2360 years ago, Mount Meager, just north of Pemberton, BC, and part of the Garibaldi Volcanic Belt in the Cascade Volcanic Arc, created the province's largest Holecene volcanic eruption, as well as two of the coolest outcrops I've ever seen.
This post continues the GSA 2014 Field Trip through the southern portion of the Canadian Cascade Volcanics. For the entire trip, until the final hour of our time in Pemberton, the clouds kept all of us from seeing even a flash of the jagged peaks... resulting in many jokes about the "meager view" we got each day (harhar) ;)
Our consolation, however, was some amazing outcrops of pumice and pyroclastic flows. What follows here are details from Day Two, and the explosive phase of the most recent eruption.
Here's the lowdown on the 2360 BP eruption:
- The Bridge River Vent, lying in Plinth Peak in the Mount Meager Massif, and the youngest vent from this complex, is responsible for releasing these materials.
- The eruption sent a 20km high Pilian column into the air - evidence of this stretches as far East as Alberta, the neighboring province, about 530km away.
- The first explosive phase (detailed in this post) resulted large pumice deposits and pyroclastic flows, including a welded block and ash flow.
- The second, effusive phase involved a lava flow that dammed the Lilloeet River, creating a lake up-river from present day Keyhole Falls (details in the next post to be posted!).
First stop: the pumice pit.
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| Happy as geologists at a pumice outcrop! Wait... |
After a short briefing at our first stop, our guides let us loose to explore the outcrop with an assignment (usually given to the UBC undergraduates) to collect as many different types of rocks as we could find. We would then recap and talk about why there were such a wide range of samples here...
...Half an hour later, there were 12 geologists still rummaging and pocketing various samples, ooh'ing and ahhh-ing over their finds, with only 3 representative samples on display. Dr. Russell was not impressed, but managed to kick us into gear and discuss what we found.
Firstly, think of pumice as the frothy, extremely hot lava bubbling away at the top of the vent. As it gets ejected from the vent, it cools quickly, with the end result being a light, ragged form of volcanic glass. Highly vesicular, it can easily get airborne - this outcrop, as well as the open pit mine in the following photos, is within 5km of the vent.
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| Dave for scale - showing the pumice outcrop. Note the band running through the centre l-r, which indicates a slower phase or "easing" of the explosive eruption (made up of finer particles) and the various holes in the pumice, which where once filled with tree branches. |
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| Pumice in hand sample - the color changes may indicate differentiating in cooling rates, although this requires more investigation on my part. |
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| "View" of Mount Meager and the Bridge River Vent from the open pit pumice mine, ~ 70km north of Pemberton, BC. See http://garibaldipumice.com/ for more information on this specific location. |
Our next stop was one of the most amazing outcrops I've seen (next to the folded limestone on Quadra Island, which will be detailed in a future post). It was a welded block and ash flow, representing a violent pyroclastic flow that tore away chunks of the peak as it erupted, and is seen here overlain by the effusive phase (lava flows).
Enjoy the following pictures, that do not do this outcrop justice...but try.
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| The group at the base of the welded block and ash flow outcrop, complete with waterfall in background! |
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| Closer examination of the deposit. The light grey you are seeing here is the fine grained ash, and the black portions are chunks of the Plinth formation, which makes up the original country rock underlaying the vent. |
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| Dr. Graham Andrews gives us the details about the events that lead to this formation. |
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| This was the first geologizing trip for my new boots - covered in volcanic ash seems a great way to start off our relationship! |
Next up: The effusive phase of the 2360 BP eurption and the Meager Creek Landslide of 2010!
