Buoyancy–driven calving events are caused by the fracture of ice due to the upward buoyant forces applied to the terminus as a result of the density differences between ice and water.
Buoyancy–driven calving events were the least common of all events, with only six identified throughout the entire study period. However, buoyancy–driven calving accounted for the largest and most energetic events, including the largest recorded event during the study period on 30/01/12 (Figure 5.34). Uplift of the terminus was evident in the days and weeks prior to calving on 30/01/12 (Glacier Explorers, pers. comm., 2012). Given their nature, high–magnitude buoyancy–driven events had a significant impact on the retreat of the glacier. For example, the large buoyancy driven calving events on 30 January and 29 April 2012 (Figure 5.34) resulted in a maximum retreat of 120 m and 60 m, respectively. This accounted for almost all of the retreat of the southern ice cliff over the two periods shown in Figure 5.13.
The size and extent of the calved section associated with buoyancy–driven calving, makes it unclear whether these calving events occurred as a single coherent section or as a series of events. Given that several large icebergs were present in the time after calving on 30/01/2012 (Figure 5.34), and that the geometry of the terminus prior to calving was in the form of two peninsulas, it is unlikely that the ice all calved as a single section. Rather, the calving of one section of the terminus likely triggered calving along the entire length of the terminus.
Figure 5.34: The high magnitude buoyancy–driven calving events observed throughout the study period occurring along (A) the entire width of the terminus overnight of 30/04/12 and (B) from the eastern section of the southern ice cliff on 29/04/12. The resulting icebergs have a mixture of white ice through to deep blue icebergs indicating that calving has occurred throughout the majority of the ice column. The dotted line indicates the calved section in the upper image. Image quality is degraded for the 30/01/12 event due to scratches on the lens. As noted above, high magnitude buoyancy driven calving events have the effect of increasing the potential for over–steepening events to occur, but also limit notching events by resetting any notches that had formed prior to the event. Over short time–scales (months) calving variability can be strongly influenced by the occurrence of larger buoyancy–driven calving events. The
result of this can be an increase in smaller over–steepening events and the limiting of notching events in the short–term.
Interestingly, both of the large magnitude buoyancy–driven calving events appear to be preceded by subaqueous events. The January event was preceded by subaqueous calving in the boundary region between the southern ice cliff and the western embayment, with the April event preceded by a large subaqueous event on 02/04/12. In the time between the subaqueous calving event and subsequent failure of the large section of the ice cliff, extensive crevasses developed in the region of the line of failure (Figure 5.35). These crevasses were clearly obvious by the 13–14 April, had become more prominent by the 21 April, and were quite large by 25 April. The formation of crevasses in this manner appears to be linked with the subaqueous event, which could have decreased the support at the base of the subaqueous ice cliff. The result of this would have been the development of surface crevasses due to increased tensile stress at the surface of the glacier. Consequently, support at the base of the ice column was lost. Increased basal crevassing (van der Veen, 1998a) may also have resulted if the isolated section was subject to increased torque due to lake–level changes, as it was no longer supported and would be able to oscillate in isolation of the remaining ice cliff. It is unclear to what extent subaqueous events prime the large calving events, but given their potential and the observations presented here, they may play a significant role.
Also, the two high–magnitude buoyancy–driven events were not directly associated with increased lake level as has been found in other studies (e.g., Boyce et al., 2007), but occurred after periods of relatively stable lake level. However, significant fluctuations in lake level were recorded prior to the period of lake level stability, which would have increased stress at the juncture with grounded ice, increasing fracture propagation as the floating sections of the glacier oscillated. When this is coupled with increased surface lowering and calving due to melt and subaerial calving, the bending moment applied to the isolated sections of the glacier may have been great enough to initiate failure. This indicates that thinning of the glacier and the presence of deep water drives the terminus to the point of flotation (Warren et al., 2001; Boyce et al., 2007), but that short–term processes (lake level fluctuations, subaerial melt and calving) play a critical role in inducing fracture propagation and calving failure.
Figure 5.35: The development of surface crevasses immediately up glacier prior to the calving event on 29/04/12 (shown in Figure 5.34). Black arrows indicate the region in which crevasses formed throughout the month of April after a subaqueous calving event. White box indicates the area shown in the close up images on 29/04/12 and 02/05/12. White arrow indicates an exposed thermo–erosional notch present prior to the calving event. Note the highly irregular surface present in the 02/05/12 image highlighting the potential for large events to increase smaller over–steepening calving.
Although buoyancy–driven calving events were dominated by the two high–magnitude events described above, relatively smaller events were also identified where small sections of the glacier became isolated and subjected to buoyant forces. Figure 5.36 shows two examples from within the western embayment and the peninsula that developed between the southern ice cliff and western embayment. These calving events indicate that although buoyancy–driven calving is typically associated with high–magnitude events, it can initiate calving across a wide range of spatial scales.
Figure 5.36: Lower magnitude buoyancy–induced calving events on the (A) 03/01/12 and (B) 05/01/12, indicating that buoyant forces affect calving at a range of spatial scales.