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Glaciers and Continental Ice

Subj: Glaciers and Continental Ice

Date. 11/13/007:43:06 AM Pacific Standard lime

From: [email protected] (Gordon Bain (on Aurora)


I've been a bit remiss of late -only sending you the daily sitreps. My last descriptive report was about how this ship broke ice without breaking itself and before that was about our Mawson exploits. Then of course Gordon told you about OzGold and my mishap when we fell into the minus 80 degree freezer and suffered bad hypothermia. We are both fully recovered now and definitely not wandering off without getting Gordon's OK first.

I promised to tell you something about glaciers, continental ice and the continental ice sheet. I also need to write about our Heard Island experiences and our last journey back to Freemantle.

Things are pretty busy around here so I need to keep these remaining episodes short. So here goes -

Let's start with few terms and definitions.

Glacier ice - Ice in, or originating from, a glacier whether on land or floating in the sea such as icebergs, bergy bits or growlers.

Glacier- a mass of snow and ice continuously moving from higher to lower ground or, if afloat, continuously spreading. The principal forms of glacier are -inland ice sheets, ice shelves, ice streams, ice caps, ice piedmonts, cirque glaciers and various types of mountain (valley) glaciers.

Ice wall - an ice cliff forming the seaward margin of a glacier which is not afloat. An ice wall is aground, the rock basement being at or below sea level.

Ice stream -part of an inland ice sheet in which the ice flows rapidly and not necessarily in the same direction as the surrounding ice. The margins are sometimes clearly marked by a change in direction of the surface slope but may be indistinct.

Glacier tongue - the projecting seaward extension of a glacier, usually afloat. In the Antarctic glacier tongues may extend over many tens of kilometres.

Ice shelf - a floating ice sheet of considerable thickness showing 2 to 50 metres or more above sea level, attached to a coast. Usually of great horizontal extent with a level or gently undulating surface. Nourished by annual snow accumulation and often also by the seaward extension of land glaciers. Limited areas may be aground. The seaward edge is termed an ice front.

Ice front -the vertical cliff forming the seaward face of an ice shelf or other floating glacier varying in height from 2 to 50 metres or more above sea level.

When ice of land origin breaks away from a mass of ice from an ice wall, ice front or iceberg, the process is known as calving. The ice which breaks away has a number of terms -

Iceberg - a massive piece of ice of greatly varying shape, protruding more than 5 metres above sea level, which has broken away from a glacier, and which may be afloat or aground. Icebergs may be described as tabular, dome shaped, sloping, pinnacled, weathered or glacier bergs.

A glacier berg is an irregularly shaped iceberg.

A tabular berg is a flat sloped iceberg - most tabular bergs form by calving from an ice shelf and show horizontal banding.

An iceberg tongue is a major accumulation of icebergs projecting from the coast, held in place by grounding and joined together by fast ice, for example the beautiful Iceberg Alley on the approach to Mawson.

A bergy bit is a large piece of floating glacier ice generally showing less than 5 metres above sea level, but more than one metre and normally 100 to 300 square metres in area.

A growler is a smaller piece of ice than a bergy bit or floeberg, often transparent but appearing green or almost black in colour, extending less that a metre above the sea surface and normally occupying an area of around 20 square metres. These are a real danger because of their small size and colour but are generally of very hard ice making them a real hazard to shipping.

Icebergs go through a continuous cycle of weathering both on the parts exposed to air but also on the major part under the water - by the effect of currents and erosion. Icebergs often tip upside down several times in their lives and take on quite spectacular shapes. Icebergs can also come in various colours, from pure white, to deep electric blue, to green to jade to black; they can be banded or contain rocks and gravel.

So, this is a fascinating area of study and is one of the major areas of research conducted by scientists not only from Australia but also from all around the world.

The world's climate is not, and never has been, constant. Ice ages come, sea Ievels rise and fall, and temperatures fluctuate for reasons we still do not fully understand.

The Antarctic continent's ice sheet contains a record of past climates that stretches back over hundreds of thousands of years. Glaciologists drill down through the ice sheet to retrieve and analyze ice cores. These ice cores provide information such as average temperatures, amounts of dust, ash and other forms of atmospheric contamination over various periods. Glaciologists can detect seasonal, annual and long-term variations in these factors. Air bubbles are also trapped in the ice, providing samples of the atmosphere from the time when the snow fell. These air bubbles can be extracted and analyzed for greenhouse gases.

Scientists predict that global warming may be a result of the greenhouse effect. One of the consequences of this could be that the oceans of the world will expand and sea Ievels rise. An increase in snow falling over Antarctica might also be expected. The effect of this is would be to transfer water from the oceans to the ice sheet. This transfer of water reduces the magnitude of the sea level rise. Australian scientists are studying Antarctic ice to see if they can detect changes taking place and, in the long-term, decide whether they are natural variations in climate or caused by human actions.

Glaciologists set up instruments in strategic locations to measure the amount of snow falling in the interior of the continent. They can then compare the amount of interior snow to the ice lost when icebergs calve and melt at the edge. If there is more snow accumulating than is being lost (that is, if there is a positive mass balance) then any change in sea level due to the greenhouse effect will not be as great as first thought. If, on the other hand, there is less snow accumulating than being released, there will be a negative mass balance and sea levels will rise more than anticipated. Australian glaciologists have been working on the East

Antarctica Lambert Glacier and Amery Ice Shelf Basin, which together drain an area of almost one million square kilometres, or about eight percent of the Antarctic ice sheet. Their task is to determine whether there is a positive or negative mass balance, but they are not yet in a position to draw any firm conclusions about changes in ice flow, and what effect this might have on sea level changes. The situation is complex, because it takes thousands of years for snow falling in the interior to reach the sea, and any imbalance is probably due to the ice sheet movement responding to lower snowfall rates in the past. There is still a lot of work for glaciologists to do in the Lambert Glacier and Amery Ice Shelf system.

The area of Antarctica is 13 million square kilometres. In winter, a band of sea ice surrounding the continent reaches its maximum extent, covering some 20 million square kilometres. For millions of years snow and ice have built up on the land so that now all but two percent of the land is buried under a permanent ice sheet. The ice sheet holds 90 percent of the world's ice - 30 million square kilometres - which is 60 to 70 percent of its fresh water. The thickness of the ice sheet averages 2.4 kilometres and has a measured maximum thickness of 4.7 kilometres - over twice the height of Mt. Kosciusko.

If all the ice were to melt,  the level of the world's oceans would rise by 65 to 70 metres and if the weight of the ice were to be removed it is estimated that the underlying rock would rise by 700 to 1000 metres.

Lambert Glacier is the world's largest at around 40 kilometres wide and 400 kilometres long - the tabular bergs spawning off the Amery Shelf at the seaward end of the Lambert Glacier are sometimes up to 20 times the size of Uluru (Ayers Rock).

This is a short treatment of the subject but, for the students among you, there is ample to start you on your own search for more information.

You might, for example, like to consider why icebergs can be 'banded', and also why there are so many colours possible. Another question is - isn't all ice the same whether it has been formed from freezing of the sea, from the freezing of fresh water or that which comes from glaciers or icebergs? The ice is in fact very different in these three broad types -you might like to do some research here.

That must be all for now. I have to help Gordon with some paper work in the morning but should find some time later in the morning or afternoon to write to you about Heard Island visit (and, a sneak preview,  we also had a look at the McDonald Islands, some 20 miles to the west of Heard Island. Check your maps. The McDonald Islands are the furthest western edge of Australia.

Take care




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