Unit 3 – Gradational Processes Glacial Processes Unit 3 – Gradational Processes GG282 2006

Glacial Processes Types of Glaciation Mass Balance Temperature Motion Water Erosion GG282 2006

Types of Glaciation Continental Alpine Enormous sheets of ice, up to 2km thick, that carve and rework land masses Responsible for most of the features we see in Canada Niagara escarpment, Great Lakes, hills in Peterborough area, bump with the Shneiders sign (401 before Guelph), Oak Ridge Moraine… Alpine Smaller scale and still occurring in many high elevation areas Temperature is cold enough that snow does not melt during the summer, the accumulated snow gradually turns to ice, forming an alpine glacier Many in the Western Cordillera and Innuition Mountains

Glacial Landscapes Laurentide and Cordilleran Ice Sheets (18 kaBP) GG282 2006

Introduction: Glacial Landscapes Late 20th century distribution of glacial ice is shown in Fig 7.3 GG282 2006

Late Wisconsinan Glaciation in Canada Consider the last glaciation Decay of the Laurentide Ice Sheet (Fig 7.9) GG282 2006

Late Wisconsinan Glaciation in Canada Great Lakes region note the changes in ice position and the drainage GG282 2006

Late Wisconsinan Glaciation in Canada Prairie region Ice positions, drainage GG282 2006

Late Cenozoic Glaciations Depositional Feature GG282 2006

Glacial Landscapes: Models Movement of ice inferred from streamlined features (Fig 7.8) note Keewatin and Quebec spreading centres GG282 2006

Glacial Landscapes: Models Erosion Zone Transition Active Zone Wastage Zone GG282 2006

Late Cenozoic Glaciations Theory for the cycles of glacial periods: Milenkovich Cycles Causes Climate Change is required to generate the conditions needed to produce large ice sheets in the mid-latitudes for example at the height of the last glaciation, global temperature was ~6oC cooler than the 20th century average at the time and spatial scales in question there are several causes: mainly tectonics and orbital forcing GG282 2006

Late Cenozoic Glaciations Causes Tectonics (slow gradual change) Movement of large land masses into both polar regions (albedo effects) Closing of the Arctic ocean (ocean and albedo effects) Mountain building (uplift) Gradual removal of carbon dioxide from the atmosphere by chemical weathering processes on land GG282 2006

Late Cenozoic Glaciations Causes Tectonics (slow gradual change) due largely to tectonics and weathering there was a slow and gradual cooling through the Tertiary GG282 2006

Late Cenozoic Glaciations Causes Orbital Forcing The Earths orbit around the Sun varies in its eccentricity, also the Earths axis changes in its angle of tilt and ‘wobbles’ These changes cause the Earth to receive different amounts of radiation from the Sun at the time scales of tens of thousands of years GG282 2006

Late Cenozoic Glaciations Causes Orbital forcings cause variations in temperature that operate at the time scales of tens of thousands of years, they are superimposed on climate changes caused by other factors GG282 2006

Glacial Processes: Mass Balance Dobbin Bay, Ellesmere Island, Nunavut, July 2000 Accumulation and Ablation Zones Equilibrium Line GG282 2006

Mass Balance Alpine and Continental Ablation and Accumulation Zones GG282 2006

Mass Balance Accumulation Processes Snowfall Rainfall (freezes in glacier) Condensation Drifting Snow and Snow Avlanches (in alpine environments) GG282 2006

Mass Balance Ablation Processes Dobbin Bay Melting Evaporation Sublimation Streams Calving Dobbin Bay GG282 2006

Mass Balance Mass Balance is the budget between accumulation and ablation May be determined for a part of a glacier or the entire glacier For the whole glacier: when positive: glacial advance when negative: glacial retreat GG282 2006

Mass Balance Techniques Direct Measures Indirect Measures Snow and ice stakes Cores, trenches, surveying Indirect Measures Remote Sensing Aerial Photography Satellite Imagery GG282 2006

Mass Balance Global warming??? Global Data from National Snow and Ice Data Center State of the Cryosphere GG282 2006

Mass Balance Bhutan Example GG282 2006

Glacial Processes: Ice Temperature Temperature (Thermal Regime) Importance of ice temperature Influences Motion and Erosion Controls on Ice Temperature Surface Air Temperature Snowfall Geothermal Heat GG282 2006

Ice Temperature Basal Thermal Regime Cold Based or Polar Glaciers Ice at the bottom of the glacier Melting Point is depressed at high pressures Cold Based or Polar Glaciers Basal Ice is frozen to bed Warm Based or Temperate Glaciers Basal Ice is at or near the pressure melting point GG282 2006

Glacial Processes: Motion Ice Velocities are Variable between several metres to hundreds or thousands of metres per year influenced by several variables ice thickness slope ice temperature water GG282 2006

Motion Glaciers move in response to stress where Shear Stress (τ) τ = ρgh sinβ where τ = basal shear stress ρ = density of ice g = acceleration due to gravity h = ice thickness β = slope of ice surface GG282 2006

Motion Stress causes the ice crystals to deform and recrystallize crystals slide (glide) past one another This process of motion is called Creep GG282 2006

Motion Mechanisms Creep Sliding Enhanced Basal Creep Regelation Basal Slip Bed Deformation GG282 2006

Motion Sliding Enhanced Basal Creep creep rate increases around bed obstacles because: GG282 2006

Motion Sliding Regelation net transfer of ice downslope due to melting and refreezing of basal ice around bedrock obstacles GG282 2006

Motion Sliding Basal Slip Bed Deformation slipping of basal ice on bed aided by liquid water at the bed Bed Deformation Materials in the bed may deform and carry the glacier along more common when bed is comprised of non-frozen saturated sediments GG282 2006

Motion Extending and Compressive Flow extending (accelerating flow) compressive (decelerating flow) GG282 2006

Motion Surging Sudden increase in velocity to potentially km per year may occurs where ice is warm based may be related to an increase in the amount of basal water reduces friction: causes basal slip GG282 2006

Motion Crevasses and Structures Crevasses may develop where the ice fractures ablation zone GG282 2006

Glacial Processes: Water Water influences motion and erosion processes Water is more important in ablation zones (particularly for temperate ice) Supraglacial Englacial Subglacial GG282 2006

Glacial Processes: Erosion Erosional Processes Abrasion (scour) Plucking (quarrying) Fracturing Meltwater GG282 2006

Glacial Erosion: Abrasion Abrasion rate is influenced by: basal sliding velocity high velocity = higher abrasion frequency, size, and hardness of the clasts a relatively small number of large, angular, hard clasts are most effective the efficient removal of fine materials (silt) by meltwater at the bed abundant fine material reduces contact with the bed GG282 2006

Glacial Erosion: Abrasion Distribution of Abrasion? Evidence of abrasion: striations and related forms GG282 2006

Glacial Erosion: Plucking Plucking (quarrying) pressure distribution at bed (Fig 5.7) GG282 2006

Glacial Erosion: Plucking Plucking (quarrying) on up ice side of obstacle (b) there is scour and abrasion and partial melting on down ice side of the obstacle (c) there is freezing of water (thus regelation) and the erosion of broken fragments in a process called plucking (quarrying) the clasts may be entrained into the ice (frozen in) or moved by meltwater GG282 2006

Glacial Erosion: Plucking Evidence of Process basal ice (regelation ice) often has a high debris content GG282 2006

Glacial Erosion: Plucking Evidence of Process variety of landforms result from plucking (stoss and lee forms) GG282 2006

Erosion and Ice Temperature Erosion most effective under warm ice, there is very little or no abrasion under cold ice Distribution of erosion may change with changes in temperature GG282 2006

Glacial Processes Entrainment: How is debris incorporated into ice: debris may be picked up (entrained) in basal ice as regelation occurs materials can be thrust up into the ice when subglacial cavities fill with water in alpine environments materials may be deposited on the ice and move into the glacier through crevasses and moulins GG282 2006

Glacial Processes Transport Materials may be carried (transported) at the bed or within the basal ice (subglacial environment) within the glacier (englacial environment) on the surface of the glacial (supraglacial environment) GG282 2006

Glacial Sediments (Deposits) GG282 2006

Glacial Deposits Depositional Environments Glacial Till Glaciofluvial deposits Glaciolacustrine, Glaciomarine deposits GG282 2006

Depositional Environments Materials may be deposited beneath the glacier (subglacial) at the margin or on the surface of the glacier (supraglacial) as the ice melts out (e.g. englacial materials deposited at bed) into bodies of water near the ice into stream channels draining the ice GG282 2006

Depositional Processes: Glacial Till Till may be deposited beneath an actively sliding glacier through a process called Lodgement (Lodgement Tills) Till may be deposited as ice melts out producing Melt Out Tills (Ablation Tills) GG282 2006

Glacial Till GG282 2006

Depositional Processes Materials may be deposited by glacial ice directly by flowing water GG282 2006

Glacial Deposits: Glaciofluvial Glaciofluvial (or fluvioglacial or outwash) sediments are materials that have been transported and deposited by glacial meltwater streams These streams may be subglacial, supraglacial or proglacial GG282 2006

Glaciofluvial Deposits What are the general characteristics of glaciofluvial materials? GG282 2006

Glaciofluvial Deposits What are the general characteristics of glaciofluvial materials? sand and gravel moderately well sorted (can vary) stratified (beds, laminations, structures that indicate deposition by flowing water) GG282 2006

Glaciolacustrine and Glaciomarine deposition of glacial sediments in a lake (standing water) Glaciomarine deposition of glacial sediments in a sea (standing water) GG282 2006

Glaciolacustrine and Glaciomarine Environment of Deposition (Figs 5.10, 5.11) GG282 2006

Glaciolacustrine and Glaciomarine Materials are delivered to the lake or sea by: streams draining the glacier glacial ice melting in the water body icebergs that carry materials out into the lake or sea GG282 2006

Glaciolacustrine and Glaciomarine Characteristics of Glaciolacustrine and Glaciomarine sediments GG282 2006

Glaciolacustrine and Glaciomarine Characteristics of Glaciolacustrine and Glaciomarine sediments mixtures of coarse and fine material near the glacier on the lake or sea bed, far removed from the glacier, the materials are fine grained, well sorted and stratified (layered) GG282 2006