January 27, 2015

Crevasse Reduction and Retreat of Salisbury Snowfield-Almer Glacier, New Zealand

Posted by Mauri Pelto

Almer Glacier is fed by the Salisbury Snowfield which also has its own terminus, and both are former tributaries to the Franz Josef Glacier.  In 2007 the Almer Glacier almost reconnected with Franz Josef Glacier.  The glaciers of the southern Alps have some of the highest recorded accumulation rates in their upper sections and highest ablation in the lower reaches. Anderson et al (2006), note accumulation rates exceeding 6 m on Franz Josef Glacier.  This combined with the steep slopes lead to higher velocity and extensive crevassing on even smaller alpine glaciers.  Purdie et al (2014)  examined modern and historic length change for Franz Josef and noted a ~ 3 km loss in length since the 1800s, with the greatest retreat from 1934 and 1983, despite two periods of advance in that 50 year period.  The retreat particularly since 1983 has been punctuated by advances 1983–1999 (1420 m) and 2004–2008 (280 m), with the current retreat up to 2014 being the fastest rate of retreat during the period of record. (Purdie et al , 2014). The annual end of summer snowline surveys by NIWA monitors the Salisbury Snowfield, the snowline was 140 m or more above the equilibrium altitude in 4 of the last six years and 20-30 m below the equilibrium line altitude in the other two.  The net result is significant mass loss in the last six years driven by exceptional melt, driving the retreat.

Salisbury snowfield

Topographic Map of Salisbury Snowfield-Almer Glacier area

Here we examine changes particularly in crevassing as well as retreat of Salisbury Snowfield and Almer Glacier from 2000-2015. In the Google Earth images from 2007 and 2013 the green arrows indicate crevassed areas and the red arrows the terminus of the Almer Glacier above and Salisbury Snowfield below.  The decrease in the amount of crevassing is evident at each location.  This indicates not just a reduction in velocity, but in glacier thickness that is driving flow.  The thinning is evident with the emergence of a bedrock knob at the pink arrow in 2013 that had been covered by crevassed ice in 2007.  The red arrow indicates the terminus where the main Almer Glacier is within 75 m of the Franz Josef Glacier.  By  2013 the terminus is much dirtier and is 200 m from Franz Josef Glacier. The icefall comparison image from 2007 and 2013 indicates the reduction in width and number of open crevasses, probably in depth too. This is something Jill Pelto (UMaine) has been measuring crevasses in the field on Easton Glacier in the North Cascades over the last few years to see how crevasses are changing as a glacier thins and slows (image below).

In 2014 New Zealand had a warm year and snowlines are high for early summer in January 2015 which will continue the retreat.  The Landsat image from January, 2015 suggests further retreat has occurred since 2013, but given the dirty terminus, it is to hard to determine a specific amount.  The retreat here follows the pattern of glaciers across the Southern Alps of New Zealand- Lyell Glacier and Tasman Glacier

salisbury 2007

2007 Google Earth image

salisbury 2013

2013 Google Earth Image.

salisbury icefall comparison

2007-2013 icefall closeup 

crevase depth
2015 Crevasse Assessment, Jill Pelto, North Cascades

salisbury 2000








2000 Landsat image

salisbury 2015





2015 Landsat image