The ice cones described below were observed during the summer of 1948 hi the course of an expedition undertaken as part of the Beach Reconnaisance program of the United States Navy, Office of Naval Research, Amphibious Branch, at Cornell University, Ithaca, New York, while the author was a member of that University. Although they were not studied in as much detail as desired, they seemed unusual enough to warrant this brief report.
Umnak, the third largest of the Aleutian Islands, holds the huge caldera of Mount Okmok, a volcano which suffered a catastrophic eruption and collapse in Post-Pleistocene timeReference Byers, Hopkins, Wier and Fischer 1 . Within this caldera, 12 km. across, there are nine volcanic cones, of which Cone A is the most active, having undergone a vigorous eruption in 1945 and some minor activity since. Between Cone A and the caldera wall is an active glacier about 8 km. long, and arcuate in shape, stagnant at its eastern extremity. The area discussed in this paper is located approximately 1.6 km. from the north-east end of the glacier and about 1 km. from the crater of Cone A. An almost continuous blanket of low clouds covers the caldera throughout the year, and precipitation occurs almost every day.
Under a uniform blanket of ash perhaps 10 to 13 cm. deep the glacier was covered with névé (or ice) cones as shown in Figs. 1 and 2 (p. 633). The ash was scraped from about twenty of these cones and each one had at its summit a small pit or crater giving it the appearance of a small ice “volcano.” The cones varied in basal diameter from 50 to 125 cm. and in height up to a maximum of 62 cm. The craters studied ranged from 2 to 16 cm. across the top, and from 1.3 to more than 77 cm. in depth.
Fig. 1. Two connected ice cones with ash cover removed. Note technician’s hand in crater. Hummocky surface with tension cracks at crests of cones clearly shown (see p. 631)
Fig. 2. Cone free of ash, showing crater, and surrounding cones beneath ash cover. Lighter colored crests are probably due to downward drainage of water in the ash
Several of the shallower craters were floored with ice and ash solidly frozen together, in one case the frozen mixture reaching within 1 cm. of the crest, and overlain with loose moist ash. Others contained loose ash to the full depth of the crater. All the craters were round or nearly so, and at least the larger ones tapered slightly inward toward the bottom. They were situated at the summit of the névé cones, the sides of which slope at 25 to 35 degrees. The cones did not appear to be aligned in any direction. No volcanic ejecta, other than the ash blanket, were observed in the vicinity.
These cratered cones represent peculiar surface features which apparently have not been previously reported. Possibly they are relict features, predating the ash fall, although no evidence for this was seen. LewisReference Lewis 2 , SwithinbankReference Swithinbank 3 and WilsonReference Wilson 4 propose that an uneven debris cover might have caused such a hummocky ice surface. These cones, however, seem to have developed under a uniform insulating ash cover, thick enough to have preserved until 1948 the snow beneath the ash fall of 1945, as exposed elsewhere. The cracking of the ash cover over them suggests that the cones postdate the ash fall.
No reference to craters of the sort described here was found in the literature. Swithinbank and Lewis both depict a cleft extending across the crest of a cone, effectively breaching it and wholly unlike the central crater shown in the figures in this paper (see also LewisReference Lewis 2 , Fig. 6, p. 22). The debris-filled cleft described by Swithinbank and by Lewis is the remnant of a crevasse filling, and there is no evidence of crevassing on the Okmok glacier near the cones, nor are the cones aligned in any way suggestive of a crevasse pattern.
Colleagues have suggested that the craters represent points of rapid ablation where, at the bottom of tension cracks in the ash at the summit of the cones, insolation can reach the névé without encountering a 10 cm. thick insulating blanket. However, such a proposal fails to explain how more than 77 cm. of ash came to be in one crater measured and how, if infall of ash were responsible for the fill, the overlying ash still maintained its normal 10 to 13 cm. thickness. It is doubtful that wind blown ash could have filled the craters since no evidence of wind shifting of the ash was noted, and the depressions between hummocks were not drifted full. The moist ash would not have been easily blown about.
An effective explanation of the cratered debris-covered ice cones must await further observations, and the author would appreciate information from others who have observed similar phenomena.
An interesting sidelight is provided by the preservation of the snow of 1944–45 as névé under the ash while subsequent snowfalls melted completely.
