Rockfall at Franz Josef glacier

27 10 2011

Glaciers transport material down-valley in a variety of different ways.  It can be carried in meltwater (glaciofluvial), entrained and dragged along the base of a glacier (subglacial), buried within the ice (englacial), or carried on the glacier surface (supraglacial).

Of course these processes vary both spatially and temporally, a single rock or grain of sand may experience all of these transport mechanisms during its journey from the top of a mountain to a glacier foreland.  For example, a rock may fall onto the surface of the ice and be carried supraglacially, before being buried and becoming englacial, brought to the base of a glacier and transported subglacially, and finally melting out and transported down valley glaciofluvially.

Supraglacial material is derived from mass movement events on free face valley walls or nunataks adjacent to the glacier. The video at the top of this post shows one such event on the Franz Josef glacier, South Island, New Zealand. This was captured on video by a lucky (no one was hurt) tour party that visited the glacier that day.

Debris covered glaciers have a lower albedo than clean-ice glaciers.  This insulating debris layer means ablation is lower; therefore debris-covered glaciers are less sensitive to climatic change.  It has been hypothesised a rockfall event at the Franz Josef glacier ~13,000 years ago caused a non-climatic glacial advance, responsible for depositing a large terminal moraine, named the Waiho Loop, 14km downvalley of the current glacier terminus.

This theory originated when it was found that a lot of the Waiho Loop moraine was made of supraglacial debris.  Supraglacial debris is typically very angular (see the rock the tour guide is holding at 1 min 40 seconds), as it has undergone very little active transport, compared to debris that is rounded at the base of a glacier .

This is a controversial theory, as it proposes that the moraine represents a non-climatic glacier signal.  However, many glaciologists still believe that the Waiho Loop was deposited during glacier response to cooling events in the aftermath of deglaciation from the last glacial maximum.  However, current efforts to date the precise age of this advance, using radiocarbon and cosmogenic nuclides, suggest that the dynamics of the Franz Josef glacier were out of sync with other South Island glaciers and local climate at this time. There are many potential sources of error involved with these techniques, so the debate continues.  This is just one of many examples of equifinality we find in the natural world; where an end result can potentially be reached in many ways.  As Quaternary scientists, it’s our job to pick apart the causative mechanisms, and come to conclusions.Who knows, maybe this event will cause another glacial advance…

In the mean time, enjoy the video.

This video was found on The Landslides Blog. A story from the local paper can also be found here.




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