Archive for the ‘Rockslide’ Category

In a recent review of the mechanics of cirque glacier dynamics, erosion, etc., I came across an interesting photograph in the Process Geomorphology textbook (John Ritter, et al., 3rd edition). The black and white, undated or referenced photograph contained only caption to describe its purpose. In a few words, the authors describe one large and one much smaller rockslide onto the body of the Schwan Glacier of Alaska’s Chugach Range. From the photograph you can observe the very steep terrain of the Chugach Mountains, carved over thousands of year to produce numerous cirques and arete ridges. You’ll notice in the upper right  hand corner (which translates to west in the photo) that there is a significantly large, and fresh, landslide deposit covering almost all of the glacier. ANother, smaller slide is located in the bottom center of the photograph (Northwest-facing).

South-looking view of Schwan Glacier and rockslides.

[Scan from page 372, Ritter, et al., 1995 Ed.]

Based on the photograph quality, I figured that the photo was taken some time ago – as the first edition was published in 1995. Extrapolating this, I am led to believe the photograph was taken somewhere between 1960 and 1980.

Now, as we all know, glaciers move around a lot. Luckily though, only in one direction. Thinking back towards a Quaternary Geology seminar almost one year ago – I remember a discussion relating to rockslides and landslides on top of glaciers and how they may or may not effect mass wasting balances, advance/retreat, surges, etc. The seminar group discussed this for a while, but ultimately no real conclusion was reached. Segueing this back into my recent page-flipping, I wondered: what evidence may exist of the Schwan Glacier’s reaction be being buried by to separate, but closely related rockslides? Of course, the quick-and-dirty method of analysis rests at the end of my fingertips and on almost every geologist’s desktop – GoogleEarth.

GoogleEarth is the lifeblood of visual geomorphology. The entire world is covered in ongoing surface processes and GoogleEarth provides an incredibly easy what to check out anything that’s going on anywhere on the earth’s surface. Within seconds of typing “Schwan Glacier, Alaska”, I was there.

[As always, click on the images for a larger view.]

A look south towards Schwan Glacier (marked with the crosshairs) shows two converging ice streams melding into one larger, debris covered stream. The farther down the glacier (closer to the “camera”) you travel, the debris looks more and more separated into medial moraine ridges rather than unorganized piles. This is normal. However, looking upstream (up glacier) there seems to be an awkward, dark bulge in the middle of the ice stream:

A disturbance in the ice stream?

A little closer look of the slide debris.

I’ve annotated this closer look to highlight what has happened here over a few decades. The light orange polygon is the landslide scarp – the area exposed after the slide moved downhill. In a lighter blue, I have what I have interpreted as the deformed slide deposit that is shown in the first image of this post. This shows a truly excellent view at ice-stream convergence dynamics. When two water streams join in a temperature climate, we’ll sometimes see a cloudy, brownish stream (high sediment load) mix in with a clearer, blue stream (one with less sediment load). Since it is still topical, I will use an example of mixing from the Connecticut River and Long Island Sound just after Hurricane Irene:

The contexts are slightly different – but the idea is basically the same. Notice how eventually the turbid water mixes with the clearer ocean water over the course of a few days. Now in contrast, the “load” entering the Schwan Glacier (which is a river in itself, just of ice) does not mix in.

We can also see in the annotated (and unedited, for that matter) image that as the debris was transported downglacier, the second incoming ice stream from the east (left of the image) is actually deforming the sediment deposited on the Schwan. Very, very cool stuff.

Now this was a very simple, quick exercise  in glacier dynamics and playing around in GoogleEarth, but even more information can be extrapolated from this method. In particular is ice velocity and landslide-triggered stagnation. If one were to know the timing of the slide on the Schwan Glacier, along with the time taken between that and the most recent imagery date – a simple length measurement (from the original landslide position to the new extent) divided by the elapsed time can give up the ice velocity. Unfortunately, I have not yet been able to find a date of the landslide photograph, and historical imagery via GoogleEarth was not available.

Whether or not ice stagnates upon burial by landslide events is also of debate within the field. One idea is that the lowered albedo due to rock debris on the glacier causes more heat absorption, rather than reflection – possibly causing melting. In contrast, it is also believed by some that the veil of debris caused by the landslide actually insulates the icefield, stabilizing it, and causing the velocity to decrease.

The Schwan Glacier images do not support the second statement – as it is incredibly obvious, based on the deformation of the debris field, that the glacier continued to waste downstream. To go as far as to say that the slide actually sped up glacier flow would be irresponsible. For one, it would be taken completely out of context (as we’re only looking at a few images, here) and two, so much more research and data collection would be needed to confirm or deny this. Also – the instantaneous point just after the landslide is when the albedo would be the lowest – once snow comes again, the debris field would soon be covered with a new blanket of highly reflective snow. But still – a fun debate.


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