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Chapter 19 Glaciation
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Overview Glacier Types Glacial Processes Landforms
Periglacial Landscapes & Permafrost Paleoclimatology & Ice Ages
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Glaciers Large mass of ice resting on land or floating as an ice-shelf next to land Form by continual accumulation of snow Move slowly under the pressure of their own weight Gravity driven Move in stream-like patterns Form in high latitudes and high altitudes Snowline – lowest elevation where snow can survive year-round Two categories of glaciers: Alpine Continental
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Alpine Glaciers Glaciers found within mountain ranges
Several subtypes exist: Valley glacier – river of ice confined within a valley Cirque glacier – originate in a bowl-shaped recess called a cirque (erosional landform at the head of a valley) Piedmont glacier – formed where several valley glaciers pour out of their confining valleys Tidal glacier – ends in the sea; calving occurs to form floating pieces of ice
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Icebergs Form wherever glaciers meet oceans, bays, or fjords
Inherently unstable Center of gravity shifts with melting & breaking Typically 14% is visible above water
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Continental Glaciers A continuous mass of ice as opposed to individual channels of ice Larger in scale than alpine glaciers Antarctica & Greenland Ice Sheet – most extensive form of a continental glacier Drained by ice streams Rapidly moving, solid ice streams formed around a continental glacier periphery Moves to sea or lowlands
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Ice Caps Found on continental glaciers Circular in shape
Cover areas less than 50,000 square km Completely buries the underlying landscape Iceland
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Ice Fields Not extensive enough to form the dome of an ice cap
Has an elongated pattern in a mountainous region Patagonian Ice Field in Argentina
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Glacial Processes Glacial ice formation shares similarities with rock formation Freezing of ice is similar to cooling of igneous rock Ice & snow build in layers similar to sedimentary rock Snow and ice are transformed under compaction and crystallization like metamorphic rock Known as Glacial Ice Essential element to formation is snow that accumulates in a snowfield Melt & rain water refreeze to form new layers of glacial ice Firn, a granular compacted snow, develops as a result of surviving the summer
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Mass Balance Inputs: snow Outputs: ice, meltwater, water vapor
Input in the accumulation zone Ends at the firn line Summer snow & ice did not melt Outputs: ice, meltwater, water vapor Occurs at a glacier's lower end Surface & internal melt Ice removal via wind, calving of blocks, sublimation Collectively known as ablation Equilibrium line – zone where accumulation gain balances ablation loss Glacier grows larger during colder periods Glacier retreats during warmer periods
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Glacial Movement Glacial ice behaves in a plastic (pliable) way
Distorts and flows in lower portions as a direct result of weight and pressure from above Rates of movement can vary from practically nothing to a couple of kilometers per year Upper portion is brittle and prone to cracking These cracks form crevasses These result from friction with valley walls or tension from stretching or compression Basal slip occurs between the upper and lower layers of the glacier movement Upper portion flows ahead of the lower portion
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Erosion from Glaciation
Glaciers haul debris for deposition Rocks, sediments, etc. Mechanically plucks rock material for transport Can be done on the surface & internally Glaciers can scour and smooth the landscape Process of abrasion Produces smooth surfaces on exposed rock Shines with glacial polish when the glacier retreats
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Glacial Landforms Erosional features of alpine glaciers
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Glacial erosion removes regolith & soil
As cirque walls erode, sharp ridges form that divide cirque basins Ridges are called aretes (French for knife-edge or fish-bone) Horns form as a result when cirque glaciers gouge a mountain summit on all sides Pyramidally shaped Matterhorn in Swiss Alps Glaciated valleys change from a V-shape to a U-shape
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Post-Glacial Retreat Retreating ice leaves behind transported rocks as erratics Tarns form in cirques where valley glaciers originated Small mountain lakes Paternoster Lakes can form as well Small, circular, stair-stepped lakes Hanging valleys form Sites of waterfalls Hang steeply above the valley floor
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Erosion can continue where a glacier meets the ocean
Landscapes can continue to erode below sea level Fjords can form when a glacier retreats Mixing of fresh and saltwater Common in Alaska, Norway, Greenland, British Columbia
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