Page 600 - Geosystems An Introduction to Physical Geography 4th Canadian Edition

564 part III The earth–atmosphere interface
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Glacier Mass Balance
Winter Summer
Net Balance
Cumulative Net Balance
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▲Figure AQS 17.1 Glacier mass balance graph. Positive values indicate net accumulation; negative values indicate net ablation. [Figure by P. giles using data from gulkana glacier, alaska, 1966–1980, acquired from USgS alaska Science Center, Water resources Office at ak.water.usgs.gov/glaciology/ gulkana/balance/index.html.]
concepts review
Key Learning
Metres of water equivalent
■ Explain the process by which snow becomes glacial ice.
More than 77% of Earth’s freshwater is frozen. Ice cov- ers about 11% of Earth’s surface, and periglacial features occupy another 20%, in ice-free but cold-dominated landscapes. Earth’s cryosphere is the portion of the hy- drosphere and ground that is perennially frozen, gener- ally at high latitudes and elevations.
A snowline is the lowest elevation where snow oc- curs year-round, and its elevation varies by latitude— higher near the equator, lower poleward. Snow becomes glacial ice through stages of accumulation, increasing thickness, pressure on underlying layers, and recrystal- lization. Snow progresses through transitional steps from firn (compact, granular) to a denser glacial ice after many years.
cryosphere (p. 534) snowline (p. 535) firn (p. 535)
glacial ice (p. 535)
1. Describe the location of most freshwater on Earth today.
2. Trace the evolution of glacial ice from fresh fallen snow.
■ Differentiate between alpine glaciers and continental ice sheets, and describe ice caps and ice fields.
A glacier is a mass of ice sitting on land or floating as an ice shelf in the ocean next to land. Glaciers form in areas of permanent snow. A glacier in a mountain range is an alpine glacier. If confined within a valley, it is termed a valley glacier. The area of origin is a snowfield, usually in a bowl-shaped erosional landform called a cirque. Where alpine glaciers flow down to the sea, the process of calving occurs as masses of ice break off the glacier into the sea and form icebergs. An ice sheet is an extensive continu- ous mass of ice that may occur on a continental scale. An ice cap is a smaller, roughly circular ice mass, less than 50000 km2 in size. An ice mass covering a mountainous region is an ice field.
glacier (p. 535)
alpine glacier (p. 536) cirque (p. 536) calving (p. 537)
ice sheet (p. 538)
ice cap (p. 538)
ice field (p. 538)
3. What is a glacier? What can we learn about existing climate patterns from conditions in glacial regions and glacial mass balances?
4. Differentiate between an alpine glacier, an ice sheet, an ice cap, and an ice field. Which occurs in moun- tains? Which covers Antarctica and Greenland?