FACTORES QUE INCIDEN EN EL EMPRENDIMIENTO DE LOS

S.2.1 The Mechanism of Glacier Flow

The snow which falls in the winter typically accumulates in snow basins between mountain peaks forming what is called the neve. The new snow is light and airy as the individual crystals are widely spaced. As further snow fall s on top, the crystals below become more and more densely packed and the trapped air is forced out. This process continues as more snow falls each winter, and with the increasing pressure from the snow above, and summer meltwater soaking into the snow, the crystals fuse into a solid mass of clear ice. When the pressure of this ice mass in the neve becomes great enough, it will begin to flow out and downhill, forming the glacier tongue or trunk. Sara ( 1 974) states that about 20 metres depth of snow is required to produce clear ice, and 45 metres is required to create pressure enough for the glacier to flow.

The maj or component of movement for many glaciers in temperate regions is considered to be basal sliding; that is the glacier slides over its bedrock. This process is not well understood and is extremely hard to measure (Drewry, 1 986). It is affected by such factors as the temperature at the glacier bed which determi nes whether the ice is frozen to the bedrock, the presence (or absence) of meltwater which forms a lower-friction interface between rock and ice, and the roughness and undulations of the bedrock. The other component of ice movement is creep, that is, the ice deforms downhill under its own weight like a viscous fluid. This ice deformation can be seen on the surface as the centre of the glacier moves at a faster rate than the edges where it is held back by friction. Drewry ( 1 986) discusses the process of creep in detail. In addition, the direction of ice flow is not parallel to the glacier surface as shown in figure 5-2.

Accumulation Zone where snowfall Ablation Zone where melting exceeds snowfall ...

Directiori·... Line

of ice flow ···...

. ... ..,

... -... _-... _-_ ... -_.

Figure 5-2: Glacier Dynamics; from Bishop & Forsyth (1 988).

In the accumulation zone at higher altitudes, snow i being added on top so there is a downward component (relative to the glacier surface) in the ice motion. Further down the glacier trunk in the ablation zone, ice is being (primarily) lost by melting so there is an upward component (Bishop & Forsyth, 1 988). A relatively thick, complete layer of rock debris tends to protect the glacier from ablation (Sara, 1 974).

The lower end of the glacier is known as the terminus which occurs where the volume of ice being pushed down from above is not sufficient to replace that lost by ablation and other processes. Usually there will be a river of meltwater flowing out from under the terminal face, or in the case of other glaciers like those in the Godley Valley, a lake of meltwater as pictured in figure 5-3.

Figure 5-3: The terminalface of the Godley Glacier, finishing at the Godley Lake.

Large chunks of ice frequently fall from this face.

The ice moves quickly through steep, narrow sections of vall ey and more slowly at other

places; this can cause what is known as an icefall, which is a section of fast-moving

chaotic ice. Because of the sudden increase in speed of the ice at the top of the i cefall ,

the ice is, i n effect, stretched, so that crevasses are formed i n this area of extending flow.

By the middle of the icefall , the crevassing will be intense; the blades or pinnacles

between them are called seracs. At the lower end of the icefall where the valley widens

or the gradient decreases, the ice velocity suddenly drops, causing an area of compressive flow so that the crevasses close and the ice thickens.

It can be seen that ice movement varies greatly at different locations on the glacier surface, and it varies from one glacier to another depending on many factors such as the size and altitude of the neve at the top of the glacier, the width and gradient of the valley down which the glacier flows, the weather patterns and amount of precipitation in the area (note that the climate in New Zeal and is temperate and maritime with generally high precipitation), the time of year and other factors.

5.2.2 Conventional Measurement Techniques

The most common type of glacier study (Bishop & Forsyth, 1 988) is aimed at determination of a glacier's mass balance which involves estimating the volume of water added in the form of snow, and the volume lost by ablation. Studies occur at a number of different levels. On the l argest scale, satellite i mages and aerial photographs are used to compile a glacier inventory and then to monitor their area and terminus position from year to year (WilJiams & Ferrigno, 1 989; Chinn, 1 996). On the level of individual glaciers, yearly photographs are taken of some glaciers from permanent photographic sites and these are used for visual inspection of terminus position and movement of glacial features.

Measurements of ablation and ice movement are made to provide data for mass balance studies. These are generally carried out by drilling lines of poles into the glacier surface which are then used for two purposes. One is the periodic monitoring of the length of pole protruding from the ice. This allows measurement of the amount of abl ation of the glacier surface, for example, Kirkbride ( 1 995). A 1 992 study by the Department of Geography, Auckland University on the Fox Glacier, which the author participated in, was measuring ablation rates of around 1 00mm per day during the summer.

In order to measure glacier flow, the position of the poles is tracked, usually by surveying their position from fixed sites on the sides of the vaJley (Gunn, 1 965). The poles are normally arranged in lines, or traverses, across the width of the glacier, so that the variation in flow rate from the centre to the edges of the glacier can be monitored. This provides displacement estimation for a number of points on the glacier surface. Drilling the poles into the ice is quite an exercise in itself. They may be drilled to a depth of around 1 0m, so that they last all summer before the high rate of ablation causes them to melt completely out.

It is extremely difficult for glaciologists to estimate the flow velocity of icefalls as it is usually not possible (or, at least, is very difficult) to move across the icefaII surface. Sometimes the ice velocity at the sides can be estimated by suspending a weight from an icescrew placed under an overhanging section of ice at the edge of the glacier. The position of the suspended weight relative to the bedrock can be marked on a daily basis.