Peak Flood Glacier Discharge Percolation Zone
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PERIGLACIAL 827 Arctic system. Permafrost regions occupy approximately PEAK FLOOD GLACIER DISCHARGE 24% of the terrestrial surface of the Northern Hemisphere. Today, a considerable area of the Arctic is covered by per- Monohar Arora mafrost (including discontinuous permafrost). National Institute of Hydrology (NIH), Roorkee, UA, India Sudden release of glacially impounded water causes cata- PERIGLACIAL strophic floods (sometimes called by the Icelandic term “jôkulhlaup”), known as outburst floods, and occasionally H. M. French spawn debris flow that pose significant hazards in moun- University of Ottawa (retired), North Saanich, BC, tainous areas. Commonly, the peak flow of an outburst Canada flood may substantially exceed local conventional bench- marks, such as the 100-year flood peak, but predicting the Synonyms peak discharge of these subglacial outburst floods is a very Cryogenic difficult task. Increasing human habitation and recrea- tional use of alpine regions has significantly increased Definition the hazard posed by such floods. Outburst floods released “ ” in steep, mountainous terrain commonly entrain loose sed- Periglacial : an adjective used to refer to cold, non- iment and transform into destructive debris flows. glacial landforms, climates, geomorphic processes, or environments. “Periglaciation”: the degree or intensity to which periglacial conditions either dominate or affect a specific landscape or PERCOLATION ZONE environment. Prem Datt Origin Research and Development Center (RDC), The term periglacial was first used by a Polish geologist, Snow and Avalanche Study Establishment, Himparisar, Walery von Łozinski, in the context of the mechanical dis- Chandigarh, India integration of sandstones in the Gorgany Range of the southern Carpathian Mountains, a region now part of The area on a glacier or ice sheet or in a snowpack where central Romania. Łozinski described the angular rock- a meltwater percolates are known as percolation zone. rubble surfaces that characterize the mountain summits In case of glaciers, the upper part of the glacier (accumula- as periglacial facies formed by the previous action of tion zone) where ice is covered by snow represents intense frost (Łozinzki, 1909). Following the XI Geologi- the percolation zone. As such water percolates through cal Congress in Stockholm in 1910 and the subsequent the snowpack because snow behaves like a porous media, field excursion to Svalbard in 1911 (Łozinzki, 1912), the while in the lower part of glacier (ablation zone) water concept of a periglacial zone was introduced to refer to flows over the ice because ice is not permeable and hardly the climatic and geomorphic conditions of areas periph- allows any percolation. This is the reason the water chan- eral to Pleistocene ice sheets and glaciers. Theoretically, nels are found in the ablation part of the glaciers where this was a tundra zone that extended as far south as the exposed ice surface is available. tree-line. In the mountains, it was a zone between timber- line and snow line (Figure 1). Today, Łozinski’s original definition is regarded as unnecessarily restricting; few, if any, modern analogs exist PERENNIALLY FROZEN GROUND (French, 2000). There are two main reasons. First, frost action phenomena are known to occur at great distances Monohar Arora from both present-day and Pleistocene ice margins. In National Institute of Hydrology (NIH), Roorkee, UA, fact, frost action phenomena can be completely unrelated India to ice-marginal conditions. For example, parts of central Siberia and interior central Yukon remained unglaciated Perennially frozen ground occurs wherever the ground during the Pleistocene, yet these are regions in which frost temperatures remain continuously below 0C for 2 or action was, and is, very important. Second, although more years. Most permafrost is located in high latitudes Łozinski used the term to refer primarily to areas rather (i.e., land in close proximity to the North and South poles), than processes, the term has increasingly been understood but alpine permafrost may exist at high altitudes in much to refer to a complex of cold-dominated geomorphic pro- lower latitudes. The extent of permafrost can vary as the cesses. These include not only unique frost action and per- climate changes. Permafrost, or perennially frozen mafrost-related processes but also the range of azonal ground, is a critical component of the cryosphere and the processes, such as those associated with snow, running 828 PERIGLACIAL X7 X6 Ice X5 Periglacial zone X4 Relict X3 periglacial zone X2 X1 (1) Theoretical limit of (2) Pleistocene periglacial (3) Present-day periglacial zone periglacial zone, as zone, displaced includes (a) climatically determined by climate southward and induced periglacial zone and peripheral to ice sheets (b) relict periglacial zone: X1–X7 = Climate zones northern part of boreal X4 = Treeline forest zones a X6 = snowline ∗ ∗ ∗ ∗ Continuous Timberline Continuous Discontinuous (Timberline) Widespread Discontinuous Periglacial zone Periglacial zone Patchy Sporadic Permafrost ∗ Snow and ice b Periglacial, Figure 1 Schematic diagram illustrating the concept of the periglacial zone in (a) high-latitude and (b) high-altitude (alpine) areas. (From French, 2007.) water and wind, which demand neither a peripheral ice- Modern periglacial geomorphology is a branch not only marginal location nor excessive cold. Instead, they assume of mainstream geomorphology but also of permafrost sci- distinctive or extreme characteristics under cold, non- ence or geocryology (Washburn, 1979; Romanovskii, glacial (i.e., periglacial) conditions. 1980; Williams and Smith, 1989; Yershov, 1990; Zhou et al., 2000). Periglacial areas are regarded as cold-climate Historical context “zones” in which seasonal and perennial frost, snow, and Periglacial geomorphology developed in a relatively normal azonal processes are all present to greater or lesser rapid fashion in the 2 decades after 1945 as a branch of degrees (French, 2007). The reality is that many so-called a European-dominated, but somewhat unscientific, cli- periglacial landscapes inherit the imprint, in varying matic geomorphology. It was aimed largely at Late- degrees, of previous glacial conditions. Pleistocene paleo-climatic reconstruction. This changed in the latter half of the twentieth century when isotopic Extent and significance of periglacial dating techniques and the explosion of the Quaternary sci- environments ences came to dominate paleo-environmental reconstruc- Periglacial environments are restricted to areas that experi- tion. At the same time, the growth of permafrost studies ence cold, but essentially non-glacial, climates. They occur in Arctic North America and the emergence of Russian not only as tundra zones in the high latitudes, as defined geocryology liberated periglacial geomorphology from by Łozinski`s concept, but also as forested areas south of its Pleistocene heritage. tree-line and in the high-altitude (i.e., alpine) regions of PERIGLACIAL 829 mid-latitudes (Figure 2). They include (a) the polar deserts Permafrost and/or intense frost action would have charac- and semi-deserts of the High Arctic, (b) the extensive tun- terized an additional 20–25% of the earth’s land surface at dra zones of high northern latitudes, (c) the northern parts some time during the Pleistocene. of the boreal forests of North America and Eurasia, and As regards human occupance, the periglacial environ- (d) the alpine zones that lie above timberline and below ments are relatively sparsely populated. A reasonable esti- snowline in mid-latitude and low-latitude mountains. To mate is just seven to nine million people, mostly living in these must be added: (a) the ice-free areas of Antarctica, Russia, or only 0.3% of the world’s population. Thus, the (b) the high-elevation montane environments of central larger importance of periglacial environments lies not in Asia, the largest of which is the Qinghai-Xizang (Tibet) their spatial extent, their snow and ice, or their proximity Plateau of China, and (c) small oceanic islands in the high to glaciers but in their environment and their natural latitudes of both Polar Regions. resources. For example, the Precambrian basement rocks Periglacial environments occur over approximately that outcrop as huge tablelands in both Canada and Siberia one quarter of the Earth’s land surface. During the Pleisto- contain precious minerals, such as gold and diamonds, and cene glacial periods, large areas of now-temperate mid- sizable deposits of lead, zinc, and copper, while the sedi- latitude experienced reduced temperatures because of mentary basins of western Siberia, northern Alaska, and their proximity to the continental ice sheets and glaciers. the Canadian High Arctic contain large hydrocarbon Limit of continuous permafrost Limit of discontinuous permafrost Limit of sporadic permafrost Treeline Glaciers Alpine periglacial zone Subarctic- maritime periglacial zone Subarctic- continental periglacial zone Boreal periglacial zone Tundra zone Arctic frost- debris zone High arctic frost- debris zone 0 500 1000 1500 km Periglacial, Figure 2 Map showing the extent of the current periglacial domain in the northern hemisphere. Not included are the alpine areas of mid-latitude mountains and the high-altitude montane environment of central Asia. (From Karte and Liedtke, 1981. Reproduced in French, 2007.) 830 PERIGLACIAL reserves. In the more distant future, the exploitation of gas The surface offset reflects primarily