superficial thermal regime and increases ablation. In contrast, a thicker layer
reduces ablation by shielding the ice underneath the debris mantle (e.g. Østrem
1959; Mattson et al. 1993). We compared the differential ablation at the glacier
tongue below the icefall (Fig. 9.8 for extent). As shown in Table 9.5, the mean
surface lowering for the debris-covered area was about 4.0 m less than for the bareice area in both periods. The annual difference in mean surface lowering amounts to
1.32 m a
À1
. Hence, the shielding effect of the debris cover was clearly dominant in
both periods.
9.4.3 Morphological Changes
We quantified two major glacier-related morphological changes in our study area.
First, we focused on areal extent of the rock windows in the massive icefall of
Pasterze Glacier. Second, we quantified the volume of the relocated rock mass
released in a large rock fall event in 2007.
Figure 9.9 depicts the evolution of several large rock windows in the icefall. In
this figure, areas 1 and 2 covered 0.072 km
2 in 2003, 0.131 km
2 in 2006 and
0.190 km
2 in 2009. The rock outcrops in this area thus increased 2.6 times in the
6 years of observation. The ice-free areas at the left (area 4) and right (area 3)
margins of the glacier in the icefall also increased substantially. The deglaciation
caused a substantial reduction in ice transport through the icefall. By 2009 only two
minor ice streams at the southwest side of the icefall were still connected with the
glacier tongue. Both are insignificant for glacier ice transport and hence for the
nourishment of the glacier tongue. Only the main glacier ice stream transports
significant amounts of glacier ice to the glacier tongue.
The mass relocation of a major rock fall event in 2007 at Mittlerer Burgstall is
depicted in Fig. 9.10. We quantified the total area influenced by the rock fall as
89,300 m
2 (detachment area 13,800 m
2 , deposition area 75,500 m
2 ). A rock volume
of 428,000 m
3 was detached by this event at the sharp and distinct mountain ridge.
The highest change in elevation in this area was À67 m and the mean was À31 m. In
contrast, the total volume of the deposited rock material was 523,000 m
3 , resulting
Table 9.5 Mean surface elevation changes below the icefall (for extent see Fig. 9.8) for debriscovered and bare ice parts for the periods 2003–2006 and 2006–2009
Period
Surface type during both stages
Area
[km
2
]
Mean surface elevation change
[m]
2003–
2006
Bare ice
0.89
16.1
Debris-covered
2.28
12.1
Difference
4.0 (24.9 %)
2006–
2009
Bare ice
0.66
14.3
Debris-covered
2.33
10.4
Difference
3.9 (27.1 %)
190
V. Kaufmann et al.
reduces ablation by shielding the ice underneath the debris mantle (e.g. Østrem
1959; Mattson et al. 1993). We compared the differential ablation at the glacier
tongue below the icefall (Fig. 9.8 for extent). As shown in Table 9.5, the mean
surface lowering for the debris-covered area was about 4.0 m less than for the bareice area in both periods. The annual difference in mean surface lowering amounts to
1.32 m a
À1
. Hence, the shielding effect of the debris cover was clearly dominant in
both periods.
9.4.3 Morphological Changes
We quantified two major glacier-related morphological changes in our study area.
First, we focused on areal extent of the rock windows in the massive icefall of
Pasterze Glacier. Second, we quantified the volume of the relocated rock mass
released in a large rock fall event in 2007.
Figure 9.9 depicts the evolution of several large rock windows in the icefall. In
this figure, areas 1 and 2 covered 0.072 km
2 in 2003, 0.131 km
2 in 2006 and
0.190 km
2 in 2009. The rock outcrops in this area thus increased 2.6 times in the
6 years of observation. The ice-free areas at the left (area 4) and right (area 3)
margins of the glacier in the icefall also increased substantially. The deglaciation
caused a substantial reduction in ice transport through the icefall. By 2009 only two
minor ice streams at the southwest side of the icefall were still connected with the
glacier tongue. Both are insignificant for glacier ice transport and hence for the
nourishment of the glacier tongue. Only the main glacier ice stream transports
significant amounts of glacier ice to the glacier tongue.
The mass relocation of a major rock fall event in 2007 at Mittlerer Burgstall is
depicted in Fig. 9.10. We quantified the total area influenced by the rock fall as
89,300 m
2 (detachment area 13,800 m
2 , deposition area 75,500 m
2 ). A rock volume
of 428,000 m
3 was detached by this event at the sharp and distinct mountain ridge.
The highest change in elevation in this area was À67 m and the mean was À31 m. In
contrast, the total volume of the deposited rock material was 523,000 m
3 , resulting
Table 9.5 Mean surface elevation changes below the icefall (for extent see Fig. 9.8) for debriscovered and bare ice parts for the periods 2003–2006 and 2006–2009
Period
Surface type during both stages
Area
[km
2
]
Mean surface elevation change
[m]
2003–
2006
Bare ice
0.89
16.1
Debris-covered
2.28
12.1
Difference
4.0 (24.9 %)
2006–
2009
Bare ice
0.66
14.3
Debris-covered
2.33
10.4
Difference
3.9 (27.1 %)
190
V. Kaufmann et al.