Sir, Calving from floating glaciers: comments on Dr N. Reeh’s paper
Reeh’s treatment of glacier calving (Reference ReehReeh, 1968) invites some comment. He chooses for corroboration of his theory in the first place the Rink Gletscher, but seems to be unaware that the catastrophic calvings of this glacier have been described in detail, photographed and even filmed (Reference SorgeSorge, 1933[a], Reference Sorge[b]). Jakobshavn Isbræ might also occasionally calve like Rink Gletscher; this is a possible explanation of the big waves which sometimes enter the harbour of Jakobshavn. But nobody has to my knowledge yet seen one of its big calvings, and this glacier cannot safely be adduced (Reference ReehReeh, 1968, p. 231) as a support of the author’s theory. it might also be pointed out that the manner of calving of the Rink Gletscher, a big detachment at roughly fortnightly intervals, is not typical for the fast-moving ice streams of Greenland which generally shed their ice surplus day by day through the formation of individual icebergs in a manner not covered by the theory.
Apart from the fact that Rink Gletscher is in a special position, the question arises whether it represents a “floating glacier” at all. The depth of the fjord into which Rink Gletscher calves, diminishes from 1 100 m at 12 km to 850 m at 3 km and 650 m at 500 m from the calving front (Reference SorgeSorge, 1933[b]). If the fjord retains the same depth, the thickness of the floating glacier cannot exceed 740 m of which 90 m can be above sea-level. According to Reeh’s figs. 12 and 13 this height is already approached at 1 km or less from the front. The longitudinal section (fig. 14) deviates about 30° from the axis of the glacier; in the direction of the flow lines (Reference Carbonnell and BauerCarbonnell and Bauer, 1968) the near-horizontal part would be considerably shorter. (In a publication received after the first submission of this Correspondence it is flatly stated (Reference Carbonnell and BauerCarbonnell and Bauer, 1968) that “le front [of Rinks Isbræ] ne flotte pas”. The same is claimed about Jakobshavn Isbræ (Reference BauerBauer and others, 1968; Reference Carbonnell and BauerCarbonnell and Bauer, 1968) which one might rather expect to float in view of the very small inclination of only 24′ of its lowest part.) It would have been interesting if the author had tried to apply his theory to the undoubtedly floating glacier tongues of north Greenland (Reference KochKoch, 1928).
The theory demands that the front should bend downward before calving. This is true in some cases whilst in others the outermost part of a floating ice shelf is raised (Reference SwithinbankSwithinbank, 1957). A picture of a detaching part of the front of Jakobshavn Isbræ which seems partly higher and partly lower than the intact part, is shown by Reference BauerBauer and others (1968, fig. 24). It might in this connection be pointed out that, contrary to a perhaps naive expectation of what a forward bending frontal part would do, at both observed big calvings of Rink Gletscher the detached part of the front of about 500 Gg tilted backward (Reference SorgeSorge, 1933[b]). The other big calving that has been described (Drygalski, 1897, Bd. 1, p. 392) also showed an original backward tilt of the iceberg. Pager and ethers (1968, p. 89) shows icebergs close to the front of Jakobshavns Isbra which have tilted backward. Reeh does not deal with the question, but a backward tilt would probably have to be expected if the break was near the line at which the ice starts to float.
The author states himself in his Conclusion that his theory is not verified by the calving of the Antarctic ice shelves although one should expect them to be more likely to conform than the laterally confined ice streams of Greenland. We find indeed floating glacier tongues like those of the Mertz and Ninnis Glaciers and ice shelves like the Amery Ice Shelf which attain lengths of not once but 100 times their thickness before they calve.
Incidentally, although the author is right that storm waves can hardly explain the calving of big icebergs, this might be different with tsunamis which have wavelengths of many kilometres and influence the whole water body. They occur sometimes at Antarctic ice fronts (Reference BarréBarré, [1953], [Tom.] 2, p. 179).
One might wish that Mr Reeh could develop his learned and elaborate theory to a state in which the processes leading to and happening during calving are fully covered.
20 October 1968
