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Chapter 15. Periglacial Processes in Glacial Environments

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Chapter 15. Periglacial Processes in Glacial Environments CHAPTER PERIGLACIAL PROCESSES IN GLACIAL ENVIRONMENTS 15 W. Pollard McGill University, Montreal, QC, Canada 15.1 INTRODUCTION The geologic record contains evidence of a planetary history that has seen the Earth’s climate fluc- tuate on geological time scales between conditions that were much warmer than present and other periods when it was considerably colder. These fluctuations between ‘greenhouse’ and ‘icehouse’ conditions are the result of complex global and astronomical processes. During an icehouse interval a combination of astronomical, tectonic, and geochemical events lead to global cooling and a subsequent accumulation of ice on land and in the oceans at higher latitudes and altitudes. These factors and their feedback systems exist in a complex cause and effect relationship that remains poorly understood. As highlighted throughout this volume an icehouse Earth is characterized by continental-scale ice sheets, ice caps, valley glaciers, and ice shelves that advance and retreat in cycles known as glacial and interglacial periods. However, during an icehouse regime cold temperatures extend well beyond the geographic limits of glaciation to create a periglacial zone dominated by frost action, frozen ground, snow, and various forms of nonglacial ice. Given that many of the criteria used to define ice house conditions are currently present, such as large ice sheets and widespread glacial activity, together with the fact that the Earth recently experienced full glacial conditions it can be concluded that the Earth is still under the influence of an ice house regime. In addition to glaciers and ice sheets, other conditions that define an icehouse regime such as widespread sea ice, seasonal/perennial snow cover, and frozen ground (permafrost and ground ice) are also currently active. The area of the Earth’s surface where water persists in a frozen state is the called the cryosphere (Barry and Yew Gan, 2011). The cryosphere includes all aspects of Earth’s environment dominated by cold climate (both seasonal and perennial) and include places where snow, lake and river ice, sea ice, glaciers, ice caps, ice sheets, ice shelves, and icebergs as well as the various types of cryotic ground (Fig. 15.1). The cryosphere concept provides a useful perspective because it includes both glacial and periglacial (cold nonglacial) systems. This chapter focuses on geomorphic processes and features that characterize areas dominated by periglacial conditions and their relationship with glacial environments. Fig. 15.1 shows the current extent of the Earth’s cryosphere and highlights relationships between areas dominated by ice sheets/glaciers and periglacial processes linked to permafrost, frost action, and snow cover. The fol- lowing discussion focuses on geomorphic features associated with the periglacial zone with an emphasis on their relationship to past and present glacial systems. Past Glacial Environments. DOI: http://dx.doi.org/10.1016/B978-0-08-100524-8.00016-6 © 2018 Elsevier Ltd. All rights reserved. 537 538 CHAPTER 15 PERIGLACIAL PROCESSES IN GLACIAL ENVIRONMENTS FIGURE 15.1 Map of the global cryosphere showing the distribution of glaciers and ice sheets, ice shelves, sea ice, permafrost, and snow. Based on information from the World Meteorological Organization. 15.2 COLD NONGLACIAL ENVIRONMENTS The cryosphere is not only distinguished by the presence of various forms of ice and snow but also by processes and landforms related to cold subfreezing (cryotic) temperatures. The global pattern of climate tends to reflect a strong zonal bias driven by the latitudinal variation in insolation. The negative radiation balance of the Earth’s higher latitudes helps sustain air temperatures conducive to the freezing of free water (frost action). Since geomorphology is concerned with the study of landforms, landscapes, and their genetic processes, it follows that cryospheric geomorphology can be divided into two broad categories; glacial and periglacial geomorphology. As discussed through- out this volume the geomorphology of glacial and glaciated landscapes reflects the processes and landforms directly related to the action of ice sheets and glaciers. Glacial geomorphology is a com- plex science focusing on the dynamics of flowing ice masses and their ability to erode, transport, and deposit rock and sediments as well as processes and landforms not directly related to the action of glaciers like glacial fluvial and glacial lacustrine activity. Some cryospheric landscapes owe their 15.2 COLD NONGLACIAL ENVIRONMENTS 539 origin to cold nonglacial conditions and processes; for example, frozen ground and frost action linked to cryotic temperatures and freezing soil moisture and groundwater. Under full glacial condi- tions a proglacial belt of cold nonglacial conditions will parallel the limit of active glaciation. In some cases these ice-free areas may be completely surrounded by ice (e.g., Beringia); however, most of the time it forms a broad zone that transitions from intense cold and frozen ground to pro- gressively more temperate environments where frost action and permafrost are replaced by seasonal frost and snow. In addition to subfreezing (cryotic) conditions the ‘periglacial zone’ may also be extremely dry and prone to desert conditions that subsequently influence patterns of vegetation and geomorphic processes. Areas adjacent to large ice sheets experience dry gravity (katabatic) winds flowing off the ice sheet that drive various aeolian processes and erosion. As glacier ice retreats the belt of cold periglacial conditions tends to shift with it. As Laurentide Ice disappeared from conti- nental North America it was replaced by a zone of widespread permafrost. Fresh glacial sediments and drift-covered landscapes are susceptible to rapid change linked to slope process, glacial melt- water, frost action, and wind erosion. As ice sheets retreated from their last maximum position large amounts of glacigenic material were and continue to be reworked and redeposited. This area of accelerated geomorphic activity is termed ‘paraglacial’ (Ryder, 1971a). Paraglacial and progla- cial are complementary terms that refer to areas adjacent to active and retreating glacial ice, whereas the term periglacial refers specifically to cold nonglacial conditions (French, 2007). Hence the presence of glaciers is not a prerequisite for periglacial conditions although periglacial condi- tions occur adjacent to glacial systems due to the pervasive cold that drives both systems. 15.2.1 PERIGLACIAL ENVIRONMENTS The term ‘periglacial’ describes terrain conditions, geomorphic processes and landforms that result from climates subject to prolonged and intense freezing conditions irrespective of proximity to gla- ciers. Periglacial environments are areas where landforms and geomorphic processes reflect the cumulative effects of cold subfreezing temperatures, cyclic freezing and thawing of sediments, and the volumetric expansion of soil moisture as it freezes. The defining criteria for periglacial environ- ments include: (1) intense frost action, and/or (2) the presence of permafrost. Today, periglacial conditions affect up to 35% of the Earth’s land area and have its greatest presence in the northern hemisphere (French, 2007; Williams and Smith, 1989). The term periglacial was proposed by the Polish geologist Walery von Lozinski in 1909 to describe weathering processes responsible for the widespread shattered rock surfaces in the Carpathian Mountains (French, 1996). Lozinski also introduced the concept of a ‘periglacial zone’ to describe climatic and geomorphic regimes peripheral to the Pleistocene ice sheets (Washburn, 1973). The periglacial concept has gone through a series of contextual changes; originally the periglacial concept was firmly rooted in climatic geomorphology and was constrained by geo- graphic proximity to glaciers and ice sheets. Climatic geomorphology was popular in the 1930s and 1940s and equated landscapes with climate. It was predicated on the idea that climate regime, mainly seasonal patterns and extremes in temperature and precipitation, control the geomorphic nature and intensity of process, which in turn control landform development. Even though climate remains a defining variable, the current focus of periglacial geomorphology is on the mechanics of heat flow, ice formation, the properties of ice, freezing and thawing, and the dynamic interaction between these processes and various distinctive landforms. Thorn (1992) unsuccessfully tried to 540 CHAPTER 15 PERIGLACIAL PROCESSES IN GLACIAL ENVIRONMENTS equate the term periglacial with processes and landforms associated specifically with ground ice. However, the periglacial environment remains synonymous with areas characterized by frost action and/or permafrost (French, 2007). Subtle differences in the meaning of the terms frozen (i.e., the solid phase of water) and cryotic (temperatures below 0C) have led to the popular use of the term geocryology (Washburn, 1979; Williams and Smith, 1989; Yershov, 1998). There is considerable conceptual overlap between the terms permafrost and geocryology in that both focus on the frozen and cryotic condition of earth materials. 15.2.2 PARAGLACIAL ENVIRONMENTS Paraglacial geomorphology is ‘the study of earth-surface processes, sediments, landforms, landsys- tems and landscapes that are directly conditioned by former glaciation and deglaciation’ (Ballantyne, 2002, p. 1935).The term paraglacial was formally introduced by Ryder (1971a, 1971b) to characterize the
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