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Coast and Shore Coasts are temporary structures, often subjects to rapid change. The shape of a coast is a product of many processes: uplift and subsidence, the wearing down of land by erosion, and the redistribution of mineral by sediment transport and deposition. Classifying coasts Active coasts Passive coasts Eustatic change The location of a coast depends primarily on global tectonic activity and the volume of water in the ocean. Change in sea level greatly influences coastal processes. For most of Earth’s recent history, sea level has been lower than we find today. Erosional coasts Depositional coasts Erosional coasts are new coasts in which the dominant processes are those removing coastal material. Depositional coasts are those coasts that are steady or growing because of their rate of sediment accumulation or the action of living organisms. EROSIONAL COASTS Features of an erosional coast at low tide: -wave-cut platform -notch eroded by waves -original land surface -sea cliff Features of Erosional Coasts -exposed beach -sea arch - sea cliffs -sea cave -sea stack -headland -blowhole High-energy coasts Low-energy coasts Shore Straightening Land Erosion Coasts - drowned river mouth (the Hudson River) - fjords are often formed by tectonic forces and later modified by glaciers eroding valleys (British Columbia, Greenland, Alaska, Norway, New Zealand). - Volcanic coasts. DEPOSITIONAL COASTS Beaches The Composition and slope of beaches -the swash -the backwash -the berm A typical beach profile. -high tide-low tide (dotted line) -longshore bars -longshore trough -berm crest -beach scarp Dunes Water-land: -offshore -nearshore (through breakers) -foreshore Backshore Longshore Transport A seasonal change , sand moves on and off Boomer Beach near La Jolla, CA. Gentle summer waves move sand onshore, but large winter waves remove the sand to offshore bars, exposing the basement rock. Longshore drift Longshore current – moves sediment along the shoreline between the sure zone and the upper limit of wave action. -Surf zone -upper limit of wave action -path followed by sand grains -direction of wave approach Depositional coasts often support beaches, accumulations of loose particles. Beaches change shape and volume as a function of wave energy and the balance of sediment input and removal. Coastal Cells Imput and sand outflow Coastal cells in Southern California Sand Spits and Bay Mouth Bars Tombolo Barrier island inlet Lagoon Sea islands Depositional coastspecially along subsiding continental margins, often exhibit characteristics large-scale features. Deltas -river dominated deltas Tide-dominated deltas -wave-dominated deltas Deltas form on broad continental shelves where rivers deposit sediments, tidal range is low, and wave and current action are generally mild. COASTS FORMED BY BIOLOGICAL ACTIVITY The Florida Keys – a coral reef in the US. A mangrove coast in Florida. Mangrove trees sediments , building and stabilizing the coast. Coasts can be extensively modified by the action of living organisms. ESTUARIES. Classification of Estuaries -Drovned river mouths -Fjords Bar-built -Tectonic Characteristics of Estuaries -Salt wedge estuaries -Well-mixed estuaries -Partially mixed - Fjord Estuaries can form at river mouths where fresh water mixes with seawater. Estuaries are amoung the most complex and biologically productive coasts. The value of Estuaries. Chesapeake Bay CHARACTERISTICS OF U.S. COASTS The Pacific Coast – the active margin. Most of sediments on the Pacific coast originated from erosion of relatively young granitic or volcanic rocks of nearby mountains. Wave-cut terraces on San Clemente Island The Atlantic Coast – the passive margin. Glaciers. The sea level was lower. The Gulf Coast experiences a smaller tidal range and – hurricanes expected. Reduced longshore drift and absence of interrupting submarine canyons and the great volume of accumulated sediments. HUMAN INTERFERENCE IN COASTAL PROCESSES -the breakwater in Santa Monica did beach bigger -groins -seawall -inporting sand Human interference in coastal processes rarely increases the long-term stability of a coast. Coastal classification Primary coast: Unmodified--morphology controlled by recent geologic history Land erosion coasts Ria coast drown river valleys: indented--shape controlled by drainage basin pattern Drowned glaciated coast (e.g. Deep coastal valleys--fiords) Examples: SØNDRE STRØMFJORD, Western Greenland Southwest Ireland West Faukland Island Southwestern Sweden TRÖLLASKAGI, Iceland Drowned karst topography Subaerial deposition coasts River deposition coasts Deltaic coasts (single delta complex) Compound delta coasts Compound alluvial fan coasts Glacial deposition coasts Partially submerged moraines Partially submerged drumlins (Boston Harbor drumlins) Partially submerged drift features Wind deposition coasts Landslide coast volcanic coasts shaped by diastrophic movements (faulted coasts) Ice coasts -------------------------------------------------------------------------------- Secondary Coasts (modified by coastal processes) Wave erosion coasts Wave straightened cliffs Differentially eroded coasts Marine deposition coasts Barrier beach Barrier island Barrier spit Bay barrier Coasts built by organisms coral reef coasts (fringing reef, barrier reef, atoll, keys, etc.) More about reefs: Coral reefs: Florida Keys National Marine Santuary Coral reefs from online books: Essentials of Oceanography and Marine Ecology Reefs of the Gulf of Mexico Coral reefs in Belize The Great Barrier Reef, Australia (National Geographic) Midway Atoll Serpulid reef coasts Oyster reef coasts Mangrove coast Mangroves-Florida''s Coastal Trees (University of Florida) Collection of Mangrove resources (Florida Plants Online) marsh grass coast Pros and cons of Shepard''s classsification: His classification is thorough, but cumbersome. Also many coasts may fall under more than one classif |
