A glacier is a huge river of ice, moving slowly down sloping terrain. Glaciers form above the permanent snowline, where winter snowfall exceeds the amount of snow that melts in summer. Snow builds up over many years, compacting to form glacier ice. Gravity sends a glacier creeping downwards, slowly deforming internally and sliding on its base. When it reaches lower, warmer altitudes, the ice melts as fast as it moves. Most of the melting takes place in summer.
New Zealand’s glaciers are fed by snow carried on the moisture-laden winds in the stormy latitudes known as the roaring forties. East of the main divide in the Southern Alps, up to 20 metres of fresh snow may be added each year to the upper Tasman and Hooker glaciers. Even greater volumes fall on the western side of the Southern Alps.
In 1966 it was estimated that there were about 370 glaciers in New Zealand. An inventory taken in the 1980s by Trevor Chinn and assistants listed over 3,100 glaciers larger than a hectare. The number of glaciers had not greatly increased – it was simply that they had never been systematically counted before.
In the North Island, only volcanic Mt Ruapehu (2,797 metres) has glaciers. There are 18, including one within the crater. The three largest are the Whangaehu, Mangatoetoenui and Summit Plateau glaciers. An active volcano, Ruapehu has occasionally spurted ash and rock debris over them, and its volcanic heat tends to accelerate melting.
The South Island has glaciers from Fiordland to Nelson. The headwaters of the Rakaia and Rangitātā rivers have hundreds, including Lyell and Ramsey glaciers, and the Bracken, Garden of Eden and Garden of Allah snowfields. Only a few dozen are found north of Arthur’s Pass, mostly in the Spenser Mountains and St Arnaud Range.
Most of New Zealand’s largest glaciers are in the Southern Alps, which have the highest mountains and heaviest precipitation (rain, snow, sleet or hail). Glaciers in the Aoraki/Mt Cook area include the 29-kilometre-long Tasman, the Murchison (18 kilometres), the Mueller, Hooker and Godley east of the main divide, and the Fox (13 kilometres) and Franz Josef (12 kilometres) west of the divide.
South of Aoraki/Mt Cook, major glaciers including the Bonar and Volta are centred around Mt Aspiring. Many, including the Olivine Ice Plateau, lie in the upper catchments of the Arawata, Matukituki, Dart and Hollyford rivers. They are scattered throughout Fiordland, but concentrated around Mt Tūtoko in the Darran Mountains. New Zealand’s southernmost glacier ice is on Caroline Peak above Lake Hauroko.
In 1943, a plane crashed on Franz Josef Glacier. Six years later, wreckage appeared at the glacier’s lower end, 3.6 kilometres from the crash site. In 1952, a plane was wrecked on the Mangatoetoenui Glacier on Mt Ruapehu. It emerged 31 years later, just 600 metres further down. It was clear the two glaciers were moving at different rates. Franz Josef moved very fast – about 600 metres per year. The Mangatoetoenui was much slower, moving about 20 metres per year.
Glaciers act like conveyor belts, carrying rocky debris. As the ice melts, this debris emerges at the surface. On the lower reaches of some glaciers, such as the Tasman, the ice is barely visible beneath the rocks. Some of the rocks are dumped as ridges of moraine along the sides and terminus (front end) of the glacier, outlining the limits of the ice.
Some arid eastern South Island mountains, such as the Ben Ōhau, Two Thumb and the Inland Kaikōura ranges, have developed ‘rock glaciers’. These creeping masses of ice-cemented rubble occur where winter snow mixes with rockfalls from surrounding cliffs. Deep inside they contain a core of glacier ice.
As glaciers lumber downhill over rough terrain, the brittle surface ice often forms deep cracks known as crevasses, up to 40–60 metres deep. These are a deadly hazard for alpine travellers, especially when hidden by snow. On very steep slopes, glaciers may break up into icefalls, such as Aoraki/Mt Cook’s Hochstetter Icefall, with cascades of towering, jumbled blocks of ice.
The Tasman Glacier, in the shadow of Aoraki/Mt Cook, is by far New Zealand’s largest and longest glacier – up to 600 metres thick and 29 kilometres long. Some 22,000 to 16,000 years ago, it was even more imposing. Joined by ice from the Murchison, Hooker and Mueller glaciers, the Tasman stretched for 115 kilometres. It gouged out the trough filled by Lake Pūkaki, which lies behind hills of moraine (rocks, soil and clay carried by glaciers).
The present outline of the Tasman Glacier has changed little over the last 2,000 years. Insulated by a thick cover of rock debris over its slow-moving lower reaches, it has responded to shifts in climate with changes in thickness rather than in the position of its terminus (front edge). A century ago travellers clambered 10 metres up from moraines alongside to reach the Tasman’s surface. The ice level has now fallen 130 metres below the moraines.
The ice has thinned so much that in the mid-1980s, ponds at the Tasman’s terminus joined to form a lake. The lakes are increasing in size as icebergs calve (break off) into the lake.
Two spectacular Westland glaciers, Franz Josef and Fox, are New Zealand icons, and a seeming paradox – ice that descends into rainforest. Of the many glaciers in the Southern Alps, why do only these two reach so close to sea level?
Franz Josef Glacier has a high-altitude snowfield of 20 square kilometres, and Fox Glacier 32 square kilometres. These funnel vast amounts of ice into narrow valleys. The effect is like pressing on a tube of toothpaste. The ice is pushed right down the steep valleys to the coast, at speeds of several metres per day. The fast-moving ice does not melt until it is near sea level, where there is warmer air and frequent heavy rain.
The Māori name for the Fox Glacier is Te Moeka o Tuawe. The ancestor Tuawe fell to his death while exploring the area, and the bed of the glacier became his final resting place (moeka). His lover Hine Hukatere wept, and her everlasting tears formed the Franz Josef Glacier – referred to in the name, Kā Roimata o Hine Hukatere.
Both the Franz Josef and the Fox glaciers are very sensitive to variations in climate. Even small changes in snowfall can result in substantial changes in the position of the terminus. There is a 5–6-year delay between a change in snowfall in their upper basins, and an advance or retreat of their terminus.
When first visited by geologist and explorer Julius Haast in 1864, the front edge of the Franz Josef stood near Sentinel Rock, several kilometres further downvalley than today. Franz Josef began a rapid retreat in the mid-1930s. A lake formed in front of the glacier between 1939 and 1949, but rapidly filled with rock debris. Since then, the Franz Josef and the Fox have been retreating, with occasional brief advances. The Franz Josef was at its smallest extent in 1982, but since then its front has once again advanced about a kilometre.
Part of the terminus of the Fox Glacier is hidden. Large masses of stagnant ice have been buried for over half a century under the gravel riverbed in front of the glacier. As the underlying ice melts, the gravel collapses, forming ponds.
In recent years both the Franz Josef and Fox glaciers have been the scene of ‘jökulhlaups’ or outbreak floods. Water from rainstorms and melting ice sometimes gets trapped in tunnels inside the glaciers. When the ice around the tunnels eventually gives way, the water, along with chunks of ice and rock debris, floods the valley downstream. One such flood in 1989 destroyed a bridge on the access route to the Franz Josef Glacier.
During the last 2.5 million years, earth has been plunged into a series of ice ages lasting many tens of thousands of years. In an ice age, global average temperatures drop markedly, major ice sheets build up in the northern hemisphere, and glaciers expand. At times, ice covered more than a quarter of the earth’s land area. As water became locked up in the massive ice sheets, the level of the oceans fell, often by more than 100 metres.
Between ice ages the climate resembled that of today, and at times was even warmer. For example, in New Zealand kauri forests and red subtropical soils extended as far south as Wellington. However, these balmy interludes rarely lasted more than 10,000 years.
In New Zealand, the surrounding oceans moderated the harsh glacial climate, so average temperatures were only around 4° to 5°C lower than today. Auckland’s glacial climate probably resembled that of present-day Dunedin, and Wellington’s that of Norway.
In the mountains, the permanent snowline was over 800 metres lower. Huge glaciers covered the peaks and filled the large river valleys from Fiordland to west Nelson. Smaller glaciers even formed on Stewart Island and the North Island’s Tararua Range, as well as on the central volcanoes.
The earliest traces of New Zealand glaciation are the layers of sediment at Ross, in Westland. They may have been laid down more than 2.5 million years ago, in what is known as the Ross Glaciation. Deep-ocean deposits drilled offshore show that the climate has alternated between warm and cold as many as 20 times since then, with nine South Island glaciations in the last 700,000 years.
Only patchy traces of the early glaciers remain. They were eroded on the rapidly rising mountains, or were overrun and destroyed by later glaciers.
During glacial periods the sea level dropped and wide areas of sea floor were exposed around the coast. New Zealand became a single large island. The North and South islands were joined across Cook Strait, and Stewart Island was joined to Southland. Large inlets such as Hawke’s Bay, the Hauraki Gulf and Firth of Thames were dry land.
Most is known about the most recent glacial period – the Ōtira Glaciation, between 75,000 and 14,000 years ago. By this time, the rising Southern Alps had developed their present pattern of ranges and valleys, down which ice moved. The landscapes of the Southern Alps were mostly sculpted by ice as it advanced and retreated during this period.
The last advance of Ōtiran glaciers reached its greatest extent around 18,000 years ago. The area they covered is evident from moraine ridges – the heaps of rock debris that built up along the glacier’s edge. On the South Island’s West Coast, ridges hundreds of metres high reach the coast, and sonar mapping shows they extend many kilometres from shore under the Tasman Sea.
In the eastern South Island, moraine remnants show that ice once reached the top of the Canterbury Plains.
Fiordland appears to have developed an extensive ice cap, as the glaciers have rounded off the mountain summits, as well as scouring deep valleys westward to well below today’s sea level. Large amounts of ice from the Fiordland region also filled Southland’s Te Anau and Manapōuri basins, now filled by spectacular lakes.
From about 14,000 years ago, as the climate warmed, ice age glaciers shrank rapidly. But 12,000–11,000 years ago they advanced again, building prominent rocky moraines such the Waiho Loop. This semi-circular ridge marks how far the Franz Josef Glacier advanced into the flats of the Waiho River.
Since then, the glaciers of the Southern Alps have continued an irregular retreat. They reached their minimum size about 6,000 years ago, when the climate was warmer than now. There have been minor advances during cooler periods such as the ‘Little Ice Age’ from the 15th to the mid-19th century.
New Zealand’s mountain scenery bears the unmistakable imprint of the ice age. Armoured with rock debris, moving ice scours the rock it moves over. Valleys develop U-shapes, with steep walls and rounded bottoms, while the ridges between them are often sharpened to knife edges. Bowl-shaped areas called cirques are scooped out at the top of glaciers. Where many cirques form on a mountain it may be reduced to a ‘horn’ peak. Mt Aspiring has developed its distinctive ‘Matterhorn’ profile in this way.
Features created by ice age glaciers are best preserved in areas with hard bedrock, such as Fiordland. Here are vertigo-inducing valleys, such as those crossed by the Milford Track, and a maze of majestic fiords – glacial valleys now filled by the sea.
By contrast, the greywacke bedrock slopes of the Aoraki/Mt Cook region have crumbled rapidly as the glaciers retreated. The steep valley sides have become covered with sloping screes and the valley bottoms buried under river gravel.
The South Island has hundreds of lakes formed by glacier activity, ranging from tiny mountain tarns to 60-kilometre-long Te Anau, which is over 400 metres deep.
Most small lakes at high altitude form in glacier-dug rock basins, known as cirques. One example in Fiordland is Lake Quill, which overflows to form Sutherland Falls, arguably New Zealand’s highest waterfall.
The great scenic lakes of the South Island are long and narrow, as they fill valleys carved out by major ice age glaciers. They include:
The glaciers have carved out these lakes to a great depth (the floors of Manapōuri, Te Anau and Wakatipu lie below sea level). Most are partly dammed by glacier rock debris.
Ice age glaciers carried gold from the Southern Alps into the gravel beds of rivers and streams. Fine gold was also flushed downriver and spread along West Coast beaches. The glittering bonanza sparked the 1860s Otago and West Coast gold rushes. Glaciers moving down valleys such as the Taramakau and Arahura carried another treasure – pounamu or greenstone – to valleys more accessible to Māori.
Glacier meltwater rivers carry ‘rock flour’ – fine, ground-up sediment that gives the water a milky appearance. Lakes such as Tekapo and Pūkaki are still fed by rivers draining from glaciers, and the rock flour turns them a striking milky turquoise.
During the ice ages, wind picked up fine silt from the open flats of glacier-fed rivers. It was spread across the countryside, covering it with thick blankets of sediment known as loess. Banks Peninsula, for example, became coated with metres of loess blown from the river beds of the Canterbury Plains during the glacial periods.
It is not known how far Māori ventured onto the upper reaches of glaciers, but they had several words for the snow and ice of high alpine areas, usually including the term huka (frost, snow, cold):
Abel Tasman and James Cook both sailed along the South Island’s West Coast, but the first description of a New Zealand glacier – probably Franz Josef – is from the log of the ship Mary Louisa in 1859. Over the next few decades, men such as Thomas Brunner, Julius Haast, Charlie Douglas and A. P. Harper explored and mapped the South Island’s glaciers.
Glaciers, and the spectacular scenery shaped by glacial erosion, form the heart of most South Island national parks. The national parks of Aoraki/Mt Cook, Westland Tai Poutini, Mt Aspiring and Fiordland are recognised internationally. Together they constitute the 2.6 million-hectare Te Wāhipounamu South West New Zealand World Heritage Area.
The glaciers of Aoraki/Mt Cook, and the Franz Josef and Fox glaciers in Westland Tai Poutini National Park, have been major visitor attractions from the earliest days of tourism.
More than 40% of Aoraki/Mt Cook National Park is buried beneath glacier ice. Aoraki/Mt Cook’s first tourist hotel, the Hermitage, was built in 1884 next to the Mueller Glacier. It was destroyed in 1913 by floods of debris-laden water that broke through the Mueller’s moraine wall. The two successors of the hotel were sited on safer ground. The Ball Glacier, which joins the Tasman, was popular for skiing, and several national championships were held there in the 1930s.
Today, visitors enjoy guided trips and mountaineering on the glacier-clad peaks. Also on offer are scenic flights and ski-plane landings on the upper Tasman Glacier.
The Franz Josef and Fox glaciers are major attractions in the park. At Franz Josef, tourists flock to see the startling cascades of white ice, framed by lush forest. The glacier is visited by about 250,000 people each year.
As at Aoraki/Mt Cook, guided glacier walks and climbs, scenic flights and heli-skiing are popular at the Franz Josef and Fox glaciers.
Geologist Julius Haast, exploring the Aoraki/Mt Cook region in 1862, was awed by the grandeur of its peaks and glaciers:
‘As far as the eye could reach everywhere snow and ice and rock appeared around us, and in such gigantic proportions that I sometimes thought I was dreaming, and instead of being in New Zealand, I found myself in the Arctic or Antarctic mountain regions.’ 1
Glaciers are a valuable resource, holding vast volumes of fresh water locked up as ice. In the present warming climate, New Zealand’s eastern glaciers are releasing more water to rivers than they are gaining each year from snowfall.
Most glacier melting takes place in summer. This boosts the seasonal flow of a number of rivers used for electricity generation and irrigation. It is especially important for maintaining river flow and the levels of hydro storage lakes during dry summers. For example, 39% of New Zealand’s glacier ice is concentrated in the glaciers that drain into Lake Pūkaki. The Pūkaki catchment is part of the Waitaki River system, which has eight hydroelectric power stations.
Chinn, Trevor, and Glen Coates. The glaciated landscape of Mount Cook. Information Series 21. Lower Hutt: Institute of Geological & Nuclear Sciences, 1993.
Coates, Glen. The rise and fall of the Southern Alps. Christchurch: Canterbury University Press, 2002.
Coates, Glen, and Trevor Chinn. The Franz Josef and Fox glaciers. Information Series 2. Lower Hutt: Institute of Geological & Nuclear Sciences, 1993.
Dennis, Andy, and Jane Pearson, ed. The story of Mount Cook National Park. 6th ed. Arthur’s Pass: Department of Lands and Survey, 1986.
Potton, Craig. From mountains to sea – the story of Westland National Park. Wellington: Department of Lands and Survey, 1985.