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• Glaciers are a part of two fundamental cycles in the Earth system - the hydrologic cycle and the rock cycle
• Approximately 160,000 glaciers presently occupy Earth's polar regions and high mountain environments
• Today's glaciers cover only about one third of the area that was covered at the height of the most recent Ice Age
• Glacier


  A thick ice mass that forms over hundreds or thousands of years, originating on land from the accumulation, compaction, and recrystallization of snow
• Glaciers


  • They appear motionless but actually move very slowly
  • They are dynamic erosional agents that accumulate, transport, and deposit sediment
• Valley (Alpine) Glaciers


  Relatively small glaciers that exist in lofty mountain areas, following valleys that were originally occupied by streams
• Valley Glaciers


  • They advance slowly, perhaps only a few centimeters per day
  • They are bounded by precipitous rock walls and flow downvalley from a snow accumulation center near their head
• Ice Sheets


  Massive glaciers that exist on a much larger scale than valley glaciers, covering large areas of land
• The Greenland Ice Sheet occupies 1.7 million square kilometers, or about 80 percent of the island
• The Antarctic Ice Sheet covers an area of more than 13.6 million square kilometers
• Ice Sheets


  • They flow out in all directions from one or more snow-accumulation centers and completely obscure all but the highest areas of underlying terrain
  • Topographic differences beneath the glaciers affect their behavior, guiding flow in certain directions and creating zones of faster and slower movement
• Ice Shelves


  Large, relatively flat masses of floating ice that extend seaward from the coast but remain attached to the land along one or more sides
• Antarctica's ice shelves extend over approximately 1.4 million square kilometers
• Since 1974 the extent of seven ice shelves surrounding the Antarctic Peninsula declined by about 13,500 square kilometers
• Ice Caps


  Masses of glacial ice covering some uplands and plateaus, smaller than continental-scale ice sheets
• Outlet Glaciers


  Tongues of ice that flow down valleys extending outward from the margins of ice caps or ice sheets
• Piedmont Glaciers


  Glaciers that occupy broad lowlands at the bases of steep mountains, formed when one or more valley glaciers emerge from confining mountain valleys and spread out to form a broad sheet
• Only slightly more than 2 percent of the world's water is tied up in glaciers
• Antarctica's ice sheet represents 80 percent of the world's ice and an estimated 65 percent of Earth's fresh water
• If the Antarctic ice sheet melted, sea level would rise an estimated 60 to 70 meters and inundate many densely populated coastal areas
• Snowline


  The elevation above which snow remains throughout the year
• Glacial Ice Formation


  1. Snow accumulation

  2. Air infiltration and crystal changes

  3. Snowflakes becoming smaller, thicker, and more spherical

  4. Large pore spaces disappearing
• Latitude


  Affects the elevation of the snowline
• Near the equator, the snowline occurs high in the mountains, whereas in the vicinity of the 60th parallel it is at or near sea level
• Glacial ice formation


  1. Snow accumulation

  2. Air infiltration and evaporation

  3. Recrystallization into firn

  4. Compaction into glacial ice
• The rate of transformation from snow to glacial ice varies depending on the annual snow accumulation
• Plastic flow


  Movement within the ice when pressure exceeds the weight of 50 meters of ice
• Basal slip


  Entire ice mass slipping along the ground
• Zone of fracture


  Upper 50 meters of a glacier where ice is brittle and carried along "piggyback" style
• Crevasses


  • Cracks that form in the zone of fracture when the glacier moves over irregular terrain
• Glacial movement is not uniform, with the center moving faster than the edges due to friction with the valley walls
• Glacial ice can move at rates ranging from very slow to several meters per day
• Glacial surge


  Periods of extremely rapid glacial movement, up to 100 times the normal rate
• Zone of accumulation


  Area above the snowline where more snow falls in winter than melts in summer
• Zone of wastage


  Area below the snowline where snow from the previous winter completely melts along with some glacial ice
• Calving


  Process where large pieces of ice break off the front of a glacier into a water body
• Icebergs are just slightly less dense than seawater, with about 80% of their mass submerged
• The glacial budget is the balance between accumulation at the upper end and loss (ablation) at the lower end
• If accumulation exceeds ablation, the glacier advances, if ablation exceeds accumulation, the glacier retreats
• Glaciers are sensitive to changes in temperature and precipitation and can provide clues about changes in climate