BC Volcanology 101: Mount Meager, Part 2: Dams!

A conclusion to this trip with the GSA is long overdue - bear with me, as several university courses as well as work have been taking up most of my time these days!

On Sunday I had the pleasure of giving a talk to the Vancouver Island Paleontology Society in Qualicum Beach, BC. The title suggested by Dr. Graham Beard was "Everything You Wanted to Know About Eruptions, But Were Afraid to Ask" aka "BC Volcanism". It was such a pleasure to present to such an enthusiastic, wonderful group of people I've known for several years, as well as the public and several local geologists, and to dispel some common myths regarding volcanoes in British Columbia and the Northwestern USA. 

Most of the talk, however, covered the GSA trip I took in October 2014, so with this post, I aim to conclude the tale of Mount Meager's 2360 BP eruption, and then I can settle into the homework that no doubt will consume me until the year is out.

But here we go! Get ready for one hell of a dam, more columns (NOT in basalt this time...) and the Meager Creek Landslide of 2010.

Our "view" (sigh) of Mount Meager near Keyhole Falls, morning of Day 3.

First stop on this day was at Keyhole Falls, the scene of a large lava dam that was created during the effusive stage of the 2360 BP eruption. The falls gets their name from the canyon shape through which it flows, shaped at the falls just like an old-fashioned keyhole, which then opens up to allow the Lillooet River a chance to meander along the base of Mount Meager. Below is a picture of the falls from an angle we didn't get to see, followed by a photo of our view of the canyon below:

Upstream view of Keyhole Falls. We were positioned at the top right of the cliff in the trees, directly above the falls. Photo credit The Wilderness Committee, 2009.

At about 14 days after the initial eruption, as our welded block and ash flow continued to be deposited in the valley below, a lava dam across what is now the Lillooet River began to form. The dam would eventually reach >1000m in thickness, and was capped by a non-welded version of the block and ash flow deposit. 

This dam created has been termed paleo-"Salal Lake", given that present day Salal Creek now flows into this ancient lake bed. The evidence for Salal Lake is given in both the underlying river sediment deposits that have been covered by the pyroclastic flows and by layers of volcaniclastic sediment washed downhill into it after it's formation.

After the initial creation of the dam, volcanism slowly continued, waned, and then resumed, adding layers to the top of the dam, and keeping the river at bay. The nature of this deposit, although being welded to some degree due to heat, was not enough to hold the river back for very long, and within a month, the dam failed catastrophically, creating a downstream debris torrent that washed out the canyon. 

Keyhole Falls, facing downriver, from the cliff directly above the 'keyhole'.

Which leaves us with the canyon today, plus about 2300 years of fluvial erosion. 

It should be noted that the EXACT nature of the deposit forming the dam is not firmly known, due to the deposits inaccessibility within the steep canyon. Dr. Andrews is looking for a sucker... I mean brave grad student, who would be willing to rappel over the edge and get a good look at these deposits...any takers??

One of our final stops, and perhaps the most interesting to me, was an outcrop along the road further downstream from Keyhole Falls which showed columns forming within the block and ash flows. This was the first time I'd ever seen columns in something other than the basalt, and I was overwhelmed and FULL of questions!

In the following photographs, you can see the edge of the flow as it reached a cooling surface. Note the small columns on the right hand-side of the deposit that grow into larger columns toward the left of the deposit, and enjoy:

Dave (UK - left) and Graham (USA - right) investigate the outcrop. The flat cooling edge is to the right of the outcrop, just above Graham's head, with small columns. As you go to the left in this photo, you will see progressively larger columns forming. 

Dave at the cooling edge of the outcrop. Here you can really see the formation of the columns! The columns here are mm-cm in scale, growing to 0.5m and larger in diameter as they move further from the cooling edge.

My biggest question here, while everyone else was preoccupied with the other, likely-more-important-science, was why is the cooling edge SO FLAT? When I got up the courage to ask the team about this, a lot of ideas were thrown around, before Graham eventually said: "Well, I don't have a firm answer for you: but that could make a great thesis!"

Our final stop was at the campsite along the Lillooet River, which, in 2010, avoided destruction by mere metres when an estimated 40,000,000 cubic metres of debris rushed down from Capricorn Mountain (the peak adjacent to the east of Meager). This landslide was deemed a lahar, and is known as the second largest landslide in Canadian History. It also created yet another dam along the Lillooet River, which remained only a matter of days before becoming washed out and sending even more debris further downstream at an astonishing rate. 

That's me - for scale! View facing towards Mount Meager/Capricorn Mountain and the Meager Creek Valley. Photo credit Dr. Dave MacGarvie, 2014.

The UBC EOS van faces the campsite, in the break in the trees, that escaped washout by meters!! Landslide deposit can be see in the background.

In British Columbia, the most frequent hazard related to volcanic activity ends up, indirectly, being landslides and slope instability in general. When working with Dr. John Clague from SFU on a seismic hazards project for my undergraduate thesis, he mentioned that, if I had aspirations to become a volcanologist, I should be prepared to work in more of a geo-engineering capacity if I wanted to stay in BC. And it's true, the better one understands how the geology is put together, the better you know how it's likely to fall apart. I won't get into anything such as funding for Canadian scientists here (or should I say, extreme lack of funding... ahem), suffice it to say that although we don't necessarily need to monitor volcanic activity in southwestern BC, we should be paying closer attention to the landforms around us in order to stay safe. 

ANYWAYS

Thus concluded our fabulous tour of Mount Meager! Now, all that was left, was to get a damn picture with the iconic peak!! On our way back, we stopped in the flat fields adjacent to the Lillooet River, and, waiting for the last of our team to join us for the drive back to Vancouver, we were rewarded with only a few precious minutes for a photo op - for the first time during this entire trip! Here is my only portrait with Meager:

At last! A view of the peak in the final moments of the trip! 

And so, with my dream of becoming a volcanologist at once more clear in my head, and more intimidating than ever to achieve, I sat in silence on the drive back to the ferry. The crew kindly dropped me off, and while waiting for the boat home to my 'rock' (Vancouver Island), I sat and had dinner at The Boathouse Restuarant in Horseshoe Bay, sipped a well-deserved drink, reviewed my maps and notes, and felt more inspired than ever. 

Thank you for reading :) Keep in touch for preparations (2014) for adventures (2015) to Mount Edziza and Mount Garibaldi! Until next time!