Every few years, a debris flow descends Boulder Glacier on the east flank of Mount Baker. These are muddy masses of snow, ice and rock debris that begin as ‘debris avalanches’ at around 9800 feet (2987 m) elevation on the north or northeast flank of Sherman Peak, the pointed 10,160 foot (3096 m) summit on the southeast rim of Sherman Crater. These large debris flows rush down the glacier, filling crevasses on the way. While they usually peter out before reaching the glacier’s terminus, at least one in recent years swept beyond the Boulder Glacier well down into Boulder Creek. MBVRC has photographic evidence for flows in June 2016, October, 2013, July 2010, July 2006. We know of many others in previous years.
At first glance, these may appear to be simple snow avalanches. However, the sliding surface is always at the ground–ice interface, rather than at some intermediate layer within the snow pack. Also, a notable proportion of the flow deposit consists of fragmental volcanic debris mantling the rim of Sherman Crater and underlying Sherman Peak. This material is principally volcanic ejecta (‘tephra’) blown out of the crater; much of it may date to the reported 1843 ‘eruption’. This was a large steam blast, as there is no geologic evidence for a true magma-producing eruption. The tephra erupted in 1843 was principally hydrothermally altered clay and rock reamed out of steam vents by the energy of the steam blast. (Magmatic tephra, tephra Layer BA, was erupted around 6500 years ago during the last geologically verifiable ash eruption at Mount Baker.) That eruption showered many meters of ash and blocks around the crater and sent a thin layer of BA ash as far east as Cascade Pass and at least to the north end of Chilliwack Lake to the northeast. The tephra on the crater rim consists of a mix of fine-grained and blocky fragments. The fine-grained tephra in particular has been decomposed to water-absorbent clay by fumarolic (hydrothermal) activity, and tends to be water saturated with little cohesion. So, when the overlying snow and ice periodically cut loose, the slide incorporates a fair amount of this unconsolidated volcaniclastic material that carpets the ground beneath.
A 1975 research paper focused on these events, and compares the extent of these flows from air photos taken in 1960, 1962, 1969 and 1973:
Frank, D., Post, A. and Friedman, J.D., 1975, Recurrent geothermally induced debris avalanches on Boulder Glacier, Mount Baker, Washington; Journal of Research, US Geological Survey, v. 3 n. 1, pp. 77-87. Click this link 1975 Frank,Post,Friedman- Boulder Glacier debris flows to read the 1975 USGS study of these debris flows. It is a 2 MB pdf file. Be patient, takes a bit of time to download.
In 1973, Frank and colleagues were on the ground and tried to get a closer view of the release area. The source area is very steep and subject to slides from higher up the slope, so those observers did not venture into this exposed area. A few weak vapor emissions were seen along the margin of the source area (Frank and others, 1975), and could conceivably indicate that sliding is related to ground heating. They also describe the mud and rocks included in the deposit on the glacier surface, and at the terminus. Their paper argues for a geothermal cause for these debris avalanches, rather than gravity release once sufficient snow and ice accumulate on the steep slope, gravity does the rest.
There is a way to further resolve this issue of heat vs. gravity. When the next debris avalanche occurs, the source area can be examined remotely by using a thermal imaging camera (‘FLIR’, or Forward Looking Infra Red). This could be done from an aircraft (calling John Scurlock!). Heat anomalies from fumaroles would hopefully show up on images obtained in this manner. A properly calibrated infrared camera would even reveal the temperature of fumaroles in the source area. See an article here about use of these systems in volcanology. http://www.industrysearch.com.au/Features/FLIR-Infrared-Cameras-Used-to-Monitor-Active-Volcanoes-1180; This photo shows a fumarole using FLIR and the temperature calibration to go with it: http://www.volcano-explorer.alaska.edu/46/flir/veni.html
Are these lahars? A ‘lahar’ is a debris flow initiating in volcanic deposits, but not necessarily
due to eruptive activity. (Lahars are also called ‘volcanic mudflows’, a misnomer since deposits may include any particle size). Since the Boulder Glacier deposits consist of material that was, at on time, erupted, the argument can be made that these debris flows are lahars. However, loose use of the term carries a certain gravitas, and may invoke actual eruptions in the public mind. To avoid argument and public (or media) over-reaction, it is safe to use the generic term ‘debris flow’.
These debris flows can be hazardous if climbers are on the Boulder Glacier. On July 26, 2006, mountaineers on a National Outdoor Leadership School (NOLS) course were just getting ready to cross the glacier from south to north, when a debris flow roared down from Sherman Peak. For the first time on record, there is an eyewitness account of one of these occasional events. According to one of the NOLS leaders, two guides were scouting the route across the Boulder Glacier with 2 student climbers. At 3:00pm they decided they had done enough scouting and the chosen route across the glacier was feasible. They turned back across the glacier toward the main party waiting further south on the the glacier. At 3:13 PM a huge debris flow consisting of snow, ice blocks, and rock ran down the Boulder Glacier. At the time of the avalanche, the four in the scouting party were at the 6,700 ft level and a few hundred feet south of the avalanche path. A group of students were waiting on the Boulder Glacier near the Talum/Boulder moraine and were 0.25 miles south of the avalanche path. Niether group could see the intiation point, and after spotting it roaring down the glacier, they lost sight of its path behind a rise in the glacier. It initially seemed was coming in their direction, but then veered down the center of the glacier. The sound was described as ‘thunderous’. By the time the flow reached a point level with the lead group, the flow, which was “very dirty and dark” according to Sean Bowditch, one of the trip leaders, went by “slowly, like lava, and made a rumbling sound. It contained many ice blocks the size of cars and even boxcars. It took several minutes for the whole mass to stop flowing”. The debris on the glacier had levees along the flow margin as much as “10 meters high”. The group was so disturbed by the near-miss they turned back, avoiding a crossing of the glacier all together.
This debris flow terminated at the 5,900 foot level, where it ran into crevasses. Aerial photographs and on site investigation show that the initiation point was at 9800’, so the total run of the debris flow was 3900 feet (1188 m) vertically and 1.63 miles (2.63 km).
The 2006 flow blocked the east breach of Sherman Crater with a 20-m-thick deposit of ice blocks and mud. This is usual after one of these flows. The East Breach is the lowest point on the crater rim, directly below the initiation zone. Heavier-than-air fumarole gases, especially hydrogen sulphide (H2S) and CO2, usually flow out of the crater by this route. When blocked by ice avalanche debris, the gases are unable to exit the East Breach and can build up on the floor of the crater, creating potentially hazardous conditions for volcanologists working in the crater. Also, an ice dam could impound melt water in the crater during elevated heat flow at the volcano. Frank and others (1975) discuss this potential hazard.
The 2006 debris flow was documented from the air by pilot John Scurlock; photos were taken by others, as well. A gallery of photos from the slide is on John’s website: http://www.pbase.com/nolock/mtbaker&page=4 along with other great views of Mount Baker.
A smaller debris flow on the glacier in 2010 was described elsewhere on this website:
Do you have photographs or first-hand knowledge of one of these events? Please email information to: