62
Climatic Geomorphology
also the rock fracturing by means of differential pressure generation. A fundamental factor
within the glacial system is the ice mass thickness, because it affects the friction
mechanism at the ice-rock interface.
Characteristics of the rocky substrate, such as rock hardness and the presence of
discontinuity planes (stratification, joints, foliation, and so forth), undoubtedly affect the
action of erosive processes. Another important variable is the rocky bed permeability,
because it conditions the penetration of melt waters, with the consequent variation in its
erosive capacity and their possible absence in future regelation.
Finally, the geometry characteristics of the rocky bed, as to form, roughness and slope
have to be added to the previously indicated factors with regard to variations in the
intensity of the glacial erosive processes.
Not only the characteristics of the ice and the rocky substratum should be considered
but also the modifications that glacial erosion undergoes in the course of time within a
glacial age. Greater erosion intensity occurs in the first stages of the glacial advance
(Tricart and Cailleux, 1962), in which the weathering mantle is easily exported by the ice
masses in its first pulses. Once the regolith is evacuated, the ice needs a greater effort to be
able to erode the non-altered substratum.
Approximate calculations of glacial erosion rates have been made (Andrews, 1975).
On the one hand, the material transport (bottom, suspension and dissolution load) by
glacial melt waters next to the glacier terminus has been determined, supposing that this
material has originated in the glacial environment. Therefore, approximate calculations of
glacial erosion can be made. Average rates seem to vary between 1 and 5 m every 1000
years, but this is perhaps too high and it is necessary to divide them by a factor of 2 to 10.
In order to make estimations of glacial erosion rates for long time periods, theoretical
calculations are also made. They have been applied to cirques, glacier valleys and fjords.
The variables considered are volume, area and age of the analysed form. Volume is
divided by the area and the erosion or reduction for the time undergoing the action of ice is
obtained, to obtain the erosion rate. For cirques this value varies between 5 cm/1000 yr for
polar cirques and 40 cm/1000 yr for the cirques of western Scotland. In any case, these
calculations are to be considered as orientative only because true rates of glacial erosion
are a major research topic at the present time.
1.1. Process types
The erosion mechanisms of glaciers are various and the following ones can be
differentiated.
1.1.1. Abrasion
The wearing of rocks by the action of ice passing is clearly shown by its effects,
represented, among others, by the microforms of glacial erosion such as striations,
grooves, and other features. The abrasion process can be studied directly by means of
tunnels into the contact with the ice-rock interface, by direct observation in natural
subglacial cavities and also, indirectly, by laboratory experiments, analysing the behaviour
of the ice sliding on different rocks and circumstances (Embleton, 1979).
Climatic Geomorphology
also the rock fracturing by means of differential pressure generation. A fundamental factor
within the glacial system is the ice mass thickness, because it affects the friction
mechanism at the ice-rock interface.
Characteristics of the rocky substrate, such as rock hardness and the presence of
discontinuity planes (stratification, joints, foliation, and so forth), undoubtedly affect the
action of erosive processes. Another important variable is the rocky bed permeability,
because it conditions the penetration of melt waters, with the consequent variation in its
erosive capacity and their possible absence in future regelation.
Finally, the geometry characteristics of the rocky bed, as to form, roughness and slope
have to be added to the previously indicated factors with regard to variations in the
intensity of the glacial erosive processes.
Not only the characteristics of the ice and the rocky substratum should be considered
but also the modifications that glacial erosion undergoes in the course of time within a
glacial age. Greater erosion intensity occurs in the first stages of the glacial advance
(Tricart and Cailleux, 1962), in which the weathering mantle is easily exported by the ice
masses in its first pulses. Once the regolith is evacuated, the ice needs a greater effort to be
able to erode the non-altered substratum.
Approximate calculations of glacial erosion rates have been made (Andrews, 1975).
On the one hand, the material transport (bottom, suspension and dissolution load) by
glacial melt waters next to the glacier terminus has been determined, supposing that this
material has originated in the glacial environment. Therefore, approximate calculations of
glacial erosion can be made. Average rates seem to vary between 1 and 5 m every 1000
years, but this is perhaps too high and it is necessary to divide them by a factor of 2 to 10.
In order to make estimations of glacial erosion rates for long time periods, theoretical
calculations are also made. They have been applied to cirques, glacier valleys and fjords.
The variables considered are volume, area and age of the analysed form. Volume is
divided by the area and the erosion or reduction for the time undergoing the action of ice is
obtained, to obtain the erosion rate. For cirques this value varies between 5 cm/1000 yr for
polar cirques and 40 cm/1000 yr for the cirques of western Scotland. In any case, these
calculations are to be considered as orientative only because true rates of glacial erosion
are a major research topic at the present time.
1.1. Process types
The erosion mechanisms of glaciers are various and the following ones can be
differentiated.
1.1.1. Abrasion
The wearing of rocks by the action of ice passing is clearly shown by its effects,
represented, among others, by the microforms of glacial erosion such as striations,
grooves, and other features. The abrasion process can be studied directly by means of
tunnels into the contact with the ice-rock interface, by direct observation in natural
subglacial cavities and also, indirectly, by laboratory experiments, analysing the behaviour
of the ice sliding on different rocks and circumstances (Embleton, 1979).