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Chapter 20 Ocean Basins and Plates Ocean and Coastal Processes 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 Ocean Basins 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 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 • 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 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?? Beach Environments 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 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 (SiO2). • In tropical climates the beach sand mineral is calcite (CaCO3). • Beach sands may also be volcanic glass. •A 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 Sea Level Change 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 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 –A spit is a small peninsula of sand. barrier island is a long sand island closes a small bay. –A 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 tombolo is a sand bar connecting to a rock or off-shore outcrop. –A Emergent Coast Seastacks Seastacks Tombolo, Chiloe 6 Holei Sea Arch, HI Sea Arch, Co. Antrim, Ireland 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 improvement of seismographic network in Indonesia (Germany) Suggested global distribution of deep sea pressure sensors |49 Atlantic Ocean Basin |50 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