chapter_13.f10.pdf

Visualizing Physical Geography Copyright © 2008 John Wiley and Sons Publishers Inc. Chapter 13 Landforms Made by Waves and Wind Visualizing Physical Geography by Alan Strahler and Zeeya Merali Visualizing Physical Geography Copyright © 2008 John Wiley and Sons Publishers Inc. Chapter Overview The Work of Waves and Tides Types of Coastlines Wind Action Eolian* Landforms * sometimes spelled Aeolian Singing sand dunes, Dunhuang, Gansu Province, China Visualizing Physical Geography Copyright © 2008 John Wiley and Sons Publishers Inc. The Work of Waves and Tides The Work of Waves Waves are the most important agent shaping coastal landforms Waves: ? Are generated by wind ? Erode shorelines ? Move sediment Shoreline: shifting line of contact between water and land Coastline: zone in which coastal processes operate or have a strong influence Bays: bodies of water sheltered from strong wave action Estuary: where a river empties into a bay Hanauma Bay, Oahu, Hawaii Visualizing Physical Geography Copyright © 2008 John Wiley and Sons Publishers Inc. The Work of Waves and Tides The Work of Waves When a wave reaches the shore, the drag of the bottom slows and steepens it The wave leans forward, and eventually breaks Visualizing Physical Geography Copyright © 2008 John Wiley and Sons Publishers Inc. The Work of Waves and Tides Marine Scarps and Cliffs Marine cliff: rock cliff shaped and maintained by the undermining action of the waves Wave action creates steep banks or scarps, as well as sea caves, arches and stacks Visualizing Physical Geography Copyright © 2008 John Wiley and Sons Publishers Inc. The Work of Waves and Tides Beaches and Littoral Drift Beach: thick, wedge-shaped deposit of sand, gravel, or cobbles in the zone of breaking waves Beaches absorb energy of waves Material moves on to and off the beach with the waves over time Visualizing Physical Geography Copyright © 2008 John Wiley and Sons Publishers Inc. The Work of Waves and Tides Beaches and Littoral Drift Littoral Drift: transport of sediment parallel with the shoreline by the combined action of beach drift and longshore current transportation Progradation: building out; beach is widened Retrogradation: sand is removed; beach is narrowed Beach sand may come from sediment brought by rivers; dams on the river cause retrogradation The Work of Waves and Tides Visualizing Physical Geography Copyright © 2008 John Wiley and Sons Publishers Inc. Groin: wall or embankment built at right angles to the shoreline to trap sediment Considerations: ? What damage will the proposed structures cause in the long run? ? Are there less costly alternative approaches? ? Is the cost of the structure worth it? ? How long will the structure be effective.? Visualizing Physical Geography Copyright © 2008 John Wiley and Sons Publishers Inc. The Work of Waves and Tides Beaches and Littoral Drift Littoral drift transports sand along the shore. On a straight beach, littoral drift forms bars and sand spits. Where the coast has headlands, littoral drift builds pocket beaches in bays. Spit may form a baymouth bar, closing off the bay to create a lagoon. Visualizing Physical Geography Copyright © 2008 John Wiley and Sons Publishers Inc. The Work of Waves and Tides Tidal Currents Ocean tide: rhythmic rise and fall of sea level under the gravitational influence of sun and moon ? Ebb current: tide is falling ? Flood current: tide is rising ? Tidal currents ?Keep inlets open ?Redistribute sediments to form tidal mud flats and marshes Visualizing Physical Geography Copyright © 2008 John Wiley and Sons Publishers Inc. Types of Coastlines Shorelines of Submergence: ? sea levels have risen, or land has subsided Ria coast, fiord coast Shorelines of Emergence: ? sea level falls, or crust rises Barrier-island coasts, volcano coasts, delta coasts, coral reef coasts, fault coasts Visualizing Physical Geography Copyright © 2008 John Wiley and Sons Publishers Inc. Types of Coastlines Shorelines of Submergence ? Ria coast: rise in sea level or crustal sinking brings ocean up to fill existing river valleys ?The new embayments are estuaries ? Fiord coast: ocean fills valleys created by glaciers Fjordland National Park, New Zealand Visualizing Physical Geography Copyright © 2008 John Wiley and Sons Publishers Inc. Types of Coastlines Shorelines of Submergence Evolution of a Ria Coastline Visualizing Physical Geography Copyright © 2008 John Wiley and Sons Publishers Inc. Types of Coastlines Barrier-Island Coasts ? Barrier island of sand lies a short distance from the coast ? Associated with recently emerged coastal plain ? Ridges of sand built by storm waves ? Behind barrier island is a lagoon ? Tidal inlet: gaps in the barrier island through which tide flows Outer Banks of North Carolina Visualizing Physical Geography Copyright © 2008 John Wiley and Sons Publishers Inc. Types of Coastlines Delta Coasts Delta: sediment deposit built by a stream entering a body of standing water ? Current of water slows as it enters ocean, and sediment is deposited ? River channel divides into distributaries ? Deltas have a variety of shapes ? Deltas can grow and shrink rapidly Visualizing Physical Geography Copyright © 2008 John Wiley and Sons Publishers Inc. Types of Coastlines Volcano and Coral-Reef Coasts Volcano coasts: lava and ash from active volcanoes deposited in ocean ? Wave actions erodes the deposits ? Forms cliffs and narrow beaches White Island, New Zealand Visualizing Physical Geography Copyright © 2008 John Wiley and Sons Publishers Inc. Types of Coastlines Volcano and Coral-Reef Coasts Coral reef: rock-like accumulation of carbonates secreted by corals and algae in shallow water along a marine shoreline ? New land is made by organisms ? Warm tropical and equatorial waters ? Coral needs warm, clean water, good aeration ? Reefs exposed at low tide, covered at high tide Fringing reefs: reef platforms attached to shore Barrier reefs: separated from mainland by a lagoon Atolls: circular reefs enclosing a lagoon; no land inside; most grow on top of old sunken volcanoes Types of Coastlines Volcano and Coral-Reef Coasts Visualizing Physical Geography Copyright © 2008 John Wiley and Sons Publishers Inc. Types of Coastlines Volcano and Coral-Reef Coasts ? Algae necessary for coral survival ? Corals can recover from temporary heat stress and bleaching ? Permanent increase in water temperature can cause coral death ? Global warming threatens coral reef survival ? Increased CO2 in ocean impedes growth of coral structure Coral bleaching: warm water temperatures force corals to expel the algae that live inside the coral structure Visualizing Physical Geography Copyright © 2008 John Wiley and Sons Publishers Inc. Types of Coastlines Fault Coast Faulting can raise cliffs along a coastline Daly City, California Visualizing Physical Geography Copyright © 2008 John Wiley and Sons Publishers Inc. Types of Coastlines Raised Shorelines and Marine Terraces Marine terrace: former abrasion platform elevated to become a step-like coastal landform San Clemente Island, California Visualizing Physical Geography Copyright © 2008 John Wiley and Sons Publishers Inc. Types of Coastlines Rising Sea Level As global temperatures increase, sea levels rise ? Expansion of water as it warms ? Melting of glaciers and snow on land ? Sea levels will likely rise 20 to 60 cm by 2100 Rising sea levels mean: ? More coastal erosion ?Higher number of severe storms ?Sea moves inland ? Land subsidence increases ? Loss of coastal wetlands Visualizing Physical Geography Copyright © 2008 John Wiley and Sons Publishers Inc. Wind Action Erosion By Wind Eolian: wind processes Aeolus was Greek god of winds Wind abrasion: particles carried by wind wear down a surface by impact Deflation: lifting and transport in turbulent suspension by wind of loose particles of soil or regolith from dry ground surfaces Deflation produces blowouts Deflation and water action produce desert pavement Visualizing Physical Geography Copyright © 2008 John Wiley and Sons Publishers Inc. Wind Action Dust Storms Strong winds in arid and semiarid regions can lift large quantities of fine dust to produce dust storms Dust cloud may be several thousand meters high, and may represent an approaching cold front Visualizing Physical Geography Copyright © 2008 John Wiley and Sons Publishers Inc. Landforms Made By Wind Eolian Landforms Visualizing Physical Geography Copyright © 2008 John Wiley and Sons Publishers Inc. Eolian Landforms Sand Dunes Sand dune: hill or ridge of loose, well- sorted sand shaped by wind and usually capable of downwind motion ? Active dunes constantly change form and move ? Dunes may be stabilized by vegetation ? Sand grains may bounce and move by saltation ? Asymmetrical; downwind slip face maintains angle of repose of about 35º ? Erg: sea of sand ? Reg: desert pavement on top of vast sheets of sand Dune Cross Section Eolian Landforms Sand Dunes Visualizing Physical Geography Copyright © 2008 John Wiley and Sons Publishers Inc. Eolian Landforms Sand Dunes Barchan dune: isolated, crescent- shaped pile of sand; points of crescent face downwind Transverse dune: wave- like ridges, transverse (perpendicular) to wind Baja California Sand sea or erg in the Namib Desert, Namibia Visualizing Physical Geography Copyright © 2008 John Wiley and Sons Publishers Inc. Eolian Landforms Sand Dunes Star dunes: large sand hills with dune crests radiating away from center Parabolic dunes: crescent- shaped, points facing toward wind Coastal blowout dunes have saucer-shaped blowout in center Visualizing Physical Geography Copyright © 2008 John Wiley and Sons Publishers Inc. Eolian Landforms Sand Dunes Longitudinal dunes: long, narrow ridges, oriented parallel to prevailing wind; May extend for kilometers Dunes adjacent to Lake Eyre, South Australia Visualizing Physical Geography Copyright © 2008 John Wiley and Sons Publishers Inc. Eolian Landforms Coastal Foredunes Foredune: ridge of irregular sand dunes typically found adjacent to beaches on low-lying coasts and bearing a partial cover of plants ? Form a protective barrier to storm waves ? If plant cover is removed, dune erodes or blowout forms; protection is lost Visualizing Physical Geography Copyright © 2008 John Wiley and Sons Publishers Inc. Eolian Landforms Loess Loess: surface deposit of wind-transported silt ? Forms vertical cliffs ? Is easily eroded ? Is used for cave dwellings ? Thickest deposits in China (source: central Asia) ? U.S. has thick loess deposits ? Forms important agricultural soil Visualizing Physical Geography Copyright © 2008 John Wiley and Sons Publishers Inc. Eolian Landforms Induced Deflation Human activities can disturb soil surface and induce deflation ? Plowing ? Removal of vegetation cover ? Grazing and trampling Dust storm approaching Stratford, Texas, Dust Bowl Era, 1935 Image ID: theb1366, NOAA's National Weather Service (NWS) Collection Location: Stratford, Texas Photo Date: April 18, 1935 Credit: NOAA George E. Marsh Album Pattie PowerPoint Presentation