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Lecture 16 - Deserts

Lecture 16 - Deserts - Chapter 19 Chapter Winds and Deserts...

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Unformatted text preview: Chapter 19 Chapter Winds and Deserts Test Thursday October 27 Chapters 3 Minerals 4 Igneous Rocks 5 Sedimentary Rocks 6 Metamorphic Rocks 16 Weathering/Mass Wasting 17 Hydrologic Cycle/Groundwater 18 Streams/Surface Waters Bring your ID Old Tests on T-square About Winds and Deserts About • Wind is a major agent of erosion and Wind deposition that moves vast quantities of sand, silt, and dust over large areas. sand, • Eolian processes shape the land surface Eolian in areas like deserts where few plants live. live • Deserts expand and contract, depending Deserts upon climatic changes. upon The Desert Environment: The Where the Deserts Are The Desert Environment The ● Where deserts are found Where ● areas of low rainfall areas ● rain shadows rain ● far from oceans far ● in polar regions in The Desert Environment ● Various factors play a role Various in the formation of deserts in ● plate tectonics plate ● climate change climate ● human actions human ● in polar regions in 1. Global Wind Patterns Global ● Wind (the flow of air) Wind ● wind is unconfined, except in narrow valleys narrow ● wind obeys all the laws of flow wind ● wind is parallel to surface, and wind may go upward too may 2. Wind as a Transport Agent ● Factors in how wind carries things Factors ● wind strength wind ● particle size particle ● surface material surface Wind as a Transport Agent: Agent: Rate of Sand Sand Movement Movement versus Wind Wind Speed Speed Wind as a Transport Agent: Agent: Sand Being Blown from Blown the Desert the in Namibia toward the South Atlantic 2. Wind as a Transport Agent ● Materials carried by the wind Materials ● windblown dust windblown ● volcanic, organic, soil volcanic, ● aerosols aerosols ● windblown sand windblown Wind as an Agent Agent of Erosion: A Ventifact Wind as an Agent Agent of Erosion: Rounded and and Frosted Frosted Grains of Sand Wind as an Agent Agent of Erosion: Desert Deflation Surface Wind Abrasion = Ventifacts 3. Wind as an Agent of Erosion: 3. Formation of Desert Pavement 3. Wind as an Agent of Erosion ● What the wind does to erode What surficial materials surficial ● sand blasting (ventifacts; frosting on sand grains) frosting ● deflation (desert pavements) deflation 4. Wind as a Depositional Agent ● Sand dunes Sand ● deserts, beaches, lake shores, deserts, floodplains, etc. floodplains, ● need a ready supply of loose need sand sand ● vegetation stabilizes dunes vegetation Wind as a as Depositional Agent: Linear Sand Dunes of the Southern Arabian Peninsula Wind as a as Depositional Agent: Wind Ripples at Stovepipe Wells, California Wind as a as Depositional Agent: Sand Dunes in Gusev Crater, Mars 4. Wind as a Depositional Agent ● How sand dunes form and move How ● saltation of sand saltation ● transverse piling of sand or sand piles up behind an obstruction obstruction ● piling and avalanche of sand piling Wind as a as Depositional Agent: Sand Piling Downwind of an of Obstruction Obstruction 4. Wind as a Depositional Agent: 4. Formation of Wind Shadow Sands 4. Wind as a Depositional Agent: 4. Formation of Sand Dunes 4. Wind as a Depositional Agent: 4. Formation of Sand Dunes 4. Wind as a Depositional Agent: 4. Formation of Sand Dunes 4. Wind as a Depositional Agent: 4. Formation of Sand Dunes 4. Wind as a Depositional Agent: 4. Formation of Sand Dunes 4. Wind as a Depositional Agent ● Types of sand dunes Types ● barchans barchans ● blowout or parabolic dunes parabolic ● transverse dunes transverse ● linear dunes linear 4. Wind as a Depositional Agent: Barchans Wind Barchans are crescent-shaped dunes, always the products of limited sand supply and unidirectional winds. supply Wind 4. Wind as a Depositional Agent: 4. Blowout or Parabolic Dunes Blowout dunes are almost the reverse of Blowout barchans (the horns point “backwards”). barchans 4. Wind as a Depositional Agent: 4. Transverse Dunes Transverse dunes form in arid regions Transverse where there is abundant sand. where 4. Wind as a Depositional Agent: 4. Linear Dunes Linear dunes occur in areas that have a Linear moderate sand supply, a rough pavement, and winds in the same general direction. and Sand Dunes 4. Wind as a Depositional Agent ● Dust falls and loess Dust ● fallen atmospheric dust creates a fallen deposit of loess (loess blanket) loess ● loess covers 10% of Earth’s surface surface Wind as a as Depositional Agent: Pleistocene Loess in the Catalina Mountains, Arizona Wind as a as Depositional Agent: Pleistocene Loess in Central China 5. The Desert Environment ● Desert weathering phenomena Desert ● desert colors (rusty, orange brown) brown) ● desert varnish desert ● stream erosion (when water is present) is The The Desert Environment: Rusty Colors and Desert Varnish (carved by aboriginal people), Canyonlands, Utah The The Desert Environment: Desert Stream Erosion Erosion (when water (when iis present), s present Saguaro National Monument, Arizona The The Desert Environment: Desert Stream Erosion Erosion (when water (when iis absent), s absent Saguaro National Monument, Arizona The The Desert Environment: Desert Playa Lake, Death Valley, California 5. The Desert Environment ● Desert sediments; sedimentation Desert ● alluvial sediments alluvial ● eolian sediments eolian ● evaporite sediments (playa and playa lakes) and 5. The Desert Environment ● Desert landscapes Desert ● playa playa ● desert pavements ● dune fields dune ● dry washes (wadis) dry ● pediments pediments The The Desert Environment: Desert Pediment, Cima Dome, Mojave Desert, California 5. The Desert Environment: Formation of a Pediment Elevated mountains Downfaulted lowlands Fault Time 1 The lowlands are downfaulted, and the mountains are elevated. 5. The Desert Environment: Formation of a Pediment Alluvial fans Early erosion of mountains Stream floodplain Time 2 Erosional debris is deposited as alluvial fan and stream floodplain sediments. 5. The Desert Environment: Formation of a Pediment Alluvial deposits Pediment Time 3 Erosion produces a pediment with thin covering of alluvial deposits. 5. The Desert Environment: Formation of a Pediment Mountain remnants Pediment Time 4 Continued erosion produces a more extensive pediment. Desert Landscapes • Basin and Range example • Erosion of mountain mass causes local relief to continually diminish • Eventually mountains are reduced to a few large bedrock knobs called inselbergs projecting above a sediment filled basin Basin and Range • Evolution of a desert landscape • Uplifted crustal blocks • Interior drainage into basins produces • Alluvial fans and bajadas • Playas and playa lakes Landscape Evolution in a Mountainous Desert – Early Stage Landscape Evolution in a Mountainous Desert – Middle Stage Landscape Evolution in a Mountainous Desert – Late Stage Key terms and concepts Deflation Deflation Desert pavement Desert varnish Desertification Dry wash Dust Eolian Inselberg Loess Pediment Playa Playa lake Sandblasting Slip face Ventifact ...
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