Page 566 - Geosystems An Introduction to Physical Geography 4th Canadian Edition

530 part III The Earth–Atmosphere Interface
accumulate in large reef structures. Corals live in a sym- biotic (mutually helpful) relationship with algae; each is dependent on the other for survival.
Wetlands are lands saturated with water that sup- port specific plants adapted to wet conditions. Coastal wetlands form as mangrove swamps equatorward of the 30th parallel in each hemisphere and as salt marshes poleward of these parallels.
coral (p. 513)
mangrove swamp (p. 516) salt marsh (p. 516)
20. How are corals able to construct reefs and islands?
21. Describe a trend in corals that is troubling scientists,
and discuss some possible causes.
22. Why are the coastal wetlands poleward of 30° N and
S latitude different from those that are equatorward? Describe the differences.
■ Describe eolian transport of dust and sand, and discuss eolian erosion and the resultant landforms.
Eolian processes modify and move sand accumulations along coastal beaches and deserts. Wind exerts a drag or frictional pull on surface particles until they become airborne. The finer material suspended in a dust storm is lifted much higher than are the coarser particles of a sandstorm; only the finest dust particles travel significant distances. Saltating particles crash into other particles, knocking them both loose and forward. The motion of sur- face creep slides and rolls particles too large for saltation.
Erosion of the ground surface from the lifting and re- moval of particles by wind is deflation. Wherever wind encounters loose sediment, deflation may remove enough material to form depressions called deflation hollows, or blowouts, ranging in size from small indentations less than a metre wide up to areas hundreds of metres wide and many metres deep. Abrasion is the “sandblasting” of rock surfaces with particles captured in the air. Rocks that bear evidence of eolian abrasion are ventifacts. On a larger scale, deflation and abrasion are capable of streamlining rock structures, leaving behind distinctive rock forma- tions or elongated ridges called yardangs. Desert pavement is the name for the hard, stony surface that forms in some deserts and protects underlying sediment from erosion.
eolian (p. 516) surface creep (p. 518) deflation (p. 518) abrasion (p. 519) ventifact (p. 519)
yardang (p. 519)
desert pavement (p. 519)
23. Explain the term eolian. How would you character- ise the ability of the wind to move material?
24. Differentiate between a dust storm and a sandstorm.
25. What is the difference between eolian saltation and
fluvial saltation?
26. Explain the concept of surface creep.
27. Who was Ralph Bagnold? What was his contribution
to eolian studies?
28. Explain deflation. What role does deflation have in
the formation of desert pavement?
29. How are ventifacts and yardangs formed by wind
processes?
■ Explain the formation of sand dunes, and describe loess deposits and their origins.
Dunes are wind-sculpted accumulations of sand that form in arid and semiarid climates and along some coastlines where sand is available. An extensive area of dunes, such as that found in North Africa, is an erg, or sand sea. When saltating sand grains encounter small patches of sand, kinetic energy is dissipated and the grains start to build into a dune. As the height of the sand pile increases above 30 cm, a steeply sloping slipface on the lee side and charac- teristic dune features are formed. Dune forms are broadly classified as crescentic, linear, and star.
Windblown loess deposits occur worldwide and can develop into good agricultural soils. These fine-grained clays and silts are moved by the wind many kilometres and are redeposited as an unstratified, homogeneous blanket of material.
dune (p. 520) erg (p. 520)
sand sea (p. 520) loess (p. 524)
30. What is an erg? Name an example of a sand sea. Where is the example located?
31. What are the three classes of dune forms? Describe an example within each class. What do you think is the major shaping force for sand dunes?
32. Which form of dune is the mountain giant of the des- ert? What are the characteristic wind patterns that produce such dunes?
33. Where does the sediment that forms the loess depos- its of China come from? What is the origin of loess in Iowa? Name several other significant loess deposits on Earth.
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