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.

Tasman Glacier and glacial lake 2007 and 2008
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.
Tasman Glacier and glacial lake 2007 and 2008
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.
Tasman Glacier and glacial lake 2007 and 2008
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.
Tasman Glacier and glacial lake 2007 and 2008
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.
Tasman Glacier and glacial lake 2007 and 2008
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.
Tasman Glacier and glacial lake 2007 and 2008
A recently overturned turquoise and scalloped iceberg contrasts with greyish, more weathered icebergs beyond. The foreground iceberg is about 3 m long (April 2007).
Tasman Glacier and glacial lake 2007 and 2008
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.
Tasman Glacier and glacial lake 2007 and 2008
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 overturning.
Tasman Glacier and glacial lake 2007 and 2008
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.
Tasman Glacier and glacial lake 2007 and 2008
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.
Tasman Glacier and glacial lake 2007 and 2008
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).
Tasman Glacier and glacial lake 2007 and 2008
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.
Tasman Glacier and glacial lake 2007 and 2008
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.
Tasman Glacier and glacial lake 2007 and 2008
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.
Tasman Glacier and glacial lake 2007 and 2008
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).
Tasman Glacier and glacial lake 2007 and 2008
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).
Tasman Glacier and glacial lake 2007 and 2008
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).
Tasman Glacier and glacial lake 2007 and 2008
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).
Tasman Glacier and glacial lake 2007 and 2008
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).
Tasman Glacier and glacial lake 2007 and 2008
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.
Tasman Glacier and glacial lake 2007 and 2008
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).
Tasman Glacier and glacial lake 2007 and 2008
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).
Tasman Glacier and glacial lake 2007 and 2008
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).
Tasman Glacier and glacial lake 2007 and 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