Tasman Glacier and glacial lake 2007 and 2008
Tasman Glacier is by far the largest glacier in New Zealand, covering three times the area of the next largest glacier, the
Franz Josef. Its area in 1988 was 101 km2, with an estimated mean depth of 160 m. Until the late 1980s the low-gradient,
debris-mantled tongue of the glacier changed little in position, abutting the Little Ice Age moraine, although it experienced
continuous down-wasting. Then, a number of supraglacial ponds near the terminus began to grow, and since that time a large proglacial
lake has formed. The lake area is growing at an exponential rate as a result of accelerating iceberg calving. Unlike the illustrated West Coast glaciers, most of the tongue of Tasman Glacier is debris covered, reflecting not only abundant rockfalls, but also a much less
dynamic behaviour. Since the Little Ice Age this glacier has not experienced any known period of advance.
The calving front of Tasman Glacier in April 2007, viewed from its right-lateral moraine, and
illustrating how a large section of the tongue has broken away into the lake. The contrast between the debris-mantled tongue and the
clean overturned icebergs is striking.
Telephoto of decaying icebergs near the front of Tasman Glacier in April 2007. Their ‘ribbed’ nature reflects
differential melting of clean and debris-rich ice layers. Note the two small yellow MAC-boats (arrowed) which take tourists to view the
icebergs at close quarters.
Icebergs congregating near the outflow at the south end of the lake in April 2007, looking towards the
increasingly distant terminus of Tasman Glacier from the Little Ice Age terminal moraine.
Looking up the full length of Tasman Glacier Lake in April 2008 from the terminal moraine. The debris-covered
calving front can just be seen in the distance.
Icebergs near the terminal moraine of Tasman Glacier in April 2008. Although by this time the glacier had
receded about 6 km, areas of debris-covered dead ice remain, adhering to the lower flanks of the right-lateral moraine, from which this
photograph was taken.
A recently overturned turquoise and scalloped iceberg contrasts with greyish, more weathered icebergs
beyond. The foreground iceberg is about 3 m long (April 2007).
Cluster of mainly debris-covered icebergs near the outlet from Tasman Glacier Lake in April 2008. Katabatic
winds down the glacier commonly drive the icebergs in this direction during fine weather, away from the calving front.
Telephoto of relatively clean icebergs in March 2007. Note the yellow dinghy in the middle of the picture,
and the three people standing on the flat iceberg to the left. Selection of icebergs for alighting onto requires care, as they are prone to
Telephoto of contrasting clean (overturned) and debris-covered icebergs wallowing in the sediment-laden
lake waters, viewed from the crest of the right-lateral moraine in April 2007.
Disintegrating clean and debris-draped icebergs near the Tasman Glacier lake-outlet in April 2008. Many
of these are grounded on the lake bottom. The suspended sediment in the lake waters is largely derived from subglacial meltwater streams
entering the lake at the terminus.
Boat trips are offered on Tasman Glacier Lake using New
Zealand-built polyethylene double-skinned pontoon boats. These versatile vessels (MAC-boats) are highly manoeuvrable and offer easy access
onto suitable icebergs (March 2007).
Group standing on a debris-striped iceberg in the middle of the lake, March 2007. The debris layers are made
up of angular gravel which originated as rockfall, then transported englacially in Tasman Glacier, before being released in an iceberg.
Weathering of ice, especially under strong sunlight, reveals the interlocking nature of the ice crystals.
This ice is bubbly from the air that was trapped in the original snowpack, before being transformed into firn and then glacier ice. The coin on
the angular boulder gives the scale.
The ice margin in March 2007, illustrating the 1-2 m-thick supraglacial debris layer, the relatively clean ice
below, and the debris-fan below that has accumulated as the ice cliff melts back. The cylindrical holes on the right are englacial tunnels that
once carried meltwater. Approaching such cliffs is dangerous as big boulders can topple off into the water.
This debris-striped ice tower, which dwarfs the MAC-boat to the left, is an up-turned iceberg. Note the thermally
carved notch at the base of the tower which has formed as a result of heating of the surface water in the lake (March 2007).
The prominent debris-striped iceberg in this photograph, with the MAC-boat at its base, resembles a ship’s sail from
this angle. Clustered around it are other icebergs, some of which still retain a mantle of debris (March 2007).
A cylindrical hole, a few metres in diameter, in this iceberg represents the remnant of an
englacial tunnel, through which meltwater ultimately reached the bed of the glacier (March 2007).
As the Tasman Glacier has receded in recent years, a long narrow arm of the lake has penetrated
the tongue along the left-lateral margin. The waters here contain a lower density of suspended sediment and take on a bluish hue,
whilst ‘bergy bits’ drift away from the ice cliff. On the left is the lateral moraine with a boulder several metres-long, whilst
beyond the boat is the receding debris-covered tongue (March 2007).
When the Tasman Glacier filled the lake basin, the Murchison River (following a major valley to the east)
was trapped between the left-lateral moraine of Tasman Glacier and the steep mountainside. In recent years, with the formation of the new
lake, the Murchison River has breached the moraine, and now flows into the lake, revealing clearly the bouldery internal structure of the
moraine (March 2007).
The source of the Tasman River is now the outflow through the terminal moraine from the lake. Icebergs are
stranded around the outflow in this April 2008 photograph.
The terminal moraine of Tasman Glacier with an abandoned vegetated braid-plain to the right.
The inner face of the moraine to the left falls away into the lake and the grey scar in the background is the present river channel,
now incised several metres below the old braid-plain. Dr John Grattan of Aberystwyth University for scale (April 2007).
The crest of the terminal moraine shows contrasting types of sediment, ranging from the rockfall-derived
angular debris on the crest where Eva Sahlin of Aberystwyth University is standing, to rounded well-sorted gravel in the foreground which represents
an older meltwater channel (April 2007).
A striated boulder in the terminal moraine of Tasman Glacier indicates that at least some of the material was
transported at the bed of the glacier, although the bulk of the coarse debris is rockfall derived and carried supraglacially (April 2008).
Intense discussion by Aberystwyth University students about the origin of the debris that makes up the terminal
moraine of Tasman Glacier (April 2008).
|Photos Michael Hambrey|