30 October 2015

The Big Horn Mountains landslide in Wyoming

Posted by Dave Petley

The Big Horn Mountains landslide

There is quite a lot of interest in the media in a landslide that has developed in the last fortnight in the Big Horn Mountains of Wyoming.  This was first reported by SNS Outfitter and Guides on their Facebook site, with the following impressive picture:-

Big Horn Mountains landslide

Big Horn Mountains landslide from SNS Outfitter and Guides

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The caption was:

This giant crack in the earth appeared in the last two weeks on a ranch we hunt in the Bighorn Mountains. Everyone here is calling it “the gash”. It’s a really incredible sight.

A couple of days later they followed this up with a further image:-

Big Horn Mountains landslide

Big Horn Mountains landslide courtesy of SNS Outfitter and Guides

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The caption this time was:

Since so many people have commented and asked questions, we wanted to post an update with a little more information. An engineer from Riverton, WY came out to shed a little light on this giant crack in the earth. Apparently, a wet spring lubricated across a cap rock. Then, a small spring on either side caused the bottom to slide out. He estimated 15 to 20 million yards of movement. By range finder, an estimate is 750 yards long and about 50 yards wide.

The interpretation is I think broadly correct, so terming this the Big Horn Mountains landslide seems reasonable.  The best image to illustrate this that I have seen is from the Sott.net:
Big Horn Mountain landslide

The Big Horn Mountain landslide via sott.net

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From this perspective the mechanisms of the landslide are clear.  The fissure is undoubtedly the rear tension crack that has opened up as the whole mass has slid forward.  On the left side as seen in the image directly above the movement has generated two new lateral shears.  On the right side the lateral shear lies within the gully to facilitate movement.  The toe of the landslide is characterised by a massive zone of buckling and compression.  There may be some evidence that the main body is going through some internal deformation – note the possible cracks in the slope on the right side in the area in shadow.
I suspect that water did play a role in this landslide, but in general the role is to change effective stress not to provide lubrication.  There is a good chance though that this is a progressive failure and that the changes in the behaviour of the springs occurred as the internal drainage of the slope replumbed itself as the deformation developed.

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