. The latter term refers to a snow-only event. A huge wall of debris, largely ice blocks, at least 30 m high now plugs the east breach of Sherman Crater. Seismic recordings suggest this event occurred October 21. More on that below.

View east out the east breach of Sherman Crater. The high wall plugging the breach is clearly visible.

View east out the east breach of Sherman Crater. The high wall plugging the breach is clearly visible. Click to enlarge.

These slides occur periodically, as snow accumulating on the steep downwind slope of Sherman Crater finally succumbs to gravity, scouring the slope right down to the ground. Typically they travel some distance down the Boulder Glacier; one of these in July 2006 narrowly missed a climbing party far below the crater. A debris flow in 1984 pitched off the end of the Boulder Glacier and went several additional kilometers down Boulder Creek.

View west into the crater. The wall of debris did not fill the deep ice pit that is the site of a large fumarole.

View west into the crater. The wall of debris did not fill the deep ice pit that is the site of a large fumarole.

This latest event was relatively small.  John says that it traveled “a mile or so” down the glacier; larger events may reach the terminus of the nearly two-mile (3 km) long glacier. John’s photos reveal a large hanging wall of unstable snow and ice remaining below the summit of Sherman Peak.

The plug of ice and rock in the East Breach pose a threat. Heavier-than-air gases, principally H2S and CO2 from the crater’s hundreds or even thousands of fumaroles can usually flow out of the crater by way of the low breach. However, the blockage now impedes this with a wall of snow and ice, which will become more substantial due to winter snowfall. With the blockage, on calm days, the gases may concentrate on the crater’s ice floor; unwary skiers and the many illegal snowmobilers are now at risk of asphyxiation if they descend below the level of the top of the plug.

The slide was recorded on one of the seismometers at Mount Baker. WWU seismologist Jackie Caplan-Auerbach wrote: “Nice catch, John!  I poked around through seismic data and found what I suspect is this event on October 21, at ~2200 UTC (1400 local time). For those of you who are unfamiliar with spectrograms (see image at right- DT),

MBW spectrogram, Oct 21, 2013. PNSN.

MBW spectrogram, Oct 21, 2013. PNSN.

the top panel of this figure shows the ground shaking at seismic station MBW, located on the west flank of Mount Baker about 9 km from Sherman Peak.  The large blue signal represents increased ground shaking associated with the avalanche.  The lower panel shows the frequencies at which the shaking was strongest.  The y-axis is frequency, from 0-30 Hz.  The warmer colors show which frequencies were strongest.  So in this case there’s a lot of energy across the spectrum between 0-25 Hz.  This is quite typical of a mass-wasting event.  The signal duration is about 71 seconds.  There’s a hint of a small signal before the avalanche (maybe a precursory slide or a tiny local event), but it’s hard to tell.”

The online signal from the other Mount Baker seismometer, SHUK, currently looks like a flat lin. The seismometer is working, but there is a scaling issue with the  seismic signals appearing on the webpage. SHUK is at the Mount Baker ski area about 14 km north-east of Sherman Peak.

In 2006, another high wall plugged the east breach, and prevented that summer's gas monitoring team from descending very low in the crater.

In 2006, another high wall plugged the east breach, and prevented that summer’s gas monitoring team from descending very low in the crater.

A dark streak of the 2010 debris flow went far down the Boulder Glacier. Phot by Jason Griffith.

A dark streak of the 2010 debris flow went far down the Boulder Glacier. Photo by Jason Griffith.


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