Chapter 15 – Restless Realm: Oceans and Coasts
• Why does ocean water circulate?
• How does the topography of the ocean floor change from a continental margin to the mid-ocean ridge?
How are the oceans mapped? • How do ocean waves and breakers form?
• How sediments are moved along a shore line and what coastal features are formed? • Be able to recognize both depositional and erosional coastal landforms
Why should we study the oceans?
• 70% of Earth’s surface
• Source for food, energy
• Used for transportation
• Drives Hydrologic Cycle
• In 1990, 50% of the U.S. population lived within 75 km of a coast; and by 2010, 75% will.
• Salinity is the concentration of salt in seawater (approximately 3.5%) • The dissolved salt content is not constant and changes with location and depth, the more evaportation the higher the concentration Temperature is stratified - varies with depth
Ocean currents move large amounts of water and heat by:
- wind-driven surface ocean circulation
- density-driven deep-ocean circulation
Cold salty water tends to sink
Warm, less salty water rises
Both redistribute heat from warmer regions to cooler regions
Landscapes Beneath the Sea
Mapping the seafloor by:
• Satellite measurements
• Echo sounding profiles
• Side-scan sonar
• Manned and unmanned submersibles
Major Physiographic Features in the Atlantic Ocean
• continental margin
– continental shelf – A broad, flat platform extending from the shoreline to the beginning of the continental slope. – continental slope - A steeper (~4º) , typically mud-draped slope marking the edge of the continental shelf. Dissected by submarine canyons and modified by turbidity currents – continental rise - A gently sloping apron of sediment formed by deposition of sands and muds at the base of the continental slope (typically at depths of 2-3 km). • abyssal plain - This plain extends beyond the continental rise typically 4-6 km below sea level. It is the flattest surface on the earth. May include submerged volcanoes called seamounts and guyots • mid-ocean ridge
– abyssal hills - Linear ridges of basalt covered with a thin veneer of deep-sea sediment on the flanks of the Mid-ocean ridge. – central rift valley
Why map the ocean floors?
Mapping is used to locate hazards that may affect communication lines, oil and gas pipelines, commercial and military transportation routes. Hazards: seamounts/guyot, areas modified by mass wasting (underwater mudflows); areas cut by faults
Coasts - the uneasy land-sea interface
Waves are due to wind
Higher waves can be caused by:
1) Higher wind speed
2) Increased storm duration (time)
3) Longer fetch (distance over which wind blows)
• Wave length – distance between crests
• Wave height – vertical distance between crest and trough • Period – time between successive waves to pass
Wave base is 1/2 wave length – there is negligible water movement due to waves below this depth
How do breakers form?
As the swell approaches the shore where the bottom shallows to less than ½ the wavelength, the wave touches the bottom causing it to slow – the wave period remains the same so the wavelength decreases and the waves height increase
(making the wave steeper). • As bottom shallows further the water can no longer support itself and the waves breaks and crashes in the surf zone
Major Parts of a Beach
Surf zone – offshore belt along which breaking waves collapse as they approach the shore Swash zone - zone where water run up on the beach from a wave Tidal flats – the area that lie above low tide but are flooded at high tide Beaches - sources of beach sediment is from rivers, cliff...
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