Calving (glaciology)

The Perito Moreno Glacier calves into Lake Argentino

As calving canceling large ice masses is by ending in the sea or inland waters glaciers referred. Many definitions of the term also include cases where the glacier does not end in the water, but the ice breaks off the glacier in a similar manner with an almost perpendicular fracture surface. In most cases, however, the process takes place on glaciers that end in water. The glacier can stand on the bottom of the water at the end of the glacier or swim on it. The glacier typically ends in the form of an ice cliff a few dozen meters high, but the height can also be up to 80 meters. Glaciers that calve in inland waters can be found in almost all glaciated mountains in the world, but glaciers that calve into the sea - so-called tidal glaciers - only occur at latitudes higher than 45 °. By far the most researched glacier in this regard is the Columbia Glacier in Alaska .

The broken blocks turn into icebergs . This process is responsible for much of the inland ice mass loss - it causes over 90 percent of the ablation in Antarctica and around half in Greenland . As a result of calving, a glacier loses significantly larger amounts of ice per unit of time than would be the case with melting processes, this is particularly evident in the case of large table icebergs detaching from the ice shelf . For this reason, understanding this process is crucial for predicting the evolution of the cryosphere in connection with climate change and forecasting sea ​​level rise .

Causes and Relevant Factors

Fundamental to the process of calving is that the glacier stretches along the direction of flow at the end of the glacier, as the flow resistance decreases when it reaches the body of water. Further crevasses break open due to the longitudinal extension , and the destabilizing effect of the crevasses “brought along” from the higher course of the glacier is also reinforced by the thinner ice cover.

In addition, numerous factors influence calving, the most important of which are:

• Melting processes below the waterline can undermine the end of the glacier. These depend on the water temperature and current and thus the amount of heat supplied, as well as the kinetic energy of the waves. On the one hand, this leads to overhanging parts breaking off. On the other hand, such melting processes can also lead to a part of the glacier lying entirely below the waterline breaking off and soaring to the surface.

• The mass loss due to calving is an order of magnitude higher in tidal glaciers than in glaciers ending in freshwater . The main reason for this is that the density of the melt water given off by tidal glaciers differs from that of sea water, which leads to considerable convection and thus increases the heat transfer.

• The flow speed of the glacier has two main influences on the process of calving: As long as the end of the glacier does not shift significantly, the flow speed of the glacier corresponds approximately to the calving speed and thus determines the loss of ice per unit of time. In addition, the flow velocity has a decisive influence on the formation of crevices in the course of the glacier. This is particularly evident in surge glaciers , where the flow velocity increases significantly at times. For example, when the surge front reached the end of the glacier in the Bering Glacier in Alaska, which normally calves at a low frequency, in 1993, it suddenly produced countless small icebergs.

• The ice temperature plays a role, i.e. whether it is a tempered , polythermal or cold glacier . On the one hand, this influences the flow rate; on the other hand, cold ice is stiffer and less malleable . The presence of melt water on the surface of the glacier is also important, as this can considerably accelerate the deepening of the crevices.

• The hydrographic features of the estuary can have a significant impact. The water depth has two effects: on the one hand, the glacier comes into contact with more water, which enables higher heat transfer. On the other hand, the buoyancy of the ice increases in deeper water, which reduces the flow resistance, which leads to longitudinal stretching and thus also promotes calving. Has a similar impact when a fjord widens. This is shown by the fact that glaciers can hardly grow beyond the end of a fjord or bay. On the other hand, shallows can represent a kind of anchor point at which the glacier front remains stable for a long time. It should be noted that such shallows can be formed by sediments and moraines , i.e. created by the glacier itself.

Modeling approaches for the calving process

Since the process of calving is responsible for a large part of the loss of mass, particularly of the ice shelf , the inland ice and many glaciers, it plays a crucial role in forecasts concerning the cryosphere and sea ​​level rise . This process does not only seem to depend on the climate, but rather to contain a certain dynamic of its own, whereby there are indications that a climate change can represent an “initial spark” and thus have a disproportionate effect. The modeling of the process is made more difficult by the countless relevant factors and also by the fact that some other glaciological problems that have not yet been satisfactorily solved also play a role, such as predictions in the area of ​​the transition zone between the overlying and floating ice. One of the key questions in modeling is whether calving is influenced by the glacier dynamics , i.e. whether a higher flow velocity causes a higher calving velocity, or whether it is the other way round, i.e. an increased flow velocity is the result of higher calving losses. In previous research, both approaches have been suggested, and significantly, this was even based on data from the same glacier, the Columbia Glacier , underscoring the intricacies of the problem. It therefore appears that a comprehensive model that has yet to be developed would also have to include glacier dynamics.

Some simpler formulas have already been suggested for isolated questions. An important variable is the Kalbungsgeschwindigkeit ( Calving rate ), commonly referred to as difference of the flow rate at the end of glaciers and the change in length per unit of time is defined. ${\displaystyle (V_{T})}$ ${\displaystyle (\Delta l)}$ ${\displaystyle (\Delta t)}$

${\displaystyle V_{K}=V_{T}-{\frac {\Delta l}{\Delta t}}}$

If the end of the glacier is stationary, the calving speed corresponds to the flow speed at the end of the glacier.

It was empirically determined that the calving speed is almost proportional to the water depth if the other influencing factors are in similar ranges. When analyzing 22 tidal glaciers in Alaska, Greenland and Spitzbergen , the following approximation formula for the calving speed (in meters per year) was developed: ${\displaystyle (H_{W})}$

${\displaystyle V_{K}=70+8,33H_{W}}$

Since glaciers that end in freshwater behave completely differently, a separate formula was determined for them on the basis of 21 glaciers:

${\displaystyle V_{K}=17,4+2,3H_{W}}$

The thickness of the ice from which a glacier no longer rests on the ground but forms a floating tongue can be estimated in the following way depending on the water depth : ${\displaystyle (H_{E})}$ ${\displaystyle (H_{W})}$

${\displaystyle H_{E}<{\frac {\rho _{w}}{\rho _{e}}}H_{W}\approx 1,1H_{W}}$

Where and are the densities of water and ice. ${\displaystyle \rho _{w}}$ ${\displaystyle \rho _{e}}$

literature

• Douglas I. Benn, Charles R. Warren, Ruth H. Mottram: Calving processes and the dynamics of calving glaciers. In: Earth Science Reviews. 82, 2007, pp. 143-179.
• Roger LeB. Hooke: Principles of Glacier Mechanics. Second edition. Cambridge University Press, Cambridge 2005, ISBN 0-521-83609-3 .

Individual evidence

1. ↑ The Brockhaus. Weather and climate. Brockhaus, Leipzig / Mannheim 2009, ISBN 978-3-7653-3381-1 , p. 165.
2. ^ Klaus KE Neuendorf: Glossary of Geology. Springer, Berlin 2010, ISBN 978-3-642-06621-4 , p. 93.
3. ↑ ^{a b c d e f} Kurt M. Cuffey, WSB Paterson: The Physics of Glaciers. 4th edition. Butterworth-Heinemnn, Burlington 2010, ISBN 978-0-12-369461-4 , pp. 121-124.
4. ^ ^{A b c} Charlese A. Warren: Calving Glaciers. In: Vijay P. Singh, Pratap Singh, Umesh K. Haritashya (Eds.): Encyclopedia of Snow, Ice and Glaciers. Springer, Dordrecht 2011, ISBN 978-90-481-2641-5 , pp. 105f.
5. ↑ DI Benn include: Calving processes and the dynamics of calving glaciers. 2007, pp. 156-159.
6. ↑ R. Hooke LeB: Principles of Glacier Mechanics. 2005, pp. 31-34.
7. ^ ^{A b} D. I. Benn et al: Calving processes and the dynamics of calving glaciers. 2007, pp. 144-147.
8. ↑ DI Benn include: Calving processes and the dynamics of calving glaciers. 2007, pp. 154f.
9. ↑ ^{a b} D. I. Benn et al.: Calving processes and the dynamics of calving glaciers. 2007, pp. 163-171.
10. ↑ DI Benn include: Calving processes and the dynamics of calving glaciers. 2007, pp. 171-174.
11. ↑ ^{a b c} D. I. Benn et al .: Calving processes and the dynamics of calving glaciers. 2007, p. 147f.

Web links

• Video of a major calving event off the Antarctic Peninsula on April 23, 2007, captured by icebreaker Nathaniel B. Palmer
• Video of a very large calving event on the Ilulissat glacier on August 28, 2008. The glacier calved over a period of 75 minutes over a width of about 5 km and thereby shortened by about 1.5 km. According to the authors, the film by Adam LeWinter and Jeff Orlowski was recorded in the Guinness Book of Records in 2016 as the largest calving ever recorded on film .