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New Zealand has a large number of glaciers, majority of these being located in the central Southern Alps of the South Island. In the South Island alone, there are over 3,100 glaciers (McSaveney, 2007). New Zealand’s largest glacier ‘The Tasman Glacier’, is located in the Southern Alps and it known to be the largest glacier in New Zealand. Since 1973, the glacier has been retreating progressively at a distressing percentage and it is predictable the lake will reach its maximum capacity within 10-19 years (Massey University, 2008). Thus, it is estimated that the glacier will eventually disappear (Mastny, 2000). In the 1950s, the process of the retreat began and there was a rapid expansion in the surrounding lake as the glacier began to melt (Kirkbride, 1993). The purpose of the laboratory was to use a stereoscope to view images of the Tasman Glacier in a 3-Dimensionial setting and then produce a geomorphic sketch of the region. From this point, the aim was to utilise the geomorphic sketch and compare it alongside a series of ASTER images of the glacier from the years 2000-2015 and also alongside aerial images from 1976 and 1986. The purpose of this being to explore and examine the changes that have occurred over time within the glacial processes of the Tasman Glacier.
To enable understanding of how changes of the Tasman Glacier and the surrounding lake have been modified over time, a stereoscope was used to construct a geomorphic sketch of the location over tracing paper with a pencil. A stereoscope is a device which is imported to view a three-dimensional image when two high resolution photographs are placed approximately six centimetres apart. Aerial images of the location in 1976 and 1986 were examined and were compared with six ASTER images from the years 2000-2015. The ASTER images were presented in three year additions. The 2010 aerial image was used to sketch the features of the geomorphic map, these features included the position of streams, alluvial fans, kettle holes and moraines- both terminal and lateral. The sketched map was compared to preceding ASTER and aerial images of the glacier to provide an understanding of the changes that have occurred throughout time within the locatio
The sketched geomorphic map (figure 1) signifies both the Tasman Glacier and how its form and physiognomies have been hindered over time due to natural processes. Results were exposed by comparing the alterations of the ASTER images and the aerial images with the hand sketched geomorphic map of the 2010 glacier; a number of observations were made.
To begin with, alongside the Tasman Glacier, bordering Tasman Lake is the lateral moraine. Through comparing the aerial images with the sketched geomorphic map, the reduction in the lateral moraine between 1976-2015 is obvious. The bottommost part of the glacier is where the terminal moraine is formed. The terminal moraine appears to be perceived as a group of small unconnected kettle holes when examining the 1976 and 1986 aerial photographs.
At the southern end of the glacier, below the terminal moraine is where the Tasman River begins to flow. Superior to the terminal moraine is the Tasman Lake which is where the glacier is found to terminate. By comparing the geological map with the ASTER photographs, it was noticed that the ice accumulation on the mountains has reduced greatly from 1976 to 2015 which is able to be observed on the lateral moraines of the glacier.
With relation to the Murchison River, in the aerial photographs it is noticeable that the river flowed around the terminal moraine close to the glacial boundary. The Murchison River joined the Tasman River on the outwash plain to an area of braided river which forms where a heavy sediment load is deposited and shift between channels giving it a braided like appearance (Harcourt, 2005). By comparing the aerial photographs with the geomorphic sketch, is also evident that over time, the Murchison River eroded part of a lateral moraine, thus moving the river closer to the Tasman lake as it disperses itself from the glacial boundary. As of now, the Murchison River feeds into the Tasman Lake and has eroded the lateral moraine bank.
A change in root of the Murchison River is noticeable when comparing the sketched geomorphic map with the ASTER images. In the ASTER images from both 1976 and 1986, the Tasman Glacier had no retreat lake whilst the ASTER image from 2000 depicts the Murchison River flowing directly into the Tasman Lake, thus contributing to the size of the lake. When comparing the 1986 aerial image with the 2000 ASTER image, a change in shape of the Tasman Valley is noticeable. It has changed from the shape of a V to the shape of a U, caused by the natural process of glaciation. It is also evident that the Tasman Lake has increased in size over the years.
When comparing the 1986 aerial images with the geomorphic sketch it is noticeable that there has been an increase in alluvial fans that have been found at the bases of hills along the glaciers path- mainly occurring on Mount Johnson and Mount Chudleigh. It is noticeable that from comparing the 1986 aerial photograph to the geomorphic sketch, that a lake has formed over time toward the eastern end of the Mackenzie basin. Further, the glacier contained more ice in the 1986 aerial photograph in comparison to the 2015 ASTER image.
Another important key geological feature on the map are the permanent snowfields which lie to the west of the lake- the Mount Cook Ranges (Aoraki). At the height of 3,724 meters, the Mount Cook ranges are high enough to hold snow throughout the year (Dennis, 2007).
By comparing the ASTER images with the sketched geomorphic map, it becomes obvious that there has been a copious quantity of changes in the Tasman Glacier and its surrounding valley between the years 1986-2015. A number of natural processes have arisen that have adapted glacier, contributing to its retreat. The Tasman Glacier is a river of ice which is shown to move slowly through time laps photography. However, it moves so slowly that one is not able to see the movement with their eyes. In 1993, the Tasman lake was shown by a bathymetric survey to be 1.95 square km whilst in 2008 it has been seen to increase in length by 4.01 square km and is now over 5.96 square km (Warren & Kirkbride, 2008).
An alluvial fan is an important feature of the maps. This feature is a triangle-shaped deposit of sediment, including gravel and sand (National Geographic Society, 2017) and are created as flowing water interacts with landforms such as mountains and (Blair and McPherson, 1994). The landform spreads out and are given a triangular shape as streams form. From 1976-2015, an increase in alluvial fans has become present on the Murchison glacier and along with the Tasman Glacier. This is noticeable when comparing the geomorphic sketch with the aerial photographs.
The Murchison river has changed direction over time, this being due to the fast rate of glacial retreat resulting in calving on the terminus. Calving refers to the breaking of ice chunks from the edge of a glacier into the water below and is a significant cause of the landforms retreat (Ferguson, 2017). The process occurs at the terminus of the Tasman Glacier and is which is normally caused by the glacier expanding (Marshak, 2009). The glacier continues to retreat in length as calving continues. Since 2007, the Tasman Glacier appears to have less frequent calving events, however when calving events happen, they have a higher magnitude in comparison to previous times. (Warren & Kirkbride, 2003)
Down wasting has occurred which is noticeable when comparing the ASTER images with the aerial images. This has happened because under the force of gravity, sediments of soil, sand and rock fall down the landscape, making the mountain decrease in size but falling into te surrounding lake increasing its depth and size (The Editors of Encyclopaedia Britannica, 2014). On the sides of the lateral moraine, the retreating glacier causes friction which produces heat and results in faster melting of the ice. The melted ice is then flown into the alluvial planes, deepening the lake. Debris of rock falls from both sides of the glacier and scrapes alongside the lateral moraine causing the glacier to retreat.
The lake surrounding the Tasman Glacier is divided into layers in relation to temperature. The upper layer of the lake has a temperature ranging from two degrees to five degrees Celsius whilst the bottom layer has a much lower temperature, sitting at around 0.1 degrees Celsius (Warren & Kirkbridge, 2010) . The temperature difference is due to uneven melting of the glacier and the sunrays partially getting passed through the top layer, causing it to be warmer than the bottom division of lake (GNS Science, 2009). Due to the temperature difference, the absorption of heat which causes the ice in the moraine to melt faster, thus increasing the size of the surrounding lake. Stress fractures are caused in the ice of the glacier due to an increase in temperature which contributes to the retreat of the landform (Davies, 2015). The sediments that have rolled down into the moraines absorb heat from the sun and result in further melting of the landform. Lakes surrounding the Tasman Glacier have combined over time, thus contributing to an increase in the size of the lake (Davies, 2015) which is made obvious whilst studying the traced geomorphic map and comparing it with the ASTER images.
The glacial ice sheets have been reducing in thickness 1976 which is present comparing the aerial images with the geomorphic map. Blocks of ice that are separated from the main glacier due to glacial retreat or a flood form kettle holes (Snorrason, 2002). This down wasting progressed over time which lead to the formation of small kettle holes across the Tasman Glacier.
A three-dimensional image of the Tasman Glacier was created with the aid of a stereoscope and a geomorphic sketch of the current glacier was conducted. This was used to identify natural landforms that make up the glacier including the lakes, lateral and terminal moraines and alluvial fans. When comparing the geomorphic sketch to the ASTER and aerial images, the changes that have occurred of the Tasman Glacier due to natural events became obvious and it shows that a number of interacting glacial processes contributed to the glaciers rapid retreat.