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 where several peaks are close together, snow gets funnelled in to the high valleys, forming 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 and eventually creates a sort of moving river of ice, called a glacier.
The ice at high altitudes does not melt much, but does when it moves down to warmer levels.. Usually there is a balance between the snow coming in, up high, 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 melting to eventually 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 then the glacier will build up, move forward and appears to lengthen – and is said to be advancing. In each case the ice is still moving downward under its own weight.
The 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 there are signs showing 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 to the end of the ice to see if they could measure if the glacier was retreating or advancing and to try and 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 cm a day This demonstrated the ice is melting faster than it is coming down and therefore 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 melting. In fact there is no precise answer to this question as it will change from month to month, and year to year and also it will 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, to establish 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 50m apart and each in a place where they could see the flag and the other protractor.
Making the Protractors
We 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 degree) of 180cm so that each cm is 1 degree. 1/10 of a degree is simply a mm on this scale and can easily be read off using a ruler. The dials were carefully painted and marked out and an angle arm was attached so that angles can be measured accurately. Drawing a line along the arms also allowed us to draw a scale diagram of the measurements we made.
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 we can work out the position and distance of the flag. If we do the same measurements the next day we will have two positions and distances for the flag corresponding to the movement of the flag from one day to the next. From this we can determine the amount of movement over one 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. This was achieved by looking along this straight edge to the other protractor and turning them until they were in line.
While 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 which was 150m distant – a great result!!
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 put into the maths (trig) to overcome any drawing errors that might have come in while sketching the scale drawing.
We got an estimate of the movement of about 1m for a day's glacier advance. Compared with European glaciers, which only move by about 10cm a day, the New Zealand glaciers are exceptional and really do move greatly. If we were to do the measurements again but without being restricted to just 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 much longer period of time, say a week or a month, so that the overall movement would have been much greater.