A glacier is a moving river of ice on land. There are two types of glacier: Continental
glaciers are massive amounts of ice that cover continents such as Antarctica or Greenland.
95 percent of ice on earth is locked up in continental glaciers.
Alpine glaciers are formed in mountainous regions. Most of New Zealand's largest
glaciers are in the Southern Alps which have the highest mountains and the heaviest precipitation.
Glaciers originate from heavy snowfalls over a long period of time.
When snow initially falls it has many air spaces between the snowflakes.
Over time the weight of new snow above it squeezes the air out of the pore spaces and compacts the
snow just like me making this snowball. Eventually this compaction causes the snow to become ice.
Because the air is being squeezed out the ice will turn a bluish colour. Glaciers
have several components: The area where the snow falls is usually the part of the glacier
with the highest elevation and is called the zone of accumulation.
Further down the glacier is the zone of ablation where most of the melting and evaporation occurs.
Here the ice shows a lot of crevasses. The end of the glacier is called the terminus or snout.
At many glaciers there are glacial lakes at the terminus,
A glacier will continue to grow as long as the rate of ice formation
in the zone of accumulation is greater than the rate of ice melting in the zone of ablation.
When more melting takes place than ice forming the glacier is said to be in retreat.
The Tasman glacier for example has been in retreat
causing the Tasman lake to grow increasingly larger. In 1973 this lake did not exist.
Over a single year a glacier may both grow and retreat depending on the rate
of accumulation compared to the rate of ablation. This is called the glacial budget.
In times when global climate has been cooler glaciers have expanded.
We call these periods glaciations. Warmer periods have seen glaciers in retreat and we
call these interglacials. Now this time in history glaciers are in retreat. Where I'm standing here
at the Franz Josef glacier was actually once hundreds of meters under ice. You can see behind
me this ah pool which is known as Peter's pool. It is what we call a kettle pool and what it is,
it is formed when a glacier that is in retreat leaves behind a huge like iceberg
and ah eventually that iceberg melts and forms this pool. The tremendous weight of the ice
combines with gravity to cause the glacier to move, At locations where a glacier flows rapidly,
cracks called crevasses form. Crevasses are very dangerous to climbers and should be avoided.
The terminus of the glacier is also very dangerous.
As glaciers move glacial erosion processes take place causing the glacier to transport huge
amounts of material with them. The main erosive process of a glacier is called plucking. As the
glacier moves along the valley the ice melts slightly around the boulders before re-freezing
them. As the glacier moves in the boulders arem then ripped out of the ground. Any rocks and
boulders embedded in the bottom and sides of the glacier became agents of the process of abrasion.
These rocks grind against the bedrock and the rock faces off the glacier as it moves. Just
like sandpaper the grinding of rocks against the bedrock causes the wearing away of the landscape.
These abrasion processes leave behind smooth polished surfaces. Sometimes these surfaces
have scratches in them called striations that have been carved out by debris embedded in the
base of the glacier. Another glacial erosion process is that of freezing and thawing.
Water enters cracks in rocks during the day. Overnight temperatures plummet and the water
freezes. When water freezes it expands placing pressure on the surrounding rock.
Over time this pressure in the rock causes it to crack and split. This process makes the rocks
more vulnerable to being plucked by the glacier. Glaciers transport enormous quantities of debris.
Most of the material is carried near the base of the glacier from where it was plucked. But
glaciers also carry frost shattered debris which has fallen from the valley walls onto the ice.
It is easy to observe the debris being carried by glaciers.
Over time glaciers can transport debris long distances.
Sometimes glaciers split at the end into smaller sections. This is called calving. These have
potential to become icebergs if the glaciers snout ends in the sea or in a glacial lake.
An hour and a half walk to the Hooker lake in the Mount Cook national park will reward the hiker
with views of the terminus of the Hooker glacier.
Calving is taking place here and numerous small icebergs float around the lake.
As glaciers reach lower altitudes temperatures rise and ice begins to melt.
As this happens the ice cannot carry as much material
so this is deposited. The main deposition features of glaciers are called moraines.
These long dark bands of debris are visible on top and along the edges of glaciers. The material
on the sides of the glacier are called lateral moraines. On the walls of many glacial valleys
are hanging glaciers which descend only partly to the main glacier before stopping at a cliff. These
glaciers at Mount Sefton lie above the larger Mueller glacier. Avalanches and ice falls are
common here and transfers snow and other debris to the Mueller glacier. Where two glaciers emerge,
their lateral moraines meet in the middle of the larger combined glacier to form a medial moraine.
The main depositional feature of a glacier is the terminal moraine.
This is the material deposited at the terminus of the glacier at the furthest extent of its growth.
The terminal moraine may act as a dam for a ribbon lake. At a Tasman glacier the terminal moraine has
acted as a dam creating the Tasman lake. Glaciers deposit a mixture of unsorted clay sandstone and
boulders. The stones are angular because they have not yet been rounded by fluvial processes.
This material is known as till. Glaciers deposit till on valley floors to form ground moraines.
Ground moraines often have a hummocky appearance and can go on to form rolling hills and plains.
Climbers have to take great care when near moraine's as there is the constant danger of rock
slides. Another deposition feature is an erratic. An erratic is a boulder transported and deposited
by a glacier having a lithology or rock type that is different from the
bedrock upon which it is sitting. For example, these erratics in the Pukaki scenic reserve.
Glaciers leave other deposition features also.
For example there are oval mounds of till called drumlins. They are commonly found in clusters.
For example, this cluster of drumlins in the Otago region.
Another feature is rock flour. This is a very fine sediment. Many glacial lakes such as Lake Pukaki
near Mount Cook have a milky turquoise colour due to glacial rivers washing the sediment into them.
Glacial erosion carves out a variety of landforms.
Even in regions where there are no glaciers many landforms are evidence
that they once existed there. The South Island of New Zealand has hundreds of fjords, ribbon lakes
and u-shaped valleys that exist as reminders of the giant glaciers that once were there.
A corrie, also known as a circque is an erosion feature formed at the starting point of a glacier.
Corries have a rounded hollow with a steep back wall. Corries form when snow accumulates
and hollows on mountainsides, slowly accumulating enough to turn into ice.
This ice slowly gouges out a steep back wall through the processes of freeze thaw and plucking.
Large crevasses on the top of the ice called a bergschrund allows water to flow onto the
ice where it freezes to create more ice. The bottom of the corrie is eroded by abrasion
as the ice moves in a circular motion known as rotational slip.
At the front of the ice the erosion is less causing a lip of rock to be left there.
As more ice accumulates it flows over the lip and into the valley below forming a glacier.
In places where glaciers have retreated the corrie was often filled by melt water to form a lake.
The rock lip and moraine act as a natural dam.
These lakes are known as tarns. When two corries form back to back they both erode backwards until
they create a narrow knife edge ridge between them. These sharp ridges are called an aretes.
Sometimes three corries back into each other. They eventually erode backwards to leave a sharp
pyramid peak. An example of this is Mount Aspiring in the Southern Alps of New Zealand.
Glaciers carve out u-shaped valleys just as a river produces a v-shaped valley.
Usually a glacier will follow the general direction of a river valley but instead of
flowing around areas of hard rock like a river does a glacier will cut straight through it.
This means that when the glacier melts it leaves behind a u-shaped valley
with very steep sides. There are numerous u-shaped valleys in the South Island of New Zealand.
The warmer North Island only has small glaciers on the summer of Mount Ruapehu.
For example the Whakapapa glacier seen here. However at Mount Tongariro you can see the
evidence that larger glaciers once existed here in the form of the u-shaped Mangetetopo valley.
Some glaciers such as the Fox and Franz Josef glacier descend to low altitudes
not far from the Tasman sea. When global temperatures and accompanying sea levels rise
low-lying u-shaped valleys get filled with water to become a fjord. This produces a coastline
with an irregular shape like those also found in Norway, Western Canada and Fiordland New Zealand.
Another feature associated with valley glaciers are hanging valleys. The floors of the tributary
valleys are eroded at a slower rate than the floor of the main valley.
So the difference between the depths of two valleys steadily increases over time.
The tributaries are left hanging high above the main valley with their rivers and streams entering
the main valley by means of either a large waterfall or a series of smaller waterfalls.
The valley sides may also have truncated spurs which are the ends of sloping ridges that have
been cut off or truncated by the valley glacier which flows straighter than a river would. You
can see these features in this diagram. Finally retreating glaciers leave behind ribbon lakes on
the valley floor with the terminal moraine acting to form a dam to hold the water in.
These are also known as finger lakes because they are long and narrow.
The South Island has numerous lakes that fill deep valleys excavated by glaciers.
For example the 42 kilometer Lake Wanaka which lies in an ancient u-shaped valley.
The ancient terminal moraine encircles the lower end of the lake.
A smaller example of a ribbon lake is Lake Alexdrina in the Mackenzie basin.
This lake has been enclosed behind moraines left behind by an ancient glacial advance.
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