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USA GY 301 - GY301 Applied Geomorphology Glacial Landforms Lab

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IntroductionGlacial FlowValley GlaciersLandform Products of Valley GlaciationContinental GlaciersContinental Glacier LandformsEXERCISE 9: GLACIAL SYSTEMSGY301 Applied GeomorphologyGlacial Landforms LabIntroductionGlacial erosion is an important erosional process capable of producing a variety oflandforms. Glacial systems can be separated into two types, continental glaciers andvalley (alpine) glaciers. Continental glaciers cover a large proportion of an entire continentor microcontinent. Today there are only two existing continental glaciers, the Antarctic andGreenland continental ice sheets. During periods of our planet's recent geological history,continental glaciers covered significant portions of several continents that are free of icetoday. In these regions landforms are predominantly glacial in origin even though glacialice no longer exists in the region.Valley or alpine glaciers occupy mountain valleys in much the same way as streamsystems. Valley glaciers, however, fill a much greater proportion of the valley volume thando typical stream systems. In contrast to valley glaciers, continental glaciers grow in aradial fashion from a continental interior, therefore, they tend to have a roughly circularshape. Glaciers can erode to a base level that is below sea level- a fact which contrastssharply with fluvial systems.The basic difference between continental glacial and valley glacial systems are thedifferences in size and shape. The mechanism by which glaciers move is the sameregardless of the type of glacier. The landforms produced by each type of glacial processare distinctive because of the difference in size and shape of each. The landformsproduced are distinct and allow determination of whether or not a glacial system hasproduced a landform, and if so, whether it was a continental or valley glacier.Glacial FlowGlaciers, especially valley glaciers, can be thought of as "rivers" of ice. In manyways the rules governing streamflow also govern the flow mechanism of glacial ice. Forexample, just as flowing water will naturally seek out the lowest elevation, so will glaciers.The rate of movement for glacial systems is several order of magnitudes less than that ofstreamflow velocities. Most glacial systems flow at the imperceptibly slow rate of less thanone meter per day.Glaciers will grow in any environment where the annual accumulation of winter snowis larger than that of the amount of snow that melts during warmer months. Note that morethan just cold temperatures are required for the accumulation of glacial ice; there must alsobe ample moisture in the environment in the form of snow. If a snowpack develops thatis more than several meters thick the delicate ice crystals of snow will crushed into moredense spherical ice pellets termed firn. This layer is also termed the "snowbase" uponwhich the un-compacted snow (powder) rests. As the layer of firn becomes thicker throughPage -1-GY301 Applied GeomorphologyGlacial Landforms Labtime the pressure on the firn increases. This leads to dramatic changes in the behavior ofthe firn ice. When a layer of snow and firn builds to more than approximately 50 metersthickness, the firn is compacted into a solid ice devoid of any void space. More importantly,however, is that the ice is under sufficient pressure to allow viscous flow in the solid state-similar to the behavior the asthenosphere relative to the lithosphere. In this state the glacialice will begin to flow in the downslope direction under the influence of gravity. In additionto the ability to flow, the glacial ice at the base of the glacier may actually become liquidwater because of the increased pressure of the building glacier. This effect can beunderstood from the Figure 12 phase diagram of water. Note that at the freezing point, ifpressure is increased that the water will convert to the liquid state because that is the moredense arrangement for water molecule. This thin film of water will allow the glacier to moveby sliding on a relatively frictionless surface.The area that annually receives more snow in winter than melts during summerperiods is termed the zone of accumulation. This area is marked on aerial photographs byvery white areas covered with fresh snow. As the glacial ice flows downslope over time thefresh snow is taken to lower elevations where temperatures are warmer. This allows thedelicate snow flakes on top of the glacial ice to melt revealing the surface of the firn or ice.This surface will appear distinctly different than the former snow because the glacial icewill reflect the blue sky if it is free of debris, or will appear "dirty" if it is covered by debris.The zone where the snow pack has melted is termed the zone of wastage or the zone ofablation. The line dividing the zone of accumulation and the zone of ablation is termed thesnowline.Valley GlaciersValley glaciers are the most common type of glaciers. They are usually found in thehigher elevation parts of mountain ranges. The higher elevations have the lowertemperatures and higher rates of precipitation that favor the growth of glacial ice. Valleyglaciers may be found quite close to the equator if elevations are high– the Andes is butone example. Once the glacial ice of a valley glacier begins to flow downslope, the glaciernormally occupies a valley that was formerly cut by stream erosion. Because the glacieris a more effective erosional agent than water the valley rapidly undergoes severaldramatic changes:! The formerly "V"-shaped valley becomes flat at the base but very steep along thevalley walls to form a "U"-shape profile.! The elevation of the base of the valley is rapidly lowered because the glacial ice willremove the entire regolith layer until bedrock is encountered. Because the solid icePage -2-GY301 Applied GeomorphologyGlacial Landforms Labdoes not float in water until approximately 98% is submerged, glacial valleys can beeroded to a depth below sea level.Topographic maps will display currently glaciated terranes as covered with a white color.In addition, the contour lines that are on top of glacial ice are colored blue. Since valleyglaciers usually exist in high-relief mountainous areas, the contour lines will often have aclose spacing giving an overall "bluish" tint to those areas covered by glacial ice.As the glacier moves downslope through is valley, mass wasting processes,especially ice-wedging, shed


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