III. Fox Glacier/Te Moeka o Tuawe

The last, very steep section of the glacier tongue below the small plateau undernearth Chancellor Ridge
The last, very steep section of the glacier tongue below the small plateau (Victoria Flat) underneath Chancellor Ridge
Mts Roon (2233m) and Anderegg (2362m) on the nortthern divide of the Fox Glacier  snowfield (neve). Note the huge accumulations of snow on the arrete on the left.
Mts Roon (2233m) and Anderegg (2362m) on the nortthern divide of the Fox Glacier snowfield (neve). Note the huge accumulations of snow on the arete on the left.

Erosion, Icefields and Rainfall

 

So if this is the timescale - say a minimum of half a million years of glacier and ice cap erosion - what about the forces at work upstream of the terminus of the Fox Glacier?

 

It's really easy to be fooled by the size of the glacier. Surrounded by massive mountains the ice river can look slightly tame from a distance and in photos.

 

The Fox Glacier is actually is 13km (8.1 miles) long and is fed by a 32 sq km snowfield (accumulation basin) and its constituent glaciers at an altitude of 2,700m. It has an estimated mean depth of 100m.

Mt Tasman (3497m). The snow field's highest point looks to be 3000m at the Engineer Col at the top of the Heemskerck Galcier (NZ Topo Map)
Mt Tasman (3497m). The snow field's highest point looks to be 3000m at the Engineer Col at the top of the Heemskerck Glacier (NZ Topo Map)

The glaciers that are connected to the Fox/Te Moeka o Tuawe are from the south to north the Castries, the Gem, the Jewel, the mighty Abel Janszoon, the Heemskerck, the Albert, the Cleeves and the Explorer.

 

The Fox differs from many of the West Coast glaciers in that it has broken through the lower mountain ranges to reach the coastal plain - even though it has subsequently retreated.

Crevasses on the Fox Galcier where the accumulation basin drops down into the steep valley occupied by the glacier tongue (SwissEduc)
Crevasses on the Fox Glacier where the accumulation basin drops down into the steep valley occupied by the glacier tongue (SwissEduc).

It and the Franz Josef flow out of the biggest and highest snowfield in the Southern Alps - with Aoraki/Mt Cook and Mt Tasman at its centre.

 

Big glaciers flow east and south from the other side of the watershed - the Tasman and Murchison - but from the map these look broader and flatter than the Fox and Franz Joseph. (The Tasman Glacier is credited with being the biggest in New Zealand.)

The extensive neve at 2,700m  of the Fox Glacier beneath Mt Tasman.
The extensive neve at 2,700m of the Fox Glacier beneath Mt Tasman.

This is not surprising as rainfall maxima occur to the west of the east-west divide (see my Hokitika page) and quickly reduce moving eastwards.

 

Further, the slope from the west side of the divide to sea level is much more acute than the eastward - piedmont - slope.

 

This is due to the position of the Alpine Fault, the zone of maximum uplift sitting adjacent to it and the proximity of the Tasman Sea (again see The Shaky Isles).

Crevasses worn down to arretes by melting and heavy rainfall at tht terminus of the Fox Glacier
Crevasses worn down to arretes by melting and heavy rainfall at tht terminus of the Fox Glacier

The Fox Glacier falls 2,600 m (8,500 ft) on its 13 km journey from the Southern Alps down to the coast. That is an average gradient of 1:4.6 which is pretty steep in a car.

 

From the map excerpt (see top page) it can be seen that this gradient of descent steepens markedly in the tongue of the Fox/Te Moeka o Tuawe.

The river running under the glacier sits below embedded ice left by the Fox Glacier's retreat
The river running under the glacier sits below embedded ice left by the Fox Glacier's retreat

The Fox and Franz Josef are particulary sensitive to changes in snowfall in the icefield, where more than 20m of snow can fall in a year.  These changes in snowfall that take 5 to 6 years to work their way through to the advance or retreat of the glacier's terminus.

 

The Fox moves up to 'several metres a day' (see below for average winter and summer movement). This keeps the ice moving down the valley, with the huge pressure behind it of the accumulated snowfied. Such is the thickness of the ice and the speed of movement that the ambient temperature and rainfall simply cannot melt the ice fast enough even though it is well above freezing point.

The Fox Glacier terminus and the glacier river flowing through the gap in the buried ice pictured above
The Fox Glacier terminus and the glacier river flowing through the gap in the buried ice pictured above

There are large amounts of ice in front of the apparent terminus of the glacier that have lain buried after retreats for half a century. These are slowly melted out by underground streams causing collapses and the formation of ponds.

 

Both the Fox and Franz Josef are also subject to jökulhlaups or outbreak floods which can amass water in blocked underground tunnels and channels only for it to eventually build up the sufficient pressure to burst out in a flood (see Te Ara Glaciers).

 

New Zealand's biggest glacier is the Tasman at 29km long and in places 600m thick - although the depth of ice on the slow moving final part of the glacier has thinned markedly.

The Fox Glacier drops from 3000m to 220m where the lower valley is surrounded by temperate rain forest
The Fox Glacier drops from 3000m to 220m where the lower valley is surrounded by temperate rain forest. View from the Chalet Lookout Point

So we have a picture of truly massive forces at work. The mountains being pushed upwards by tectonic movement and the mountains being ground down by the massive accumulation and movement of ice occassioned by the South Island's phenomenal annual rainfall.

 

And yet the glacial landscape looks so new, so pristine (in an apocalyptic fashion),  the thousands of acres of bare rock looking like they were new-laid as of yesterday.

The path to the Fox Glacier terminus
The path to the Fox Glacier terminus

But every now and then I'd note plants growing in what seemed the most inhospitable terrain. I wondered how that process of colonisation worked as the sun beat down and rockfall signs told us to get a move on.

 

The views from the Chalet Lookout point are spectacular, especially as the lookout is surrounded by rain forest. When we were there part of the path has just been washed out by the innocuous sounding Mill's Creek.

Fox Glacier valley warning sign
Fox Glacier valley warning sign

The wardens of the National Park take the risks of guiding unwary tourists to the glacier terminus with great seriousness.

 

Signs and sectioned-off paths are intended to keep people out of harms way and we saw a group of men with long steel crowbars levering teetering rocks down the valley side.

TThe devastation at the foot of the Fox Glacier: riversin Westland have among the highest sediment loads in the world.
The devastation at the foot of the Fox Glacier: rivers in Westland have among the highest sediment loads in the world. Note people on left in red for scale.

The photo below is taken from the Chalet lookout point above and back from the Fox Glacier.

 

As I was taking it I saw something about the size of an ant moving about through my telephoto lens. It turned out to be a person walking in front of the glacier terminus. He or she is marked with an 'a' on the photograph.

 

Two Australian tourists were buried alive here in 2009 under hundreds of tons of ice that collapsed off the front of the glacier. Needless to say they did not survive.

It is hard to capture the scale of glaciers. The 'a' in this photo is just to the right of a person, shown below at massive magnification. This is near where two tourists were killed by hundreds of to
It is hard to capture the scale of glaciers. The 'a' in this photo is just to the right of a person, shown below at massive magnification. This is near where two tourists were killed by hundreds of tons of falling ice in January 2009.
The person at 'a' in the photo of the terminus of Fox Glacier above.
The person at 'a' in the photo of the terminus of Fox Glacier above.

How much does it weigh?

 

I couldn’t get to sleep recently and began thinking about glaciers. I wondered how much a glacier weighs, how it really carves out solid rock, what happens at the boundary between ice and the rock bed.

 

With regard to weight I did the following calculation. The névé (snow and ice field) of the Fox Glacier is about 32 sq km and it has a depth of up to 100m.

A helicopter dwarfed by the Chancellor Ridge coming in to land on the mid-tongue plateau of the Fox Glacier and
A helicopter dwarfed by the Chancellor Ridge coming in to land on the mid-tongue plateau (Victoria Flat) of the Fox Glacier

There are one million square metres in a square kilometre.  So in 32 sq km you have 32 million sq m. Multiply that by say half the max depth of the neve (i.e. 50m) and you get 1.6 billion cubic metres of ice.

 

Glacial ice (as opposed to snow or the top 'firn' layers of a glacier) has a density of 850kg (0.85 tonnes) to the cu m.

 

So the Fox Glacier could weigh something like 1.6bn x 0.85 which equals 1.36 billion tonnes.

otten ice embedded in the lateral moraine of the Fox Glacier near its terminus
Rotten ice embedded in the lateral moraine of the Fox Glacier near its terminus

Where the glacier is 100m thick the weight exerted on a square metre is something like 85 tonnes.

 

The downward stress can be much more concentrated because contact between the glacier and bedrock is often discontinuous due to ice cavities that form along the bottom of the glacier. This is particularly so when a glacier is moving quickly and basal water pressure is low (see Harbour 2011 p. 333 below).

 

But a lot of the Fox glacier runs at a steep angle. It drops 1,400m from 1700m to 300m above sea level in 5.83 km. That is an angle of just over 25% (1 In 4). This increases the vertical column of snow to 125 metres and the weight per metre to 106 tonnes.

The path to the Fox terminus
The path to the Fox terminus with the Chancellor Ridge behind.

Due to the effects of friction glaciers move fastest in the middle – relative to the valley walls – and at the top – relative to the valley bottom. As the ice flows from the broad névé into the narrow confines of the glacier convergence causes compression in the central section of the glacier. In the Fox Glacier this is also where the ice starts to flow faster as the gradient steepens.

 

The top 50m of a glacier is known as the fracture zone. Here the ice tends to move in one piece and subsequently is quick to crack into crevasses as gradients change and underlying bedrock features cause stress fractures.

 

Below 50m the pressure on the ice allows it to move with a plastic flow like a very viscous liquid.

Eroded ice and ground rock at the terminus of the Fox Glacier: summer surface melting averages 129mm a day reducing to 22mm in winter.
Eroded ice and ground rock at the terminus of the Fox Glacier: summer surface melting averages 129mm a day reducing to 22mm in winter.

The glacier's movement in most temperate glaciers is enhanced by a process known as basal sliding. Here ‘the immense pressure caused by the weight of the overlying glacial mass causes the ice making contact with the ground to melt because of pressure, despite subzero temperatures, through a process called pressure melting'.

 

This is an uneven process – melting occurs at the points of most friction – on the ‘stoss’ or up-glacier side of a protuberance. The melt then runs to the leeside of the bump (down-glacier) and refreezes in joints and fractures (see www.physicalgeography.net).

Chaotic ice forms in the fracture zone of the Fox Chaotic ice forms in the fracture zone of the Fox Glacier 500m from the terminus. In a year the glacier advances past this point at about 275m (900ft)
Chaotic ice forms in the fracture zone of the Fox Glacier 500m from the terminus. The glacier advances past this point at about 275m (900ft) a year. Summer flows are, not surprisingly, faster than winter flows.

The forward motion and movement of the Fox Glacier is regulated by the seasons and weather as well as overall climate and climatic changes.

 

In the summer the glacier moves more per day - averaging 0.87m a day - while in the winter this is reduced to 0.64m a day.

 

So called 'short term velocity peaks' also occur due to heavy rainfall. Heavy rain in the winter can cause the glacier to speed up by 44% twenty four hours after the rainfall.

 

If we take the midpoint between these two rates the hypothetical advance over a year is 275.5m (903ft).  This does not result in a corresponding advance of the nose of the glacier due to 'net radiation and sensible heat contributing energy for surface melt' (Purdie et al. 2008 p.140).

Rock debris, perched boulder and massive chunk of rock ion the right hand side of the Fox Glacier just below Victoria Flat.
Rock debris, perched boulder and massive chunk of rock on the right hand side of the Fox Glacier just below Victoria Flat.

Not surprisingly ablation (surface melting) rates also vary with the seasons. In summer they average 129mm a day and in winter just 22mm.

 

Overall the Fox Glacier terminus position is currently relatively stable after rapid retreat in the first three-quarters of 20th century.

 

Purdie, HL., Brook, MS., and Fuller, IC (2008) Seasonal Variation in Ablation and Surface Velocity on a Temperate Maritime Glacier: Fox Glacier, New Zealand Arctic, Antarctic, and Alpine Research Vol. 40, No. 1

The steepness of the last section of the Fox Glacier tongue
The steepness of the last section of the Fox Glacier tongue seen from the valley floor

Two major erosion processes occur at the base of a glacier. The first is ‘scouring’. Here the glacier, incorporates large amounts of loose rock and sediment by partial melting and refreezing. This creates a potent ‘scouring cloth’ or more accurately a very coarse grade of sandpaper.

 

Scouring creates features like striations and glacial polishing and produces a fine clay-like sediment that when carried by meltwater is known as 'glacial milk'.

 

The second erosion process is ‘plucking’ or ‘quarrying’ where basal ice freezes in rock surface cracks and as the main body of the glacier moves the attached rock is pulled and plucked out.

 

The freezing and thawing action of the ice causes cracks in the bedrock through hydraulic wedging. This loosens and levers blocks up into the flow of the glacier.

 

Often the rock is plucked from the downside of bedrock protuberance into the glacier giving rise to roche moutonées (literally 'sheeped rock') - a smooth protruding rock on the upstream side of the glacier flow and a gap-toothed, plucked hole in the rock on the downstream side, rather like a sheep sheltering on the ground from the wind, its broad rounded back into the wind and and its legs and head away from it.

More huge boulders and roche moutonnée ('sheepback')  left by an earlier passages of the Fox Glacier. Ice thaw and melt allow the glacier to pluck the down-glacier side of the moutonée away.
More huge boulders and roche moutonnée ('sheepback') left by an earlier passages of the Fox Glacier. Ice thaw and melt allow the glacier to pluck the down-glacier side of the moutonée away.

The rocks can then become embedded in the bottom of the glacier through 'regelation' - refreezing.

 

The rocks so refrozen into the glacier act rather like the ploughshare on a plough and can leave so-called ‘chatter marks’ - wedge shaped indentations on bedrock or other rocks as they are dragged across or gouged into them by the glacier.

Scree and moraine deposits at the bottom of Cone Peak in the Fox Glacier valley
Scree and moraine deposits at the bottom of Cone Peak in the Fox Glacier valley

The more fragmented, fractured, jointed and faulted the bedrock the easier it is for water to infiltrate and start the hydraulic wedging process.

 

And the more rock entrained in the glacier the heavier the glacier gets as it chugs downhill. This gives it more downward force on the subsequent bedrock it crosses.

From Harbor, Jonathan (2011). Encyclopedia of Snow, Ice and Glaciers. Springer. pp. 332.
From Harbor, Jonathan (2011). Encyclopedia of Snow, Ice and Glaciers. Springer. pp. 332.
Crevasses and cross-crevasses  on the accumulation basin of the Fox Glacier showing uneven surface topography (SwissEdu - click fo
Crevasses and cross-crevasses on the accumulation basin of the Fox Glacier showing uneven surface topography (SwissEdu - click for link)

 

‘Subglacial fracture of bedrock occurs when the pattern of stress induced by the ice and by any clasts embedded in the basal ice causes a tensile stress component in the rock that exceeds the fracture strength of the rock, usually at the tip of a microcrack.’

 

For the above see Ping, F. and Harbor, J. (2011) 'Glacial Erosion' in . Encyclopedia of Snow, Ice and Glaciers. Springer. pp. 332.

Another view of Cone Point with its many rockfall scars and the barren landscape left by a mountain torrent in the foreground.
Another view of Cone Point with its many rockfall scars and the barren landscape left by a mountain side torrent in the foreground.

Of course, it is not only glaciers that are effecting the rapid erosion of the Southern Alps. The annual 10 metres of rainfall delivered in torrential downpours rapidly fills mountain torrents that descend with alarming speed to the valley floor and nearby Tasman Sea.

 

These carry vast quantities of debris down the mountains. The biblical rains also loosen the shallow vegetation mat that clings to the steep rock faces and tree-falls and rock-falls are common as can be seen in the photograph of the rockfall scars on Cone Point below.

The cloudy waters of the glacier fed stream in the Fox Glacier valley and rock- and tree-fall scars on Cone Rock.
The milky waters of the glacier-fed stream in the Fox Glacier valley and rock- and tree-fall scars on Cone Rock.

The huge debris and sediment load of the torrents and rivers, including those that emerge from underneath the glaciers, cause more problems when they hit the flat plain to the Tasman Sea.

 

At this point they start to drop some of their sediment and rubble, abrading and raising the river bed. This can create a severe flood hazard (see the example of the Waiho River at the Franz Josef Glacier on my page,  To Hokitika).

Scree and lake made by underground ice melting after the Fox Glacier's retreat from its highpoint in the 1890s.
Scree and kettle-hole lake made by underground stagnant ice melting after the Fox Glacier's retreat from its highpoint in the 1890s. This is at the terminal moraine deposited in the 1920s that has been since eroded away (near the main valley car park).

A kettle hole lake is formed when a retreating glacier leaves a huge - I mean like massive - block of ice as an outlier.

 

Gradually debris washed from the glacier and down the valley sides fills the valley floor around and over the block of ice. Over the years and years the block of buried ice - insulated from the heat of the sun - gradually melts leaving a steep-sided hole in the valley floor which fills up with water.

 

The video below gives a good guide to the pronunciation of kettle hole when lying down in the rain in New Zealand - it's 'kittle hole'.

 

DOC ranger, Lorraine Smith, talks about the ephemeral plants and turfs that grow in New Zealand 'kittle holes' - strangely called 'knife, fork and spoon' plants on account of their different shaped leaves specially adapted to deal with changing water levels.

tages of kettle hole lake formation (from the Department of Conservation - click for link)
Stages of kettle hole lake formation (from the Department of Conservation - click for link)
Erosion by ice is joined by erosion by mountain torrents that bring down huge amounts of debris, particularly when fed by snow meltwater in the spring and early summer
Erosion by ice is joined by erosion by mountain torrents that bring down huge amounts of debris, particularly when fed by snow meltwater in the spring and early summer

Climate Change Implications

 

Who would want to talk about the climate change implications for the Fox and Franz Josef glaciers? They are on of the top ten if not top three sites of New Zealand tourism and are vital, in particular, for West Coast tourism and the communities this industry supports.

 

One 2003 detailed study of the Franz Josef glacier came to some shocking conclusions:

 

On a local scale, the retreat of Franz Josef Glacier will have a drastic effect on tourism. Guided walking and recreation on the glacier will become increasingly difficult and parts of the glacier may become impassable.

 

Guided walks without helicopter support will be impossible after about 2015, when the glacier retreats back over two bedrock steps. The entire area of the glacier currently used for guided walks will have disappeared by 2100 (Figure 5.11).

 

As the Franz Josef Glacier is a major drawcard for tourists to the entire West Coast region, a significant effect on the local economy can be expected (Anderson 2003, p. 104).

Fox Glacier web cams

 

You can check out real time conditions on the Fox glacier with these handy web cams - if it looks black that'll be because it's night time in New Zealand.

Fox Glacier from the web cam above on October 9th 2014. Arrow shows figure walking up the Observatory Path for scale.
Fox Glacier from the web cam above on October 9th 2014. Arrow shows figure walking up the Observatory Path for scale.

Postscript 21 November 2015


Tragically a laden helicopter crashed on Fox Glacier on November 20th 2015 with the loss of seven lives. Weather had been bad.


At the same time the video clip below showed up on You Tube showing a serac collapse up near Aoraki/Mt Cook. It's worth watching all the way through.