Extrusion Flow in Glaciers: A description of tests to prove the hypothesis and some details regarding its influence on glacial erosion

Abstract

IN the middle of last century, Agassiz, Tyndall and others showed that a glacier flowed faster at its centre than at its margin. Partly from experiments and partly from the assumption that it behaved like a river, it became generally accepted that it also flowed faster at the surface than lower down. This belief was held until ten or fifteen years ago although search through earlier literature shows that evidence was accumulating which might disprove this. During the thirtit;s of this century several writers began actively to reexamine the question. I In 1937 the late Dr. M. Demorest,2 reporting on a West Greenland glacier, wrote that owing to plastic flow the ice was, as it were, forced out from under an ice cap at places where accumulation of snow lay deepest. He cited well-known laboratory experiments showing that ice tended to flow when under differential pressure. In the following year, Dr. R. Streiff-Becker published his first paper 3 asserting that in many parts of a glacier, flow was faster below than above. Demorest used the evidence of glacial striae in old glacier beds. On the other hand Streiff-Becker developed his hypothesis by certain measurements and this paper has been written from material submitted by him. He had been measuring snow accumulation on the Claridenfirn (Canton Glarus) yearly since 1916 and it occurred to him that if the accumulqted firn was not to raise the level of the glacier, it must be carried away faster than appeared from the rate of flow measured at the surface. His measurements of the glacier level showed that the firn moved in waves but that the mean level of the surface was no higher in 1937 than it had been in 1916. Fig. I, p. 13, shows the position of a post, Point 2900, through which is drawn a line EDC recorded in the 1 See for instance Hollingworth. S. E. Q.

Life, death and afterlife of the extrusion flow theory

“…This belief was held until ten or fifteen years ago although search through earlier literature shows that evidence was accumulating which might disprove this. (Seligman, 1947 )…”

“…For example, Gerald Seligman apparently was drawn to these ideas. In assessing the pros and cons of the extrusion flow theory, Seligman (1947) suggested that Holmes’s work supported the idea that…”

“…Although Seligman (1947) interpreted this as a statement of extrusion flow, it could also describe downward flow in which the upper ice is carried along by the ice below. I think that Demorest’s concept of extrusion flow may not yet have been fully formulated.…”

“…(c) Similar patterns were expected in an ice sheet or ice cap, because its lower layers were expected to be plastic. From Streiff-Becker (1942) and Seligman (1947). …”

“…Streiff- Becker used region ABCD for continuity calculation. Adapted from Streiff-Becker (1938) and Seligman (1947). …”