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Visualizing Earth Science
By Z. Merali and B. F. Skinner
Chapter 6 – Deserts, Glaciers and Ice Sheets
• Deserts and Drylands
Chapter Overview
• Glaciers and Ice Sheets
DesertsGeography
• Categorization of deserts by geography– Due to global air circulation
• Subtropical deserts– Sahara, Kalahari,
Namib• Polar
– Due to local climate conditions• Continental interior• Rain shadow• Coastal
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Hot Deserts
• Wind Erosion– Little flowing water
• Main erosion mechanism: wind blown sand
– Processes moving wind– Processes moving wind blown sediments• Largest grains: surface
creep• Medium sized grains:
saltation• Fine grains:
Suspension– Cause dust storms
• Abrasion– Air borne particles chip-off rock fragments
Mechanisms of Wind Erosion
Mechanisms of Wind Erosion • Deflation
– Removal of loose particles by wind• Desert pavement forms
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Wind Deposits and Desert Landforms
• Dunes– Wind deposited– Hill or ridge of sand– Typically asymmetrical in shape
• Gently sloping long windward face
• Steep leeward face: slip face• Interior cross-beds are
- Dune strata- Buried former slip faces
• Angle of repose– Maximum steepness of
slip-face
Wind Deposits and Desert Landforms
Dune Formation and Types
• Factors influencing dune formation and behavior
– Wind• Barchan dunes – unidirectional
windSt d N i d di ti• Star dunes – No wind direction preferred
– Vegetation• Parabolic dunes – presence of
vegetation– Sand supply
• Transverse dunes – large sand supply
• Longitudinal dunes – low sand supply or bidirectional wind
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Dune Formation and Types
The Work of Desert Streams
• Flash floods– Creates fast moving
short lived streams– Erode steep sidedErode steep sided
canyons– Deposit alluvial fans
in flat areas– Many alluvial fans
coalesce into a bajada
Desertification• Creation of desert
conditions in non-desert areas– Can be natural or
produced by human activity
– Signs• Lowering water tables• Higher salt content in
soil and water• High rates of soil
erosion• Destruction of
vegetation
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Desertification• Areas most vulnerable
– Semi-arid areas near deserts
– Poor soil management in intensely farmed areas
Desertification
Glaciers and the Cryosphere• Cryosphere is the frozen part of the
hydro sphere• Glaciers are moving layers of ice
– Snow accumulations in cold climates
– Increasing weight forces ice formation
• Movement regulated by internal temperature of ice
– temperate vs. polar glaciers
– Ice sheet flows under gravity
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Glaciers and the Cryosphere
Glaciers and the Cryosphere
Ice Sheets• Continent sized glacial
surface ice cover– Antarctica and Greenland– Expand and contract with
changing global climate– Contain ice shelves– Contain ice shelves
• Thick floating sheets of ice land-based
• Adjacent to land based ice sheet glaciers
– Sea ice• Formed by freezing sea
water• Antarctic ocean and
Arctic ocean
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Glacier Formation
• Compaction and recrystallization of snow– Gradual accumulation of snow– Increase in pressure– Reconfiguration of crystalline structured of ice
Glacier ice impermeable to air– Glacier ice – impermeable to air
Glacial Budget• Mass balance
– Net gain or loss or ice
– Gain called accumulation
– Loss calledLoss called ablation
– Motion
Glacial Motion
• Occurs due to gravity
• Usually slow• Two important
processesprocesses– Internal flow– Basal sliding
• Glacial surge
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Glacial Motion
Glacial Landscaping• Glaciers erode efficiently
– Filing rock surfaces smooth– Plowing rocks from the
ground– Transporting sediments
• Filing and plowing actionFiling and plowing action– Rock fragments carried
within the glacier– Near the base, rock
fragment scrape the ground– The action of large rock
fragments makes• Glacial striations• Glacial grooves
Glacial Landscaping
• Mountain glaciers produce distinctive land marks– Cirques form at the glacier’s
head– Ridge between two
adjacent cirques called arête
– Horns develop when cirques are cut out on all sides of a mountain
– U-shaped valleys• Depth of valley related to
volume of ice in eroding glacier
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Glacial Deposition
• Glaciers carry large loads– Varying rock sizes– Deposition usually unsorted– Rocks form glacial deposition
called tillcalled till– Glaciers can deposit in ridges
called moraines• Lateral moraines – form along
the sides of glacier• A terminal moraine – forms at
the end of a glacier• Converging glaciers leave a
ridge of till called a medial moraine
Glacial Deposition
Glacial Deposition and Periglacial Landforms
• Melt water streams in a glaciers body
– Leave behind sediment in a raised bed called an eskerWhen glaciers retreat– When glaciers retreat
• Trapped ice left behind in the deposited till makes
– Kettle lakes and ponds– Periglacial regions – near glacial ice
• Have certain distinguishing land marks
– Tundra regions• Have permafrost
covered by a thin active layer
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Glacial Deposition and Periglacial Landforms
Glacial Deposition and Periglacial Landforms
Chapter Summary
• Deserts and Drylands– Deserts: Geography– Desert Landforms
• Dunes, and their classifications
D tifi ti– Desertification• Glaciers and Icesheets
– Glaciers and the Cryosphere– Ice Sheets– Glacier Formation and Budget– Glacial Motion, Landscaping and Deposition
