Glacial erosion
63
Because ice at 0~ has a hardness of 1.5 in the Mohs scale, it is difficult for it to scratch
or wear away the rocky surfaces over which it moves, because most minerals have higher
hardness numbers than it does, from which it is deduced that the abrasive action must be
due to the friction of rocky fragments held by the glacier in contact with the rocky
substrate (Sugden and John, 1976).
Calculations of the abrasion rate have taken place at numerous sites. Values vary
between 0.001 mm/yr for small glaciers and 36 mm/yr for a crystalline limestone substrate
located in Argenti6re Glacier (French Alps) under an ice thickness of 100 m and a velocity
of the glacier of 250 m/yr (Boulton, 1974). Perhaps this last number is too high, because
the dissolution process of limestone, which is known to be very important in cold waters,
has not been considered.
Factors affecting the abrasion process can be classified in two groups: those inherent to
transported fragments and characteristics of the rocky substrate and the ones related to the
particularities of the ice mass (Menzies, 1995b). Clean ice, no doubt due to its hardness,
does not make abrasive effects on the rocky bed; on the contrary, ice at the interface shows
scratches and cavities in it indicating the wearing action of the rocky bottom. The abrasion
process requires for its action, the presence of rocky fragments held in the glacier base. In
polar ice caps and glaciers the action of abrasion is very small, partly due to the paucity of
fragments in the ice mass. In some soundings of more than two thousand meters only a few
meters have been registered containing significant debris, although in soundings made in
Greenland (Herron and Langway, 1979) and in Antarctica (Gow et al., 1979) abundant
debris appears in the basal ice of these ice sheets. In contrast, the greater amount of
material transported in temperate glaciers produces a much more important abrasive
action, although in these glaciers the fragment content factor regarding the abrasion
intensity should not be the only one to be considered. Another important circumstance is
given by the relative hardness of the fragment with respect to that of the rocky substrate
(Drewry, 1986). It is obvious that a higher hardness of fragments becomes apparent in a
greater abrasive potential. To these factors it is necessary to add the morphologic
characteristics of the particles. The constant fragment interaction with each other and with
the rocky bed has as a consequence abrasion and reduction of angularity. This decreased
effectiveness of the process can be overcome with provision of a new particle or with the
breakage of previously worn fragments. The existing erosive processes at the ice-rock
interface produces a fine material that, in some cases, stands between the ice mass and the
rocky bed thus hindering further abrasion action; in these circumstances the presence of
subglacial water favours the export of this "glacial flour" film. It is also necessary to
consider the permeability of the rocky substrate, because the water presence at the
interface produces an increase of flow velocity, which is also an influencing factor of
abrasion, as discussed below.
The other group can be denoted as glaciological factors (Embleton, 1979). If the
existing temperatures in the basal ice are very low, an adhesion between bed and glacial
ice takes place, so that very strong stresses for sliding are necessary. If we add that under
these temperature regimes the fragment proportion is, as previously indicated, very small
or null, abrasive effects will practically be negligible. Basal flow speed is a factor of
considerable importance because it determines the number of particles running past one
point at the ice-rock contact; therefore, at higher speed a greater abrasive power is
expected. Ice mass thickness has an important influence. A particle located in the ice-rock
63
Because ice at 0~ has a hardness of 1.5 in the Mohs scale, it is difficult for it to scratch
or wear away the rocky surfaces over which it moves, because most minerals have higher
hardness numbers than it does, from which it is deduced that the abrasive action must be
due to the friction of rocky fragments held by the glacier in contact with the rocky
substrate (Sugden and John, 1976).
Calculations of the abrasion rate have taken place at numerous sites. Values vary
between 0.001 mm/yr for small glaciers and 36 mm/yr for a crystalline limestone substrate
located in Argenti6re Glacier (French Alps) under an ice thickness of 100 m and a velocity
of the glacier of 250 m/yr (Boulton, 1974). Perhaps this last number is too high, because
the dissolution process of limestone, which is known to be very important in cold waters,
has not been considered.
Factors affecting the abrasion process can be classified in two groups: those inherent to
transported fragments and characteristics of the rocky substrate and the ones related to the
particularities of the ice mass (Menzies, 1995b). Clean ice, no doubt due to its hardness,
does not make abrasive effects on the rocky bed; on the contrary, ice at the interface shows
scratches and cavities in it indicating the wearing action of the rocky bottom. The abrasion
process requires for its action, the presence of rocky fragments held in the glacier base. In
polar ice caps and glaciers the action of abrasion is very small, partly due to the paucity of
fragments in the ice mass. In some soundings of more than two thousand meters only a few
meters have been registered containing significant debris, although in soundings made in
Greenland (Herron and Langway, 1979) and in Antarctica (Gow et al., 1979) abundant
debris appears in the basal ice of these ice sheets. In contrast, the greater amount of
material transported in temperate glaciers produces a much more important abrasive
action, although in these glaciers the fragment content factor regarding the abrasion
intensity should not be the only one to be considered. Another important circumstance is
given by the relative hardness of the fragment with respect to that of the rocky substrate
(Drewry, 1986). It is obvious that a higher hardness of fragments becomes apparent in a
greater abrasive potential. To these factors it is necessary to add the morphologic
characteristics of the particles. The constant fragment interaction with each other and with
the rocky bed has as a consequence abrasion and reduction of angularity. This decreased
effectiveness of the process can be overcome with provision of a new particle or with the
breakage of previously worn fragments. The existing erosive processes at the ice-rock
interface produces a fine material that, in some cases, stands between the ice mass and the
rocky bed thus hindering further abrasion action; in these circumstances the presence of
subglacial water favours the export of this "glacial flour" film. It is also necessary to
consider the permeability of the rocky substrate, because the water presence at the
interface produces an increase of flow velocity, which is also an influencing factor of
abrasion, as discussed below.
The other group can be denoted as glaciological factors (Embleton, 1979). If the
existing temperatures in the basal ice are very low, an adhesion between bed and glacial
ice takes place, so that very strong stresses for sliding are necessary. If we add that under
these temperature regimes the fragment proportion is, as previously indicated, very small
or null, abrasive effects will practically be negligible. Basal flow speed is a factor of
considerable importance because it determines the number of particles running past one
point at the ice-rock contact; therefore, at higher speed a greater abrasive power is
expected. Ice mass thickness has an important influence. A particle located in the ice-rock