Chapter 17 Ocean and Coastal Processes
Ocean Basins and Plates
Ocean Basins Ocean Basins are profoundly different from the continents. Ocean crust is thin and dense and young. New ocean crust is generated at spreading ridges and consumed is subduction zones
Ocean Basins
Tides are caused by Gravitational Attraction of the Sun and Moon
Waves Waves are driven primarily by wind. Waves may also be generated by earthquakes, submarine landslides and meteorites. Wave height is the vertical distance between crest and trough. Wave length is the horizontal distance between crests. Period is the time between crests.
Wave Size (Wind) Wave height: Ocean wave heights range from 0.3 to about 5m. Maximum recorded ocean wave height is 34m (100ft). Wavelength: Ocean wave lengths range from 40 to about 400m. Ocean waves travel at speeds of 25 to about 90 km/h Wavebase The depth at which wave motion ceases Equals about 1/2 wavelength
Wave Motion Wave motion of the water extends to about one-half the wavelength (20 to 200m) (wave base). When a wave enters shallow water (<1/2 wavelength), it is slowed by drag. Slowing causes bending of wave parallel to coast (refraction). Breakers are caused by the crest overtaking the trough.
Wave Base Wave Motion
Refraction Waves travel more slowly in shallow water (shallower than the wave base). This is called refraction This causes the wave front to bend so it is more parallel to shore. It focuses wave energy on headlands.
Refraction
Refraction
Tsunamis A tsunami is a seismic sea wave caused by an earthquake or submarine landslide. Wave height in deep water of 0.5 to 2 m. Wave height in shallow water up to 50 m. Wavelength of 100 km Wave velocity up to 700 km/h (400mph).
Wave Motion and Sediment Sorting Question Why does sand accumulate at the beach??
Wave Motion and Sediment Sorting Waves sort sediment particles by size. Fine particles (silt and clay) are kept suspended if water is moving. Sand accumulates at the beach, and fine particles can only settle out in depths below wave motion (1/2 wavelength).
Beach Environments
Beach Environments A beach is a strip of sand extending from low water line to cliff or zone of permanent vegetation. In temperate climates, the dominant beach mineral is quartz (SiO 2 ). In tropical climates the beach sand mineral is calcite (CaCO 3 ). Beach sands may also be volcanic glass.
Quartz-Sand Beach, California
Black Sand Beach, Hawaii
Rising and Falling Coasts Sea level is currently rising at about 2mm/y An emergent coast is rising faster than the water. Land may be rising due to glacial rebound (New England, Scandinavia), or tectonic activity (California) or volcanic activity (Hawaii). Coastline is rocky A submergent coast is falling relative to the water. Southeast Atlantic and Gulf of Mexico Coasts. Barrier island coast (temperate). Barrier reef coast (tropical).
Submergent-coast Beaches Waves arriving at an angle to the beach will cause a current or longshore drift which moves sand parallel to coast. A bay-mouth bar is a sand bar that closes a small bay. A spit is a small peninsula of sand. A barrier island is a long sand island enclosing a lagoon or bay.
Barrier Island
Spit
Bay-mouth Bar
Emergent-coast Beaches Emerging coasts have rocky outcrops, cliffs, arches, and caves eroded by wave action. A sea stack is an offshore rock column A sea arch is an arch eroded by wave action. A tombolo is a sand bar connecting to a rock or off-shore outcrop.
Emergent Coast
Seastacks, Tombolos
Sea Arch, Co. Antrim, Ireland
Rising and Falling Coasts Sea level is currently rising at about 2mm/y An emergent coast is rising faster than the water. Land may be rising due to glacial rebound (New England, Scandinavia), or tectonic activity (California) or volcanic activity (Hawaii). Coastline is rocky A submergent coast is falling relative to the water. Southeast Atlantic and Gulf of Mexico Coasts. Barrier island coast (temperate). Barrier reef coast (tropical).
Climate Change and Ocean Waves Ocean Wave Heights increasing since 1980 Both in Atlantic and Pacific Average storm wave heights are measured as the largest one-third of the waves. US northwest coast experienced four storms in 1998-1999 winter that exceeded 100 year norms. One storm had average deep-water wave height in excess of 14m. (~ 45 ft)
The Ocean Basins Cover about 70% of the Earth s surface. Are Geologically Young (<250 My) Are underlain by basalt and gabbro are under about 3000 m of water. If this water was spread evenly over the surface it would be about 2000 m. There has been liquid water on the Earth as far back as we can see (~4.3 By).
Submarine Canyons
Ocean Basins and Plates
Atlantic Ocean Basin
Pacific Ocean Topographic Profile
Atolls and Guyots
Black Smokers: Ocean Floor Hotsprings
Mid-Atlantic Ridge, Iceland
Ocean Basins and Plates
Ocean Terms Tide Wave Height Length Period Base Refraction Tsunami Beach Sea stack Sea Arch Bay-mouth Bar Spit Tombolo Coast Emergent Submergent Atoll Guyot