Box 64 Rock Mass Strength And Trough Morphometry

Working in the Southern Alps of New Zealand, Augustinus (1992) set out to test the hypothesis that the morphology of a glacial trough is related to the strength of the rock mass or bedrock into which it is cut. Determining the strength or resistance of bedrock to erosion is difficult because it is a function not only of the intact strength of the rock but also the density, spacing and orientation of the joints or other lines of weakness within the rock mass. To get around this problem, Augustinus used a method commonly utilised in studies of slope stability, known as the Rock Mass Strength, to estimate the geomorphological strength of the bedrock. The Rock Mass Strength classification involves scoring a rock mass in the field against a series of eight properties that collectively determine its strength. They are: (i) intact rock strength measured with a Schmidt hammer; (ii) the degree of weathering; (iii) the spacing between joints or partings; (iv) width of joints or partings; (v) the continuity of the joints or partings; (vi) the orientation of the joints or partings in relation to the slope; (vii) the presence or absence of infills along joints or partings; and (viii) the presence of the outflow of water from joints or partings. Augustinus (1992) determined trough morphometry and estimated the Rock Mass Strength of the rock mass into which each trough was cut at a series of different sample sites. The results indicate that there is a strong relationship between Rock Mass Strength and trough form. Glacial troughs appear to become narrower and the sides become steeper as the Rock Mass Strength increases. The number of troughs in a given area also appears to increase with Rock Mass Strength. This work illustrates neatly how the morphology of glacial troughs is partly controlled by the strength of the bedrock into which they are eroded.

Source: Augustinus, P.C. (1992) The influence of rock mass strength on glacial valley cross-profile morphometry: a case study from the Southern Alps, New Zealand. Earth Surface Processes and Landforms, 17, 39-51.

preglacial valleys, where ice flow is difficult, into forms that are able to comfortably and efficiently accommodate ice flow. The amount of glacial erosion needed to create a glacial trough is therefore equal to the amount of adaptation needed to modify the preglacial valley in order to discharge the available ice efficiently. Once the shape of a glacial trough is established its size should be simply a function of the amount of ice that it has to discharge. This relationship has been confirmed by studies of outlet glaciers draining ice caps in Greenland and for valley glaciers in New Zealand, where there is a strong relationship between the drainage or accumulation area that supplies a glacier and the size of its trough. The relatively simple morphology of glacial troughs and their close relationship to ice discharge has also enabled their formation and evolution to be modelled numerically (Box 6.5). This type of analysis suggests that over time glacial valleys become adjusted to the glacial systems that form them. This might imply that most erosion will occur in

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