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Episode 4 - Ice

Speed and Melt of Glacier
by Jonathan Hare

What is a glacier?
The Franz Josef Glacier: receding or advancing?
Measuring the rate of movement of the glacier
Making the Protractors
Angle survey

Franz Josef glacierChallenge
To find out if the Franz Josef Glacier is receding or advancing and to measure the speed of the glacier's movement.

What is a glacier?
In the high regions of the world where the air temperature is low, clouds are forced up and snow tends to fall instead of rain. In mountain ranges where it is not only high but also has several peaks close together, snow gets funneled into the high valleys and forms a pile of permanent snow-ice. The weight of this frozen ice eventually forces it to creep downward, while fresh snow falling all the time adds from above. Eventually it creates a sort of moving river of ice, called a glacier.

Franz Josef glacierThe ice does not melt much at high altitudes, but does when it moves down to warmer levels.

Usually there is a balance between the snow coming in at the heights and the melting near the bottom, and so although the ice is continuously moving downward the total length of the glacier remains roughly constant. Glaciers are massive, many miles in length, and it often takes years for changes in the balance between snowfall and melt to reveal changes in the size of the glacier.

If the lower parts of the glacier melt faster than the snow falls, the glacier will appear to get shorter and the glacier is said to be receding. If more snow falls than ice melts then the glacier will build up, move forward and appears to lengthen — and it said to be advancing. In each case, though, the ice is still moving downward under its own weight.

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Franz Josef glacierThe Franz Josef Glacier: receding or advancing?
The first task was for Mike B. and Ellen to determine if our glacier is advancing or receding. Walking around the glacier they saw signs that showed where the end of the glacier has been at different times. The marker for a hundred years ago was miles further forward than today's position, showing just how much the glacier's length can change.

Ellen and Mike set up some apparatus near the end of the ice to see if they could measure if the glacier was retreating or advancing, and try to determine by how much. By fixing markers in the rock and measuring the distance to the ice face they were able to show that the glacier was retreating by a few inches per day. This demonstrated the ice is melting faster than it is coming down and therefore the glacier appears to be receding.

Measuring the rate of movement of the glacier
The next step is to work out how fast the ice is moving downward on the glacier. This was done much further up the glacier where there is less melt. In fact, there is no precise answer to this question as the rate changes from month to month and year to year. The glacier will also tend to move faster in the middle of the ice than at the edges. However, we needed to get an estimate in a very short time and we did this using some very simple techniques.

Kathy planted a flag on a prominent point on the glacier ice, establishing a reference point from which to gauge how fast the ice was moving. We set up our apparatus on solid ground and observed the movement of the flag over time.

We decided to use a technique called a plane table survey to estimate the speed of the glacier. We needed to measure angles from two fixed points ('A' and 'B') to the third point (the flag) on the glacier. We set up two protractors at 'A' and 'B' on solid ground along the side of the glacier. These were placed 160 feet apart and each in a place where they could see the flag and the other protractor.

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Making the Protractors
ProtractorWe needed to make two protractors good enough to measure the small angles that we expected for the relatively small movement, and the bigger the better. We decided to make up a protractor having a circumference (180 degrees) of 180 centimeters (70 inches) so that each centimeter is 1 degree. 1/10 of a degree is simply one millimeter on this scale and can easily be read with a ruler. The dials were carefully painted and marked out, and an angle arm was attached to measure angles accurately. Drawing a line along the arms also allowed us to draw a scale diagram of the measurements we made.

Angle survey
The idea of the survey is to measure the angle of the flag as seen from point A and B using the two protractors. From this data we can determine the position and distance of the flag. If we make the same measurements the next day, we will have two positions and distances for the flag that correspond to the movement of the flag from one day to the next. That will let us determine the amount of movement per day and the speed of the moving glacier ice.

The protractors are each half a circle, a total of 180 degrees. The protractors need to be set up correctly before they can be used. This was done by aligning the base of protractor A with the base of protractor B, lining up the protractors' bottom straight edges with each other. We did this by looking along this straight edge to the other protractor and turning them until they were in line.

Kathy on glacierWhile Kathy was fixing the flag in the ice, Kate, Mike L. and Jonathan made the first day measurements on the flag. Jonathan aligned the pointer arm to the flag on both protractors and Mike L. read out the angles. We were able to draw a scale diagram using the protractors. This gave us our first estimate for the distance of the flag from almost 500 feet away — a great result!!

The next day we came back to the equipment and made the same measurements on the new position of the flag. By putting this new data on the scale drawing we were able to get a rough estimate for the amount that the flag had moved. Kathy also used the angle measurements to plug into the equations (trigonometry) and compensate for any drawing errors that might have occurred while sketching the scale drawing.

We got an estimate of the movement of about one yard for a day's glacier advance. Compared with European glaciers, which only move by about 4 inches a day, the New Zealand glaciers are exceptional and really do move fast. If we were to do the measurements again without being restricted to three days, we could do it more accurately by repeating all the measurements to get a good average. It would also be much better to take the measurements over a longer period of time, say a week or a month, so that the overall movement would have been much greater.

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Photo: Franz Josef Glacier
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