Ice calving

As the calving Cancel greater ice masses of ending in the sea or inland waters glaciers is called. Many definitions of the term also cases involve, in which the glacier does not end in the water, but the ice breaks off in a similar manner with almost vertical fracture surface of the glacier. In most cases, however, the process takes place at ending in the water glaciers. Here, the glacier on the glacier can end stand on the bottom of the water or swim in this. Typically, the glacier ends here in the form of a few tens of meters high ice cliffs, the height can also be up to 80 meters. In inland waters calving glaciers there in almost all the glaciated mountains of the world, into the sea calving glaciers - called tidewater glaciers - but occur only at higher latitudes than 45 °. The explored in this regard is the single biggest glacier is the Columbia Glacier in Alaska.

The blocks are broken apart into icebergs. This process is responsible for a large part of the mass loss of the ice sheet - about 90 percent of the ablation in Antarctica and about half in Greenland are caused. By calving glacier loses a significantly larger amounts of ice per unit of time than would be the case by melting processes, this is particularly evident in sloughing off the ice shelf large table icebergs. For this reason, the understanding of this process for predictions on the development of the cryosphere in relation to climate change and the forecast of sea level rise is crucial.

Causes and relevant factors

Fundamental to the process of calving, that the glacier flow direction of the long stretches at the glacier terminus, since the flow resistance decreases by reaching the water body. Due to the longitudinal stretching break even on other columns, also the destabilizing effect of the from the higher course of the glacier " brought " column is also reinforced by the thinned ice.

In addition, the calving is influenced by numerous factors, the most important are the following:

  • Melting processes beneath the water line can undermine the glacier terminus. This depends on both the water temperature and flow, and thus the amount of supplied heat as well as the kinetic energy of the waves. This leads on the one hand, that cancel overhanging parts. Such melting processes on the other hand can also lead to an entirely lying below the water line of the glacier breaks off and rocketing to the surface.
  • The mass loss by calving is higher in tidal glaciers of magnitude than ending in freshwater glaciers. The main reason for this is that the density of the output from tidal glaciers melt water is different from that of sea water, which leads to significant convection and thus enhances the heat transfer.
  • The flow rate of the glacier has two major influences on the process of calving: As long as the glacier terminus does not significantly shift corresponds to the flow rate of the glacier approximately the Kalbungsgeschwingkeit and thus determines the ice loss per unit of time. In addition, the Fließgeschwindigekeit decisively influenced the formation of gaps in the course of the glacier. This is where the flow rate temporarily increased significantly in surge glaciers, particularly clear. As an example, when normally calving low frequency Bering Glacier in Alaska in 1993, the surge front reached the glacier terminus, this produced at once numerous small icebergs.
  • The ice temperature plays a role, ie whether it is a temperate, polythermalen or cold glacier. This affects firstly the flow rate, secondly, cold ice is more rigid and less malleable. Also of importance is the presence of meltwater on the glacier surface, this can significantly accelerate the deepening of the columns.
  • The hydrographic characteristics of the estuary may have a significant influence. The water depth it affects in two ways, firstly, the glacier comes into contact with more water, resulting in greater heat transfer. Secondly, increases in deeper water the buoyancy of the ice, bringing the flow resistance is reduced, resulting in the longitudinal stretching and thus also favors the calving. Has a similar effect if a fjord is wider. This shows in the fact that glaciers can hardly grow beyond the end of a fjord or a bay. In contrast shallows can be a kind of anchor point at which the glacier front over a long period remains stable. It should be noted that such hollows to be formed by sediments and moraines, are thus created by the glacier itself.

Modeling approaches for the Kalbungsprozess

Since the process of calving is responsible for a large part of the mass loss of the ice shelf in particular, of the inland ice and many glaciers, it plays a crucial role in forecasting relating to the cryosphere and sea level rise. Here, this process seems not only to depend on the climate, but to include a momentum of its own, and there is evidence that climate change may represent an " initial spark " and thus affect disproportionately. The modeling of the process is complicated by the numerous relevant factors, and in addition also the fact that some hitherto not satisfactorily solved other glaciological problems also play a role, such as predictions in the transition zone between resting and floating ice. One of the key issues in modeling is whether the calving is influenced by the glacier dynamics, ie whether a higher flow rate causes a higher Kalbungsgeschwindigkeit, or whether the reverse is true, that is, an increased flow rate is the result of higher Kalbungsverluste. In previous research, both approaches have been proposed, significantly, this was even on the basis of data of the same glacier, the Columbia Glacier, which underlines intricacy of the problem. It thus seems that would have a yet to be developed, comprehensive model also include the glacier dynamics.

Some simpler formulas have been proposed for isolated issues. A key variable is the Kalbungsgeschwindigkeit ( Calving rate), which is commonly defined as the difference of the flow velocity at the glacier terminus and the change in length per unit time.

When the glacier terminus is stationary ie, corresponds to the Kalbungsgeschwindigkeit the flow rate at the glacier terminus.

It has been empirically determined that the Kalbungsgeschwindigkeit nearly proportional to the water depth when the other factors are in similar areas. For analysis of 22 tidal glaciers in Alaska, Greenland and Spitsbergen following approximate formula for the Kalbungsgeschwindigkeit was developed ( in meters per year):

Because behave in freshwater ending glaciers entirely different, was determined for this analogy on the basis of 21 glaciers, a separate formula:

The ice thickness at which a glacier no longer rests on the ground but a floating tongue forms can be estimated in the following manner depending on the water depth:

In this case, and the densities of water and ice.