Glacial Processes and Landforms I INTRODUCTION A Definitions 1 Glacier a thick mass of flowing moving ice a glaciers originate on land from the compaction and recrystallization of snow thus are generated in areas favored by a climate in which seasonal snow accumulation is greater than seasonal melting 1 2 2 polar regions high altitude mountainous regions Snowfield a region that displays a net annual accumulation of snow a snowline imaginary line defining the limits of snow accumulation in a snowfield 1 3 Water balance in general the hydrologic cycle involves water evaporated from sea carried to land precipitation water carried back to sea via rivers and underground a water becomes locked up or frozen in glaciers thus temporarily removed from the hydrologic cycle 1 II above which continuous positive snow cover thus in times of great accumulation of glacial ice sea level would tend to be lower than in times of no glacial ice FORMATION OF GLACIAL ICE A Process Formation of glacial ice snow crystallizes from atmospheric moisture accumulates on surface of earth As snow is accumulated snow crystals become compacted in density with air forced out of pack 1 2 Snow accumulates seasonally delicate frozen crystal structure a Low density 0 1 gm cu cm b Transformation snow compaction pressure solution of flakes percolation of meltwater c Freezing and recrystallization density Firn compacted snow with D 0 5D water a b 3 With further compaction D firn ice Crystal fabrics oriented and aligned under weight of compaction Ice compacted firn with density approaching 1 gm cu cm a b Snow to firn to ice density increases with increasing depth and increasing weight of compaction Rate of snow to ice transformation function of climate 1 Humid Freeze thaw climate days to weeks a Higher snow accumulation rates 142 2 Polar climate years to hundreds of years a c Freezing Process Liquid to Solid 1 4 III Low snow accumulation rates Water to ice accompanied by x 8 increase in volume Glacial Ice a Requires humid cold climates of high snow fall 1 e g Alaska b Dry cold areas may not accumulate significant ice thicknesses 1 e g Antarctica c Ice Stratification 1 Annual accumulation layers of snow ice and dust rock debris PROCESS OF GLACIAL ICE MOVEMENT A Mechanical Properties of Ice 1 2 B Pressure Melting under great overburden of pressure melting point of ice is lowered a Pressure Gradient melting point 1 degree C for every Kg sq cm pressure b Partial pressure melting of ice at base of glacier aids in sliding motion and movement Ice Deformation Stress strain Relationships a Elastic Strain deformation directly proportion to amount of stress applied ability for strain to be removed upon removing stress b Plastic Rate strain rate of stress ice permanently deformed upon removing stress c Brittle Stress exceeds internal shear strength of ice brittle rupture fracture occurs Ice Flow Mechanisms1 Basal Sliding the interface between the ice and underlying stratum is most often the site of shear slip pressure melting and hydraulic lubrication contributing factors a b c 2 Zone of intense crushing and grindng action particularly by enclosed rock debris Notable in temperate glaciers with freeze thaw cycles Absent in polar regions where ice is frozen to underlying stratum Plastic Flow plastic deformation and lateral spreading of glacier over time a Internal Deformation at Molecular Level 1 ice behaves brittly until a thickness of 150 200 ft in which the weight of ice causes it to deform plastically and flow 143 b Areal distribution of ice flow 1 2 Glacial ice flows fastest in central portions of valley glaciers and slowest along valley walls due to friction Plastic deformation controlled by a b 3 4 3 b C Ice motion along shear fracture planes Fracture planes dip up valley 1 upward thrust of ice and debris at toe of glacier Zone of Fracture uppermost portion of glacier nearest the surface where the ice deforms brittly Ogives wavy ridges and swales on glacier surface Rates of ice flow from very slow cm s per 10 s of years to very rapid several meters day 1 glacial surging periods of time in which glaciers flow very quickly up to 180 ft day a IV Extending flow extension and plastic thinning of ice Crevassing process of ice brittly fracturing into cracks tensional stress bending stress a 5 Compressing flow pile up and plastic thickening of ice Internal Shearing a 4 Thickness and weight of ice pile Steepness of slope and tangential gravitational force possibly results from hydroplaning sliding along bottom of glacier due to increase in hydraulic lift pressure reducing friction at base of glacier GLACIAL CLASSIFICATION A Thermal Classification based on temperature of glacial ice climatically and latitudinally controlled 1 2 Temperate ice at pressure melting temperature a Meltwater common throughout body of glacier b wet glaciers 1 basal sliding surging and hydroplaning common c relatively fast glacial advance Polar ice below pressure melting temperature a b 3 Subpolar a b B Meltwater absent frozen throughout basal sliding negligible frozen ice rock base Frozen to substrate seasonal melting of upper portions Morphological Classification topographic configuration 144 1 Alpine Glaciers glaciers generally confined to mountain valleys a b c Cirque Glaciers confined to circular depressions at head of valley Valley Glaciers extend down drainage valley 1 presently found in Pacific Northwest Canadian Rockies Swiss Alps and other mountainous regions 2 High mountain elevations cold temperatures abundant seasonal snowfall Outlet Glaciers 1 d Valley glacier fed by polar ice cap Tidewater Glaciers 1 Glacial advance to coastal regions a 2 Piedmont Glaciers valley glaciers that extend to mouth of valley where ice spreads out broadly along base of mountain front a 3 Glacial Fans analogous to alluvial fans Ice Sheets and Ice Caps a Continental Glaciers massive accumulations of ice that cover extensive areas of the earth s surface 1 Unconfined glaciers a b Ice Cap in size compared to Ice Sheet 3 Present examples include Greenland and Antarctica Greenland 1 7 million sq km Antarctic 13 million sq km Highland Icefields occur in mountain areas ice accumulates in a relatively unconfined sheet through coalescence of valley glaciers e g wester Canada and Alaska Dynamic Classification of Glaciers mass balance relationships a b c V Flow controlled by internal mechanisms rather than topographic slope 2 a b 4 e g Glacier Bay Alaska Advancing Glacier Neutral