GEOMORPHOLOGY EXAM #3

Transcription

1 Formation of Glaciers GEOMORPHOLOGY EXAM #3 - Transformation of snow into glacial ice - Density; SNOW = g/cc FIRN(Neve) = g/cc (Pellets) GLACIAL ICE = g/cc - Firn / Ice Boundary = change to glacial ice (50 m 160 meters deep) Fabric Types; - Pressure Melting Point melting point decreases with increased pressure. THERMAL Temperate (Warm Based) ice at or near pressure melting point through its thickness. -At pressure melting point through entire thickness. -Upper few meters frozen during winter months - Wet Glaciers melt water at the base and throughout the entire glacier. Polar (Cold Based) ice is entirely below pressure melting temperature. -Plastic flow slow, No basal sliding -Only Ablation is calving, wind erosion, and sublimation -Velocity slow (few meters a year) -Meltwater and Basal sliding are absent (no bedrock erosion, no outwash plains) Subpolar frozen to their substrate, surface melting occurs in Summer. -intermediate between temperate and polar glaciers -have characteristics of both Polar & Temperate Glaciers DYNAMIC Active vs. Stagnant Active High Rates of Movement Passive Low Rates of Movement Dead No Movement, inactive, dormant - Glacier movement rates greater near Upper Center of glacier - Thickness, Temperature, Slope, and Basal Water Pressure affect Velocity - Max Velocities happen at the Firn Line (equilibrium line) - Accumulation / Ablation Zones

2 - Plastic Flow low levels of stress cause the ice to flow as plastic, ice spreading out near margin. Types; - Infinite Yield Stress will flow at only 1 bar of stress, once elastic is reached. Interganular Shifting rotation & sliding between x ls Intraganular Shifting basal plane gliding (deck of cards, parallel/perpendicular) Recrystallization melting and refreezing down-ice - Basal Sliding greatest in Temperate Glaciers, thin glaciers with high slopes. Relatively small % of total movement, Often a stick-slip jerky motion. Greatest % of rocks at the bed, creates crescentric gouges, chatter marks, striations, ect. MORPHOLOGICAL often related to topographic influence. - Alpine small cirque glaciers limited to area near firn line more frequent on N & E facing slopes in the NH valley glaciers flow beyond their cirques - Piedmont valley glaciers that extend into valley floor below broad radiating lobes - Ice Sheet / Ice Cap Large glaciers not confined by topography Ice Sheet large Greenland (1.7 million sq km.) Antarctic (13 million sq km.) Ice Cap small >50,000 sq km. (Baffin Island,CAN. & Iceland) Dynamic Classification of Glaciers Mass Balance relation between accumulation/ablation -Advancing -Retreating -Stationary

3 Budget of a Glacier Zone of Accumulation freezing, adding to a glacier Zone of Ablation - melting, removing of a glacier Firn snow line, (equilibrium line) may not necessarily be the same, but usually are. Accumulation All processes by which snow & ice are added to the glacier. -Snow -Ice/Snow Avalanches -Drifting Ablation All processes snow & ice are lost -Melting -Evaporation -Sublimation (evap., 1% loss) -Calving (water depth & movement rate) -Wind Erosion -Heat (comes from solar, geothermal, air, rain) Net Mass Balance Difference in annual budgets of accumulation/ablation Accumulation/Ablation equal, mass balance = 0 Accumulation > Ablation balance is positive Accumulation < Ablation balance is negative - Terminus position directly affected by balance Accumulation = Forward, Ablation = Retreat - Climatic response time Lags behind Compressive/Expansive Flow Internal Shearing Occurs in upper 30 m or near terminus along Planes of slippage. Mechanisms for bringing basal debris to surface. Compressive Flow develops Crevasses

4 Glacier Surges rapid advances Dramatic thinning of ice Rapid movement Mechanisms of Glacier Surges; Rigid bed Hypothesis closure of subglacier drainage tunnels, increase H 2 0 P Deformable bed Hypothesis bulge develops in lower reaches of glacier, boundary between cold/warm based zones. H 2 0 P builds, sediment is slurry, and thickening leads to increase of basal shear stress. Glacial Hydrology Temperate Glaciers May cause surges; Frozen Toe Causes water pressure to build Floats Ice reduces effective pressure Surges often associated with Jokulhlaups (Iceland term) glacial outburst floods Ogives arrhythmic banding of differing tone and color, has seasonal/annual ridges. Ridge = 1 years ice flow Differing Colors = Winter / Summer Crevasses

5 Glacial Erosion Abrasion Plucking division based on 2 observations; - Interpretation of Roches Moutinees One side is smooth/polished, the other is jagged/blocky. - Component of glacial debris (Rock floor & Rock fragments) Abrasion Bedrock is scoured by debris carried in basal layers of ice. Evidence; Striations Rock Flour Variables; Requirements: Basal debris, Sliding basal ice, Transport of debris toward bedrock. Factors: Ice thickness, Basal water pressure, Hardness, Particle characteristics, removal of Rock Flour. Regulation incorporation of water refreezing under ice, pressure melting causes an incorporation of debris. (esp. at high pressure points) Rat-tails Striations Glacial Grove Erratic Cresentric Fracture Cresentric Gouge

6 Cirques bowl shaped top, surrounded by headwalls. Tarn lake that occupies a Cirque. Pater-Noster Lakes Tarns connected by a stream. (String of beads) Horn steep peak Arête steep walled ridge separating 2 glacial valleys, sharp, Jagged. Col separation of 2 glaciers between 2 back to back Cirques. TYPES of Erosion Abrasion large blocks in basal ice Rock Fracture large blocks in basal ice Joint Exploitation hard jointed rock, suitable bedding Entrainment (ice pressure) large fragments Entrainment (regulation) small fragments Meltwater Erosion impermeable bedrock, soluble bedrock Meltwater Evacuation impermeable bedrock Moraine ridge of glacial sediment, layers or ridges of till. Lateral Moraine sides of glacier, alpine only, parallel to flow. Medial Moraine middle of glacier, alpine only, parallel to flow. End Moraine abalation = accumulation, a location where the glacier was at equilibrium for a while, composed of sediment/till. Perpendicular to floods. Terminal or Recessional. Ground Moraine flat broad layer of sediment 1-10 meters high, near head of glacier, composed of sediement/till. No orientation, can happen in all glaciers except polar. Plastered under glacier. Interlobate Moraine- confluence of 2 glacial lobes where the Dumping of large boulders makes a valley, lakes, and steep topography. Rogen Moraine- shearing at base of ice. Cavity filled with sediments where ice sheared upward to clear an object. (Occurs in bunches) Ice Shove Moraine (Push Moraine)- unconsolidated sediments (vertical), sand was frozen Ice then advanced, bulldozing the sediment vertical. Perpendicular to flow. Ice Shove Moraine Rogan Moraine

7 Glacial Depositional Landforms Kettle depression in outwash plain Kettle Lakes depression filled with water in the outwash plain, where a block of ice stuck in the ground melts. Kettle Chains bumpy topography, Glaciers melt, covers area in sediment, But melt water collects in depressions. Outwash Plain gently sloping, braided streams, sand & gravel, well sorted, good gravel pits. Composed of stratified drift deposited by melt water leaving the glacier. Consists of sand, gravel. Stratification, alternating of size, Cross bedding can see flow directions. Kame accumulations of sand/gravel, conic shape, where debris gathered after moulins dumped it within the glacier. Ice Contact Kame Delta- if stream came out of the glacier and formed a delta. Kame Terraces- deposits of meltwater streams flowing between the ice and the adjacent valley side. Esker sub-glacial stream deposits within the glacier, elevated because of its tunnel-like shape, good gravel pits. Drumlin teardrop shape, range in size 0-5 meters, mile or so long, usually in a drumlin field, 5-10 km up ice from End Moraine. Current Hypothesis = water at bed of the glacier is responsible. Or an obstruction, collects material behind it as the glacier passes. Streamlined sub-glacial landform. Can form from pencil like to blunt. Contain till & outwash both. Rock Drumlin- caused an obstruction to the ice flow. Crag-and-Tail- A mass of rock lying in the path of a glacier which protects the softer rock in the lee beyond it. Flutes low linear pencil like ridges.

8 Glacial Sediments (drift) Unsorted & Unstratified Till- laid directly by glacial ice, not reworked. Formed underneath at margins. Can be rocky around Moraines. (Till Lodgement)Basal Till sediments at the bottom of glacier. Basal or Lodgement Ablation Melt-out Flow Sorted & Stratified (layered) Glaciofluvial (Fluvioglacial) Till Diamicton any unsorted or unstratified mix of sediment. Basal Ablation, Flow Glaciomarine, Glaciolacustrine Poorly sorted, not stratified, compacted, faceted, polished, stratified stones, fabric, Erratic lithologies, deformed sediments Glaciomarie- poorly sorted, lacks stratification, marine shells, polished, faceted, striated clasts, mollusks berried in growth position, preservation of delicate ornamentation. Plunge Pool a glacial front in a spot where heavy flow dumps(waterfall) onto the outwash plane. Isostatic Rebound weight of the glacier pushed land down, but now is rebounding back to up. WhaleBacks typical in hummocky terrain, small isolated hill, looks similar to a whales back. Hummocky Glacial Terrain rolling ridges, irregular lakes, depressions, kettles

9 Ice-Walled Lake Plain a glacial lake that had ice form its shoreline, when the glacier melted a flat plain resided. Varves(in glaciers) Rhythmites Alternating layers Summer = coarser, lighter colors (because flow increases and more sediments are moved) Winter = finer silts, darker Thickness is reliant on the stream. Varve Rhythmites Continental Glacier large Alpine Glacier small

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