Winds+and+Deserts
Page 1: Wind and Deserts Overview
Chapter 17: Discusses wind and desert dynamics.
Page 2: Exam Information
Exam 3 covers topics from:
Chapter 9: Geologic Structure
Chapter 8: Earthquakes
Interlude D: Earth’s Interior
Chapter 2: Tectonics
Chapter 17: Wind and Deserts
Requirements: Bring a scantron and a pencil.
Format: 50 questions.
Page 3: Learning Objectives
Understand wind dynamics as an agent of erosion, transport, and deposition.
Explore desert environments, types, formation, and ecological significance.
Examine how wind and desert processes influence Earth's landscapes and ecosystems.
Page 4: Causes and Nature of Wind
What Causes Winds to Blow?
Basic understanding of wind formation is introduced.
Page 5: Definition of Wind
Wind is defined as moving air within the atmosphere.
Driven primarily by convection and pressure differences due to varying densities.
Page 6: Factors Controlling Air Density
Temperature: Heating causes air expansion, reducing density and prompting rising.
Humidity: Moist air is less dense due to lower mass of water molecules compared to nitrogen and oxygen.
Warm, moist air rises; cool, dry air sinks.
Page 7: Wind Speed and Direction
Wind speeds arise from pressure differences:
Greater differences lead to higher wind speeds (e.g., Las Vegas 0-19 mph; Hurricane Katrina 174 mph).
Wind direction is indicated by where it originates (e.g., westerly winds).
Page 8: Simplistic Views of Wind Patterns
If temperature were the only factor, air motion would only be simple convection from the equator to the poles.
Page 9: Introducing Moisture
At equators, warm air promotes evaporation; rising air cools and contracts, leading to rain.
By 20-30° N/S, air is cool, dry, and dense, sinking to create distinct weather patterns.
Page 10: The Coriolis Effect
Influence of Earth's rotation: long-distance travel causes apparent curve in trajectory.
Page 11: Influence of Winds on Landscapes
Wind impacts landscapes significantly in regions:
With strong winds
Lack of vegetation
Fine-grained soils
Page 12: Sediment Movement by Wind
Where is the Sediment?
Sediments prone to wind movement are often found in:
Stream floodplains
Sandbars
Beaches
Deserts
Dry lakebeds
Key conditions for effective wind transport: strong winds, sparse vegetation, fine-grained, dry sediments.
Page 13: Desert Geology
Deserts characterized by extreme conditions and cover ~25% of Earth’s land surface.
Page 14: Definition of a Desert
A desert is defined as a region:
With little to no permanent surface water
Supporting no more than 15% vegetation cover
Receiving less than 10 inches of annual rainfall.
Page 15: Characteristics of Hot and Cold Deserts
Hot Desert Features
Found at low latitudes and elevations, far from oceans.
Temperatures often exceed 35°C.
Cold Desert Features
Located at high latitudes and elevations, near cold ocean currents.
Typically below 20°C.
Page 16: Desert Locations
Most deserts are found at ~30° N & S or downstream from major mountains.
Page 17: Types of Deserts
Geologists classify deserts into five classes:
Subtropical deserts (e.g., Sahara)
Rain-shadow deserts
Coastal deserts (e.g., Atacama)
Continental interiors (e.g., Gobi)
Polar deserts (e.g., Antarctica).
Page 18: Formation of Subtropical Deserts
At the equator, moisture-laden hot air rises and cools, leading to extensive rainfall.
Dry air then descends at subtropical latitudes (20° to 30° N and S), warming and absorbing moisture, forming deserts.
Page 19: Rain-Shadow Desert Formation
Moist ocean winds ascend over coastal mountains, cool, and release moisture creating rainforests.
Subsequently, leeward air descends, forming arid conditions.
Page 20: Coastal Desert Characteristics
Coastal deserts are influenced by cool ocean currents and experience minimal rainfall (e.g., Atacama Desert, one of the driest).
Page 21: Continental-Interior Deserts
Located far from ocean moisture, resulting in arid conditions (e.g., Gobi Desert).
Page 22: Polar Deserts
Characterized by low moisture levels due to cold temperatures at high latitudes.
Page 23: Aeolian Geomorphology
Fast winds are needed for sediment movement; smaller particles are lifted more easily.
Particle movement includes rolling, bouncing, and suspension.
Page 24: Weathering and Erosion in Deserts
Physical weathering dominates, leading to unique desert landscapes.
Water erosion, though infrequent, plays a significant role during flash floods.
Page 25: Wind Erosion Processes
Wind can transport fine sands and dust over long distances.
Surface load: grains moved by saltation.
Suspended load: finer grains carried in the air.
Page 26: Lag Deposits and Deflation
Lag deposits form when coarse material is left after finer particles are blown away.
Deflation: lowering of the land surface due to wind erosion.
Page 27: Unique Erosional Features: Ventifacts and Yardangs
Ventifacts: stones shaped by wind erosion.
Yardangs: wind-sculpted features in bedrock.
Page 28: Dune Formation Processes
Dune Deposition
Particles accumulate on windward slopes and migrate over time, forming different dune types based on sediment supply and wind direction.
Page 29: Types of Dunes
Barchan Dunes
Characterized by limited sediment supply; migrate rapidly in one direction.
Longitudinal Dunes
Formed by bi-directional winds, spaced apart, common in deserts.
Transverse Dunes
Extensive dune rows perpendicular to wind; require large sand supply.
Parabolic Dunes
Formed in vegetated areas where wind erodes a section creating a parabolic shape.
Star Dunes
Formed from winds coming from multiple directions; can reach heights over 100 m.
Page 30: Desert Varnish
Dark coating of iron and manganese oxides forming slowly on exposed surfaces; thickness indicates age.
Page 31: Alluvial Fans
Form when water exits canyons and spreads out, dropping sediment in a conical shape over time.
Page 32: Playas
Desert lakes with no outlets; collect water from rare rains, leaving behind crystallized solids upon evaporation.
Page 33: Desert Landscapes
Rocky Cliffs and Mesas
Resistant rocks (e.g., sandstone) create cliffs while erosion shapes mesas and buttes through retreat.
Hoodoos and Arches
Unique formations arise from erosion along joints in sandstone.
Cuestas and Hogbacks
Defined by the angle of bedding; cuestas have asymmetric ridges, while hogbacks present symmetric ridges.
Inselbergs
Result from erosion leading to isolated rocky remnants surrounded by alluvial deposits.
Page 34: Desert Problems
Desertification
Process of non-desert areas turning arid due to human activity, leading to consequences like famine.
Page 35: Class Questions for Review
Questions cover characteristics of hot deserts, sediment types, alluvial formations, and remnants of mesas.
Page 36: Terminology
Terms to remember include:
Abrasion, Alluvial fan, Barchan dune, Butte, Chimney, Convection, Coriolis, etc.