Chapter 20 Ocean Basins and Plates Ocean and Coastal Processes Tide Wave Height Length Period Base Refraction Tsunami Beach Sea stack Ocean Terms Sea Arch Bay-mouth Bar Spit Tombolo Coast Emergent Submergent Atoll Guyot 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 Tides are caused by gravitational attraction of the moon and sun. The tidal bulge forms on the side facing the moon and the side away from the moon. If there were no land the tide would pass around the Earth with a 12 h period. The presence of land masses can cancel or multiply the amplitude. 1
Tides are caused by Gravitational Attraction of the Sun and Moon Tides Tide maxima (spring tides) occur when sun and moon line up (new moon, full moon). Tide minima (neap tides) occur when sun and moon are at right angles (first quarter and third quarter 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 2
Refraction Refraction Waves travel more slowly in shallow water (shallower than the wave base). This is due to drag on the bottom. 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 Wave Refraction Wave 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). 3
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 4
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). Sea Level Change Submergent-coast Beaches Barrier Island 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. Spit Bay-mouth Bar 5
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. Sea Caves A tombolo is a sand bar connecting to a rock or off-shore outcrop. Emergent Coast Seastacks Seastacks Tombolo, Chiloe 6
Sea Arch, Co. Antrim, Ireland Holei Sea Arch, HI Sea Cave, OR 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). 7
Submarine Canyons Ocean Basins and Plates Deep Sea P Sensor Deep-sea pressure sensors (Germany) 47 48 8
Suggested global distribution of deep sea pressure sensors Suggested improvement of seismographic network in Indonesia (Germany) 49 50 Atlantic Ocean Basin Pacific Ocean Topographic Profile 9
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 10