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Rock Glaciers on James Ross Island, Antarctica

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Rock Glaciers on James Ross Island, Antarctica ROCK GLACIERS ON JAMES ROSS ISLAND, ANTARCTICA Jorge A. Strelin1, Toshio Sone2 1. Instituto Ant‡rtico Argentino and Centro Austral de Investigaciones Cient’ficas, Av. Malvinas Argentinas s/n¼, (9410) Ushuaia, Tierra del Fuego, Argentina e-mail: [email protected] 2. Institute of Low Temperature Science, Hokkaido University, Sapporo 060, Japan e-mail: [email protected] Abstract Lack of glacier cover in north-western James Ross Island, favours the development of a number of periglacial landforms. Ice-cored rock glaciers, protalus lobes, and recently discovered protalus ramparts are some of the most conspicuous cryogenic features. The ice-cored rock glaciers appear in a complex and genetically related landform system. Besides their mor- phological characteristics, these landforms are also differentiated by their dynamic behaviour. Mechanisms of ice and debris flow and debris extrusion are discussed in order to ascertain the initial age of the main rock glac- ier formation. Protalus lobes and protalus ramparts, formed at the base of scree slopes and ephemeral snow patches with no relation to former glaciers, are also typical features of this environment. All these landforms were probably formed after the third Neoglacial, 1300-1000 years BP. Introduction Since 1990, the joint Argentine (Instituto Ant‡rtico Argentino) - Japanese (Institute of Low Temperature Science) Group ÒCriolog’aÓ has focused its research on cryological and geomorphological topics in the northern Antarctic Peninsula area. In this paper, we present results of a study in the NW part of James Ross Island (Figure 1). About 80% of James Ross Island is ice-covered, and most of this is due to the large Mount Haddington Ice Cap. This ice cap stretches over an area 40 km in dia- meter, reaching the highest point of the island at 1628 m (a.s.l.). Most of the ice-free land is located in the NW sector of the island, where it is isolated from the main ice cap. This area corresponds to a former glacial landscape carved in friable Mesozoic sedimentary rocks covered by Cenozoic volcanics. The latter, mainly basalt and pyroclastic breccias, are preserved as 300 to 900 m (a.s.l.) high remnant plateaus, separated by wide va- lleys. The steep slopes that surround the volcanic plateaus are affected by large landslides, glacier ero- sion, nivation, and snow-debris avalanches. Uninterrupted and intense frost shattering leads to rock fall, roll, slide, and creep. Figure 1. Location and geomorphological map of the NW sector of James Ross Island. Jorge A. Strelin, Toshio Sone 1027 The climatic conditions are polar arid to semiarid, and sorted patterned ground. Most of these cryogenic fea- the location of the island within the area of seasonal tures were morphologically described by Strelin and sea-ice results in maritime influences during the sum- Malagnino (1992). mer and a more continental winter season. In the study area, the mean annual air temperature at sea level is ca. The present work focuses on the morphological and -6.5¡C and the annual precipitation, mostly snow, is morphodynamic aspects of ice-cored rock glaciers estimated to be around 200 mm water equivalent. This (Potter, 1972), protalus lobes (Whalley and Martin, low annual accumulation results in large snow-free 1992) and protalus ramparts (Bryan, 1934; Ballantyne areas during much of the year. The area is affected by and Benn, 1994). The first are partially channelled in cold and wet southwesterly winds (Schwerdtfeger, short valleys and the last two are present at the foot of 1975) and warm and dry west to northwesterly valley slopes. winds (fšhn). The first description of rock glaciers in Antarctica was Small ice caps and glaciers develop respectively at the for southern Victoria Land (Mayewski and Hassinger, top and foot of the volcanic plateaus. The equilibrium 1980). On James Ross Island, close to the present study lines in this sector of the island are at a mean altitude of area, a similar landform was described and alternative- 200 m (a.s.l.). However, variation in factors such as ly called a debris-covered polar glacier and a rock gla- insolation, wind control on snow deposition, exposure cier (Chinn and Dillon, 1987). to fšhn, katabatic winds, orographic precipitation, etc., results in a remarkable equilibrium line variability Lachman II rock glacier (from 0 to 500 m). The environment described above favours the development of periglacial landforms and Six ice-cored rock glaciers develop at the east foot of deposits such as talus slopes, ice-cored rock glaciers, Lachman Crags (Figure 1). Among these, Lachman II protalus lobes, protalus ramparts, stone banked ter- rock glacier is analysed here in detail. This rock glacier races, nivation hollows, mixed snow and debris is a component of a complex geomorphic system that avalanche deposits and several types of sorted and non- has different temporal and spatial related parts. Two Figure 2. Geomorphological map of the south-east sector of Lachman Crags. 1028 The 7th International Permafrost Conference main zones are distinguished (Figure 2): accumulation The moraines comprise recessive ice-cored ridges that and ablation zones. become smoother in the direction of the front of the glacier. ACCUMULATION ZONE This zone is subdivided into a main and a secondary An active ice-cored rock glacier (Potter, 1972) extends accumulation zone. The first one involves ice and snow downvalley of the morainic sector and is obstructed by accumulation on a series of small ice caps situated on a rock glacier of a previous stage that is still active or in the top of Lachman Crags: Norte, Central and Sur ice a steady state. The moraines and rock glaciers, which caps. The second corresponds to a regenerated glacier, enclose this morphological system at its front, consti- principally nourished by ice and debris avalanches and tute the passive ablation zone. wind-drifted snow, situated at the foot of the crag along a 3 km wide front. MORPHOLOGY OF LACHMAN II ROCK GLACIER The following characteristics were observed in the ABLATION ZONE ablation zone of the ÒLachman II glacier-rock glacier Three ice lobes, partially separated by moraines, con- systemÓ (ice tongue, ice-cored moraines and rock gla- stitute the main ablation zone. The central glacier lobe ciers) (Figures 2, 3A and 3B). is the most extended, showing a frontal sector placed much lower than its surrounding moraines. In the main ablation zone, the glacier tongue surface has an average slope of 7 to 8¼. The ice foliation is Figure 3. (A) Vertical aerial photograph (November 1992), (B) topographic map, (C) morphodynamic map (interval 1992 - 1995), and (D) morphometric map of Lachman II rock glacier. Jorge A. Strelin, Toshio Sone 1029 Table 1. Parameters measured in the Lachman II rock glacier domain clearly marked, enabling its photointerpretation and The debris cover of the rock glacier is usually 0.60 to reconnaissance in the field. Sometimes the ice gives 0.80 m thick, but at the outer limit it exceeds 1 m in way to the emergence of till. The flow pattern of the thickness. Permafrost, rather than the ice core, was glacier can be traced through mapping this ice foliation. observed 1.10 m beneath the ground surface, close to E34 (Figure 3C). The central ice tongue is depressed at its front by 10 to 15 m relative to the enclosing ice-cored moraines. The characteristics of the sediments that cover the Downvalley, the moraines lose their shape while the rock glacier depend on their origin. Shattered volcanic debris cover increases. rocks are supplied from the crags and move downslope principally by supra- and intraglacial transport, where- Ridges and furrows develop where the debris on the as Cretaceous, sandy to clayey, loose sediments are glacier ice is at least 0.60 m thick. These flow-like fea- extruded from the glacier sole. This allows identifica- tures mark approximately the transition to the rock tion of the position of the deep shear zones on the rock glacier domain. The rock glacier stretches approximate- glacier surface. The biggest volcanic blocks, that cover ly 700 m downvalley and is roughly 500 m wide. The or crop out from the rock glacier front, reach tens of upper surface dips 4 to 5¼ in the flow direction and ends cubic metres in volume, but the average size does not at a steep talus apron of 24 to 42¼. During the summer, exceed 0.15 m in diameter. Where platy, mostly a stream originating at the front of the ice tongue, dis- basaltic, debris stretches over the surface, an open fab- charges water through a steep channel cut in the ric of strongly-imbricated clasts (dipping about 75¡ moraines and in the central part of the rock glacier. The upglacier) is recorded. The matrix appears approxi- release of additional meltwater is accomplished by two mately 0.20-0.30 m below the ground surface and con- ephemeral creeks that drain off the steep sides of the sists of unsorted gravel and sand. Where the rock gla- rock glacier. Conical holes, sometimes occupied by cier surface is principally nourished by finer material melt water, are spread out on the surface of the rock (sand or tuff), patterned ground (sorted nets and cir- glacier. cles) and gelifluction lobes develop. The inner fabric of these features shows imbricated gravel, suspended in a Sporadic outcrops of glacier ice are visible at steep finer, homogenous sandy matrix. slopes developed in the central channel, in some of the bigger conical holes, and at the marginal talus. The ice MORPHODYNAMICS OF THE ROCK GLACIER core has a marked foliation, steeply dipping up-valley, Flow and ablation rates measured between January with ÒblackÓ regelated ice layers, including till, alternat- 1992 and January 1995, allow four zones to be identi- ing with prevailing white glacier ice.
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