17 January 2017

Mount Sulzer – a series of dramatic, and extremely large, debris and ice avalanches

Posted by Dave Petley

Mount Sulzer debris and ice avalanches

Mike Loso (contact via: michael_loso@nps.gov) of Wrangell-St. Elias National Park and Preserve in Alaska has kindly provided details of an amazing series of debris and ice avalanches that have descended from the flanks of Mount Sulzer in recent years. The Google Earth image below shows the location.  On the left is the site of these major landslides.  The next valley to the west (on the right in this image) has also suffered a glacier surge in the 2015-16 period, but that is not the focus here.

Mount Sulzer

Google Earth image of the Mount Sulzer debris and ice avalanches

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In the couple of years before summer 2015 this site appears to have suffered at least two major debris and ice avalanche events.  The image below, the earliest in this sequence (from summer 2015) shows the aftermath of these flows:

Mount Sulzer

An image of Mount Sulzer from Summer 2015 showing the remarkable trim line caused by an earlier landslide events. Image by Jeff Trop.

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A landslide deposit is very clear in the foreground, but note the removal of vegetation on the substantial hill on the inside of the bend in the river, including the creation of a clear trim line marking the edge of the flow as it crossed the topography. The very obvious deposit in the foreground, shown below, does not appear to be the major landslide that caused this trim line; this appears to be a second, smaller, event that is sitting on the sheet-like deposit of the larger first landslide:

Mount Sulzer

The deposit left by the second Mount Sulzer landslide, sitting on top of the earlier event. Image by Jeff Trop.

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Later in summer 2015 there was a further (the third in this sequence) very large landslide event. This appears to have run over the hill in the foreground once more, removing even more of the vegetation:

Mount Sulzer

Aerial shot of the aftermath of the summer 2015 landslide on Mount Sulzer. Image by Paul Claus

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Then in summer 2016 a further slide occurred.  This landslide, which happened on 13th August 2016, was observed and videoed by one of the rangers. This video is now online. The image below shows the aftermath of the landslide:

Mount Sulzer

The aftermath of the August 2016 Mount Sulzer landslide, with remains of the flow in the channel (image by Michael Loso)

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The source of these landslides is a steep glacier terminus that is clearly capable of discharges large ice-avalanches, but that also contains evidence of strongly altered, likely clay and ice-rich, unstable slopes beneath the active glacier face:

Mount Sulzer

The steep hanging glacier source of the Mount Sulzer landslides. Image by Mike Loso.

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Ice-rich, highly altered sediments that represent the source of much of the debris for the Mount Sulzer landslides. Image by Michael Loso

Ice-rich, highly altered sediments that represent the source of much of the debris for the Mount Sulzer landslides. Image by Mike Loso.

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Once again the incredibly dynamic landslide environment of the mountains of Alaska is clear.  Over the last three years it is become apparent that this area is the most active on Earth in terms of very large landslides.  That was not expected (by me at least).

Many thanks to Mike Loso (michael_loso@nps.gov) for providing this information, and to various others for the images.