The Tasman Glacier in Mount Cook National Park has had another large section of the terminal ice face dramatically calve. On Friday evening August 20th, 2010 this amazing event was witnessed by two people from the distant public viewpoint of Tasman Valley walking track and when they joined with Discovery Tours Guide Elayne Nilson-Reid two days later to reach the much closer guides view point directly above the Tasman Glacier, they were very excited!
Over the last two months, Discovery Tours guides noticed the tilting and lifting of the terminal ice face with several metres rising each week over a number of weeks until the strain on the Tasman Glacier became too great and resulted in the massive break up of the terminal ice face.
Observing this type of activity in winter is very unusual as it occurs more often in spring and most often during autumn when rates of ice melting from summer reach maximum flows and if they combine with heavy rain events it can result in the terminal lake leavel to rise and an increase in the amount of water under the glacier to increase. The increase in hydraulic (water) pressure builds to such an extent under the glacier's terminal area that the ice lifts to a point where it breaks (calves) away from the ice of the main glacier trunk behind.
Seeing this occur during winter has been very unusual and has been an exciting experience for both guides and clients on tour. The viewpoint the guides use is right above the glaciers terminal ice face and provides a stunning view of the lower 12km of the glacier, providing visitors with a graphic vantage point to see and understand the proccess causing such dramatic ice calving to occur. Just about every time we have been out on tour some part of the glacier has changed and all the guides have been wanting to be the one to see this latest very large calving happen. Grant Pearson was the lucky guide this time and he took some great photos showing before and after, see if you can spot the difference!
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While this is quite a large calving it is not the largest! Judging from the cracks that can be seen directly behind the freshly broken ice face, it can be expected that there will be a number of calving events to follow over the next few weeks.
There are a number of factors which influence how quickly a glacier retreats, but at the moment the biggest influence is the terminal lake, which formed in the late 1980's with increasing melt water from the Tasman Glacier dammed behind the moraine rock wall left behind by the retrearting ice. The lake slowly increased in size to reach around 2km in length by 2005 and since then has been rapidly growing larger every year to be 4km long. Over time the Tasman Glacier will continue to retreat as it finds an equalibrium between the amount of ice forming each year and the amount that melts.
Vernon Reid director and Chief Guide said it was the first time in 18 years he had not seen the glacial lake freeze entirely and perhaps this indicated a change in the lakes dynamics. He also noted extremely little rain and very little wind throughout July, which are normally ideal conditions for such surface ice to form on the lake during the cold winter months. This lack of the glacier's lake surface ice forming along with the unusual blue hue to the lake, which normally has such a high concentration of rock flour it stays a muddy colour, maybe indicators the terminal lake is now large enough to retain heat from summer warming and increase the lifting of the glacier's terminal ice face, perhaps resulting in an increase to ice calving off the Tasman Glacier Terminal?
There will no doubt be a number of interesting discussions on why it was different this year. While we have a number of theories such as the the increasing size of the Termianl Lake, without a lot of money and a brave scientist to measure the lake and terminas regularly, we cannot be exactly sure what is going on under the lake surface. But even so it is fascinating to watch the Tasman Glacier change in response to the climate change and in turn the surrounding landscape changes too!
Author Elayne Nilson-Reid (Bsc) is a senior guide for Discovery Tours Ltd based in Twizel where they conduct tours and hikes of the Aoraki Mount Cook National Park and surrounding area of New Zealand. After completing a science degree in ecology at Otago Univeristy Elayne went on to work for the Department of Conservation in the visitor centre and also act as the Historic Heritage manager for the National Park. Following several years with the Department, she worked at a number of tourism ventures then joined her husband Vernon Reid in 2005 as a guide for Discovery Tours, while also developing her art and photography interests.
