15 February 2015
Fox Glacier – a time lapse video of slope failures as the glacier retreats
Posted by Dave Petley
The spectacular Fox Glacier in New Zealand is a very popular tourist attraction on the South Island – Wikipedia estimates that around 1000 people visit per day. Sadly, the Fax Glacier is retreating rapidly, in common with most glaciers as they respond to the effects of climate change. And the effects are dramatic – this pair of images, taken ten years apart, shows just how quickly the glacier is retreating:
As the Fox Glacier retreats, the surrounding hillslopes, which consist of glacial till, lose toe support. In consequence they can slip, although this process is surprisingly poorly investigated. Kerry Leith of TU Munich, pointed out via Twitter this really interesting time lapse video of the glacier retreating, which captures beautifully the creep of the slopes as the snout of the glacier transitions through:
Fox Glacier’s spectacular retreat from Brian Anderson on Vimeo.
In the early part of the video the slopes on the right side of Fox Glacier are moving (comparatively) rapidly. By the end of the video the glacier has withdrawn from this section, and the slopes have stabilised. Thus, as the glacier snout moves up the valley a wave of slope movement moves with it. This process is being replicated worldwide as glaciers retreat in response to global warming.
The video was compiled by Victoria University of Wellington with the support of Fox Glacier Guides, Department of Conservation, Snowgrass Solutions, University of Canterbury and the Marsden Fund.
Whilst slope movements do occur due to the loss of support as a result of glacial retreat, it would appear that the localised rapid movement in the video is actually due to the melting of ice beneath the material in question rather than due to the loss of support due to the glacier
Compare the creeping slope area down-valley from the little tributary stream, with the stationary valley slope up-glacier from that stream. I think that the tributary (rather than under-glacier meltwater) is eroding the toe of the valley slope, and removing the slope’s support.
The tributary water also appears to be more active in melting the glacier ice than the under-glacier stream. (Compare the rapid changes in the ice below the tributary with the slower changes above the tributary stream.)
Meltwater derived from the overlying glacier will have its temperature buffered at 0 deg Celsius, whereas tributary streams may be several degrees warmer, and may thereby have more erosive effect on ice.
There is no snow seen in the video, even through the (Southern Hemisphere) winter, and vegetation is plentiful, so the weather wasn’t extraordinarily cold even through winter. Despite this, there is a block of ice in that right foreground that survives from 2014-05-27 (when it is isolated by the collapse of a natural bridge) until finally disappearing by about 2014-11-22 – a full six months. This block shrinks rapidly at first while in contact with the flowing water, then much more slowly once the stream migrates away from the ice-block in early July.
In short, maybe tributary streams are more significant in glacier retreat than is generally recognized.