Wednesday, December 28, 2011

The Accretionary Wedge # 41: Something's Slumping

It is time for me to participate in the Accretionary Wedge once again. This month's topic is: What is the most memorable/significant geologic event that you have experienced?

What we seek for AccretionaryWedge #41 is an account of a geologic event that you experienced firsthand. It could be an earthquake, a landslide, a flood, a volcanic eruption, etc. (but don’t feel compelled to stick to the biggies – weathering, anyone?) – some geologic process that you were able to directly observe and experience. - Ron Schott

This is a toughy. I have never seen a volcano erupt, been in an avalanche or really in any life threatening geologic events in my short career. Thus far in my life I have experienced one earthquake, in Ottawa, and a few other smaller events such as watching rivers erode banks, observing coral reefs and sedimentation processes in Bermuda, climbing a glacier in New Zealand. All of these things are cool, but I think the coolest of them all is the time I spent working in the massive retrogressive thaw slumps located in the Northwest Territories where I have done fieldwork. I think it is fair to say that these slumps are geologic events in progress. Indeed, standing in a  slump is to watch ice, rocks, trees and mud cascading from the headwall around you and if you close your eyes you can hear the cacophany of the slumping process all around you, mostly in the form of plopping and thuds.

The Charras slump. It is about 1-1.5km across.
These slumps are formed in permafrost regions and are due to melting of permafrost and a thickening of the active layer, which is the layer of soil that melts on an annual basis. This melting can result either in active layer detachments, which is when the melted surface layer slides off the permafrost below, exposing it to melting, or when the melting occurs on slopes that result in a landslide. These slumps can be extremely large and have a huge impact on the land and watersheds they form in as they dump millions of tons of sediment into nearby streams and lakes.

Basically a slump consists of the three main parts: the headwall, the body and the mudflow. The headwall is the front of the slump where is it retrograding backward into permafrost which then collapses into the hole as it melts. The body is the big hole in the landscape and the mudflow is the trail of sediment that has flowed downhill and either been carried away or is damming a stream. 
The mudflow. This one is about 2-3km long and has completely filled the stream valley.
My work in the slumps near Fort McPherson, NWT consists mainly of taking water samples in nearby creeks, samples of peat moss and samples of ice from the headwall for geochemical analysis of stable and radioisotopes. Our usual routine consists of camping in Fort McPherson and then driving out to near the slumps and then bushwhacking/hiking out over the land to reach them, which takes about 45 minutes. These slumps are an awe inspiring site and every time I go out to them I am truly overwhelmed at their size and the magnitude of destruction they have caused to the land around them. Honestly, walking into a slump is like waking onto another planet because one minute you are surrounded by green shrubs and peat moss and the next mud! Mud is everywhere, completely dominating the landscape and flowing everywhere. In fact, the inside of a thaw slump is like visiting the moon. 

Closeup of the mud with "craters" left by raindrops

The headwall. It is around 30m high. You have to be really careful when on top of the headwall since you can't tell from above if you are standing on an overhang or on solid ground!! Also, when in the slump you have to watch your footing since the mud can be pretty treacherous and suck you in. It is best to travel in groups.
Furthermore, in the summer the thaw slumps are ever changing. I alluded to the sound a slump makes as rock and ice cascade down into the slop below, but it is difficult to describe how rapidly the appearance and conditions can change. For example, a week of sunny weather will lead to lots of melting in the headwall and the collapse of a large amount of land and a rainstorm the following week will wash it all out onto the mudflow. Indeed, features that seem immense one week will be gone the next and don't even think about trying to spot the same features from year to year!! 

Week 1

Week 2. I kind of think this looks like a face. Particularly Sideshow Bob from the Simpsons. Also, notice that the bush on top in Photo 1 is now gone as are many of the trees.
Well, that is my most interesting experience of geology in action: hanging out in a thaw slump. I hope you enjoyed the read! If you have any questions feel free to ask them. Thanks for reading.


Look out for bears!!

Sampling the headwall.

1 comment:

  1. Nothing big and it was a long time ago (no photos), but, for me, it's got to be that landslide during one of Art Bloom's class field trips in Ithaca, NY. It was a rainy day. We were traversing a slope, one by one stepping across a slight surface bump with a small tree on it. I think I was the last one across, and, just as I took a couple steps beyond, I heard this loud whoosh, turned around, and saw that tree swiftly riding the bump down the slope.