Processes Of Glacial Transportation PDF

6 PROCESSES OF GLACIAL TRANSPORTATION M. P. Kirkbride 6.1. INTRODUCTION moving ice fundamentally influences the assemblages...

6 PROCESSES OF GLACIAL TRANSPORTATION M. P. Kirkbride 6.1. INTRODUCTION moving ice fundamentally influences the assemblages of landforms deposited at different stages of the mass- The entrainment and transport of sediment by glaciers balance cycle. Two examples serve to illustrate this is an important link in the sediment cascade in high association. First, the large sediment loads and higher latitudes and at high altitudes today and, in the past, than average velocities of many warm-based valley over vast areas of the mid-latitude continents. Glacial glaciers account for their huge lateral moraines action is not only a potent erosional agent, but also formed during the Neoglacial period. These moraines provides the means by which eroded debris is provide detailed sedimentary records of global Holo- removed from its source and transported to areas of cene climatic variation (Chapter 2). Secondly, the deposition. Potential transport distances vary from a streaming of basal debris around large obstacles on few hundred metres in cirque glaciers to many the glacier bed may lead to the downstream persist- hundreds of kilometres in ice sheets. Without the ence of longitudinal ‘ridges’ in the basal transport evacuation of debris by glaciers, wear processes zone that results in the deposition of fluted moraine would not continue to attack fresh bedrock and during ice retreat. sediments to create many of the characteristics of Glacier transport is remarkable in the diverse glaciated landscapes. Most wear and subsequent manner in which the sediments themselves are erosion is performed by rock particles (clasts) in modified. Much basal debris can be drastically shaped transport at the glacier sole. and comminuted over short distances, while part of Depositional landforms equally owe much to the the debris load may travel large distances with processes of sediment delivery to ice mass margins virtually no sedimentological change. In the former and interfaces (Fig. 6.1). Where, for example, deposi- case, sustained mechanical crushing and fracture tion is areally restricted around stable margins, under the relatively low regional stresses (but large continuous sediment supply allows large moraines to tractive forces and high local stresses) beneath ice be constructed; or where glaciers overrun soft sedi- masses produce sediment quite distinct from the ment or easily eroded material, large basal ‘layers’ of products of other comminuting processes. Indeed, glacial sediment may be derived, transported and glacial abrasion may be responsible for most of the subsequently deposited. The amount, nature and silt-size rock particles created over geological time. distribution of sediment within, upon and beneath Conversely, glacial transport of large boulders over 147 148 PROCESSES OF GLACIAL TRANSPORTATION (a) ICE SHEET glacial land- and bed-forms. The understanding of the Equilibrium line principles of drift exploration and in mining placer Equilibrium line deposits (Menzies, 1996, chapters 15 and 16) in the establishment of the geotechnical character of glacial Iceberg sediments and in the decisions concerning the suita- bility or otherwise of landfill sites for the disposal of Land periphery Maritime periphery toxic and other waste products hinges upon the impact and influence of glacial transport processes (De (b) ICE SHELF Mulder and Hageman, 1989). Equilibrium line Iceberg Land ice 6.2. ICE PROPERTIES AFFECTING Sea Melting Strand crack SEDIMENT TRANSPORT (c) VALLEY GLACIER Grounding line Glaciers are commonly likened to ‘rivers of ice’. Such a comparison could not be further from the truth when Equilibrium line the mechanics of flow and the nature of sediment transport are considered. An understanding of glacial transport depends on examining those physical prop- erties of ice that have a direct bearing on how rock particles are entrained and transported. Three proper- ties are important in this context: ice temperature, Accumulation Basal Ice Velocity density and viscosity. Ablation Particle Trajectory 6.2.1. Temperature FIG. 6.1. Models of (a) ice sheet, (b) ice shelf and (c) valley glacier, showing the distribution of accumulation and ablation and related flow characteristics. Basal sliding is assumed to occur in models (a) Within any ice mass, ice temperature varies over both and (c) and is at a maximum in the vicinity of the equilibrium line time and space (Chapter 3). Ice at the base of many (from Sugden and John, 1976, Glaciers and Landscape, Edward ice masses is at or close to the pressure melting point. Arnold). Pressure melting occurs over large areas of glacier beds where overburden pressures are high (Fig. 6.1) and in localized zones in response to pressure hundreds of kilometres without modification is variations caused by substrate irregularities. In terms unique in the natural sediment cascade. of sediment transport, the distribution of basal melting It is misleading to suggest that glaciers in all parts and the presence of water affects the processes and of the world are highly erosive. Glaciers frozen to effectiveness of erosion and entrainment, and has an their substrates are incapable of significant geo- overall influence on the discharge of sediment morphic work and may even have reduced erosion through ice masses. Where the ice mass bed is frozen, rates compared with neighbouring ice-free land. later transport of previously frozen debris may occur Indeed, doubt exists as to whether warm-based resulting in ‘patches’ of debris being moved long glaciers necessarily erode and transport more material distances, in some cases, with limited comminution or than non-glacial processes in landscapes of similar evidence of deformation; or in other cases frozen relief and precipitation (Hicks et al., 1990). Never- debris, after transport, may melt out and produce a theless, processes of glacial transportation have distinctive lithofacies. At glacier surfaces, the rate of played a major role in shaping landscapes, in moving melting is strongly influenced by the distribution and vast supplies of sediment both on land and in the thickness of supraglacial debris (Paul and Eyles, oceans and in the genesis and maturation of many 1990).  PROCESSES OF GLACIAL TRANSPORTATION 149 6.2.2. Density 6.3.1.1. Weertman regelation Ice is less dense in the solid phase than in the liquid The Weertman regelation mechanism operates under phase. Implications for glacial sediment transport are warm-based glaciers at the scale of bedrock obstacles important where ice is in contact with water suffi- of