16 oceans coasts_forstudents
TRANSCRIPT
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Chapter 18Chapter 18
Oceans and CoastsOceans and Coasts
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Chapter 18
Outline• Ocean exploration & oceanography
• The globes’ oceans-Lithosphere density and ocean basins-Global ocean landscape
• Submarine landscape-Major zones (shelf to Abyssal plains) – reflection of tectonics-Continental margins, submarine canyons-Ocean water composition, temperature
• Ocean dynamics-Currents, Coriolis effect, tides, waves
• Coastal landforms-beaches, barrier islands, rocky coasts, tidal flats, reefs etc..
Chapter 18
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Chapter 18Chapter 18
Oceans• Humans have explored the ocean for centuries.• Before 1800s,
• Dredged• Measured• Analyzed• Collected
Chapter 18
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Chapter 18Chapter 18
Oceans• Our knowledge of oceans has expanded greatly:
• Oceanography – • Marine geology – • Marine biology –
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Chapter 18Chapter 18
Oceanography• Instruments have greatly expanded our knowledge.
• Submarine exploration – • Bathymetric mapping – • Ocean drilling
• Major
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Chapter 18Chapter 18
Oceanography
• Seismic reflection profiling.• Sound waves• Some waves• Travel time of• Used to make images of
Chapter 18
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Chapter 18
Outline• Ocean exploration & oceanography
• The globes’ oceans-Lithosphere density and ocean basins-Global ocean landscape
• Submarine landscape-Major zones (shelf to Abyssal plains) – reflection of tectonics-Continental margins, submarine canyons-Ocean water composition, temperature
• Ocean dynamics-Currents, Coriolis effect, tides, waves
• Coastal landforms-beaches, barrier islands, rocky coasts, tidal flats, reefs etc..
Chapter 18
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Chapter 18Chapter 18
The Global Oceans• Ocean covers
• Largest reservoir
• Oceans…• Serve as• Regulate• Cycle
• Linked to
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Chapter 18Chapter 18
Global Landscape
• Oceans exist because of lithosphere differences. • Continental lithosphere• Oceanic lithosphere
• Ocean basins collect
Chapter 18
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Chapter 18Chapter 18
Global Ocean Landscape • The world ocean is
• Tectonic processes
• Biotic activity:
Chapter 18
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Chapter 18Chapter 18
Global Ocean Landscape
• Present configuration of tectonic plates…• Most continental crust in• Most oceanic crust in
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Chapter 18
Outline• Ocean exploration & oceanography
• The globes’ oceans-Lithosphere density and ocean basins-Global ocean landscape
• Submarine landscape-Major zones (shelf to Abyssal plains) – reflection of tectonics-Continental margins, submarine canyons-Ocean water composition, temperature
• Ocean dynamics-Currents, Coriolis effect, tides, waves
• Coastal landforms-beaches, barrier islands, rocky coasts, tidal flats, reefs etc..
Chapter 18
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Chapter 18Chapter 18
Undersea Landscapes• Sea floor bathymetry
• Continental shelf – • Continental slope – • Abyssal plain –
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Chapter 18Chapter 18
Undersea Landscapes• Sea floor reflects
• Continental shelf – • Continental slope – • Abyssal plain –
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Chapter 18Chapter 18
Undersea LandscapesContinental margins:• Passive• Active
Chapter 18
Eastern NA: passive
western SA: active
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Chapter 18Chapter 18
Undersea Landscapes
• Submarine canyons• Associated with• Carved during• Funnel sediments to• Submarine fans grow where
Chapter 18
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Chapter 18Chapter 18
Ocean Water Composition• Normal marine salinity, reflecting dissolved ions, is• 3.5%
• Dissolved ions derive from rock chemical weathering• Ions mostly cl, Na, SO, Mg, Ca, K
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Chapter 18Chapter 18
Ocean Water Composition
• Surface salinity can vary.• Higher salinity evaporation and sea ice formation• Lower salinity rainfall, glacial melt, river input
• Salinity becomes more uniform with depth
Chapter 18
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Chapter 18Chapter 18
Ocean Water Temperature
• Ocean surface T varies inversely with latitude• Water buffers wide T shifts (moderates climate)• Water T approaches a uniform value with depth• Ocean bottom water is near freezing <4 degree C
Chapter 18
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Chapter 18
Outline• Ocean exploration & oceanography
• The globes’ oceans-Lithosphere density and ocean basins-Global ocean landscape
• Submarine landscape-Major zones (shelf to Abyssal plains) – reflection of tectonics-Continental margins, submarine canyons-Ocean water composition, temperature
• Ocean dynamics-Currents, Coriolis effect, tides, waves
• Coastal landforms-beaches, barrier islands, rocky coasts, tidal flats, reefs etc..
Chapter 18
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Chapter 18Chapter 18
Oceanic Currents• Currents continuously move ocean water in 3d
• Surface currents (upper 100 m) - due to wind shear • Current motion creates large spirals (notice a pattern)
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Chapter 18Chapter 18
The Coriolis Effect• Earth rotation deflects prevailing winds and currents• Acts as an “apparent”force on winds/currents• Coriolis deflection sense depends upon…
• Direction of motion• Position relative to equator
• Merry-go-round analogy:
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Chapter 18Chapter 18
The Coriolis Effect• Surface winds & currents are both influenced
• North hemisphere:• S-moving winds/currents deflected to W
• N-moving winds/currents deflected to E
• South hemisphere.• N-moving winds/currents deflected to W
• S-moving winds/currents deflected to E
Chapter 18
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Chapter 18Chapter 18
Downwelling Upwelling
Vertical Oceanic Currents
• Currents also transport ocean water vertically • Downwelling – surface waters drawn downward• Upwelling – deep waters pushed upward
• Wind perpendicular to shore• Onshore – water piling up along coast drives downwelling • Offshore – upwelling replaces water moved away
Chapter 18
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Chapter 18Chapter 18
Vertical Oceanic Currents• Thermohaline contrast also drives vertical currents
• Temp – cold water is dense> sinks• Salinity – More saline water is dense> sinks
• Polar water is both cold and salty • Deep ocean waters are replenished from poles
Chapter 18
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Chapter 18Chapter 18
Oceanic Currents
• Sinking polar water is replaced by surface flows• This process carries warm water up from the tropics • These surface currents warm northern oceans
• System forms a global “conveyor belt”
Chapter 18
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Chapter 18Chapter 18
Tides
• Sea level rises and falls twice daily• High tide – max tidal flooding• Low tide – max tidal withdrawal• Tidal reach – range between high and low tides• Intertidal zone lies between tides
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Chapter 18Chapter 18
Tides• Caused by:
• Gravitational pull of Moon and sun• Centrifugal forces from Earth Moon and Sun rotations
• Orbiting moon creates strongest tidal effects.• Sublunar bulge follows Moons orbit• Smaller bulge occurs on opposite side of Earth• Bulges = high tides ; low tides = between bulges
Chapter 18
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Chapter 18Chapter 18
Tides
• Lunar & solar tidal effects interact • Positive alignment yields enhance spring tides• Negative alignment results in dampened neap tides
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Chapter 18Chapter 18
Waves• Ocean waves develop via friction from wind on water.
• Gentle wind small waves ; gales giant waves
• Waves move upper part of the water• Wave height, length, and period depend on wind speed,
wind duration, and travel distance (fetch)
Chapter 18
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Chapter 18Chapter 18
Waves• Wave anatomy:
• Crest – wave top• Trough – low between crests• Wavelength – distance between adajcent wave crest
• Depth of influence (wave base) is ½ the wavelength. • Above wave base, water moves in circular motion• Below wave base, water is not affected
Chapter 18
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Chapter 18Chapter 18
Waves
• As waves approach shore, wave base hits bottom • Friction slows wave motion near sea floor• Near surface, waves continue moving fast• Wave over steepens and
• This zone features
Chapter 18
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Chapter 18Chapter 18
Waves
• Waves that crash onto beach breakers• Wave energy dissipated by turbulence
• Creates white water in surf zone• Water surge (swash) rushes up beach face• Gravity pulls backwash back down beach slope
Chapter 18
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Chapter 18Chapter 18
Wave Refraction
• Irregular shoreline water depth varies• As waves drag on bottom, they are forced to bend• This process, wave refraction, has consequences:
• Wave attack concentrated on headlands • Wave attack is dissipated in embayments• Tend to straighten irregular shores
Chapter 18
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Chapter 18Chapter 18
Longshore Currents
• Sediment is transported along shore. • Oblique waves push sediment sideways up the beach• Gravity then pulls this sediment straight downshore• Zig-zag pattern moves sediment in one direction
Chapter 18
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Chapter 18Chapter 18
Rip Currents
• Develop when wave flow is perpendicular to shoreline• Water piles up on beach, must return seaward
• Rip current develops perpendicular to beach• Rip currents are often strong; people can get pulled under
• Rip currents dissipate away from the surf zone
Chapter 18
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Chapter 18
Outline• Ocean exploration & oceanography
• The globes’ oceans-Lithosphere density and ocean basins-Global ocean landscape
• Submarine landscape-Major zones (shelf to Abyssal plains) – reflection of tectonics-Continental margins, submarine canyons-Ocean water composition, temperature
• Ocean dynamics-Currents, Coriolis effect, tides, waves
• Coastal landforms-beaches, barrier islands, rocky coasts, tidal flats, reefs etc..
Chapter 18
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Chapter 18Chapter 18
Coastal Landforms
• A variety of landforms
Chapter 18
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Chapter 18Chapter 18
Beaches
• Dominated by sand• Gravel beaches reflect energetic surf and a rock supply• Muds absent
• Turbulent surf suspends and removes finer sediment • Muds transported to lower energy environments
Chapter 18
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Chapter 18Chapter 18
Beaches
• Beach sediment composition reflects• Quartz• Carbonates• Resistant
Chapter 18
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Chapter 18Chapter 18
Beaches
• Beaches develop distinctive cross sectional profiles• Profiles change seasonally with energy regime
• Summer – broad sandy beach• Winter – narrow gravel beach
Chapter 18
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Chapter 18Chapter 18
Beaches• Distinct zones exist along a beach profile.
• Foreshore or intertidal – region between high and low tide• Beach face – steep, concave zone formed by wave swash• Backshore – upper part
• Beyond reach of• Often exhibit
Chapter 18
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Chapter 18Chapter 18
Beaches
• Longshore currents move sediment along beaches• This process, beach drift moves tons of sand daily• Beach drift builds bars and spits
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Chapter 18Chapter 18
Beach Drift
This animation illustrates the sawtooth motion that causes sand to gradually migrate along beaches in a process called beach drift, and shows how this can create sand spits in places where the coastline indents landward. For more information, see Section 18.6 Where Land Meets Sea: Coastal Landforms starting on p. 638 and Figure 18.20 in your textbook.
Beach Drift
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Chapter 18Chapter 18
• Barrier islands are elongate, linear sand bars• Form where sand is plentiful• Protected backwater area called a lagoon• Common places for development• They are ephemeral (temporary)
Barrier Islands
Chapter 18
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Chapter 18Chapter 18
• Form in intertidal zones lacking strong waves • Common behind barrier islands or in estuaries • Thinly laminated sand and muds• Ancient tidal flat sediments
Tidal Flats
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Chapter 18Chapter 18
• Bedrock intersects• Wave action is• Wave energy acts• Develop unique
Rocky Coasts
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Chapter 18Chapter 18
• Wave-cut notches – waves erode an overhang • Cliff collapses and process resumes• Over time, cliff retreat is marked by a wave cut bench
• An erosional remnant of former cliffs• Often exposed at low tide
Rocky Coasts
Chapter 18
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Chapter 18Chapter 18
• Rocky headlands are preferentially eroded• Refracted waves focus energy to headland • Erosion from both sides• Collapse of the
Rocky Coasts
Chapter 18
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Chapter 18Chapter 18
• Wetlands cover large coastal regions• Develop in places protected from waves and currents• Fuel high biological productivities • Vegetation governed by climate
• Temperate -• Tropical –
Wetlands
Chapter 18
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Chapter 18Chapter 18
• River valleys flooded by marine water• Mixed fresh and salt water• Modern estuaries are related to glaciation
• Rivers carved canyons during sea level lows• Sea-level rise flooded the canyons
Estuaries
Chapter 18
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Chapter 18Chapter 18
• Flooded U shaped valleys carved by glaciers• Form spectacular bedrock bounded troughs • Notable examples found in…
• Norway.• British Columbia.• New Zealand.
Fjords
Chapter 18
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Chapter 18Chapter 18
• Coral reefs grow in tropical marine settings • Large structures of cemented skeletons • Most biologically productive ecosystems
Reefs
Chapter 18
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Chapter 18Chapter 18
• Coral reefs modify sediment accumulation • Reefs alter wave and current energy• Protect• Abundant debris is shed to adjacent environments.
Reefs
Chapter 18
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Chapter 18Chapter 18
• Coral atolls reefs formed on sudsiding volcano• Reef is established when is active • After extinction, volcano erodes and subsides • Reef can easily keep pace with subsidence • Reef continues long after volcano is below sea level
Reefs
Chapter 18
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Chapter 18Chapter 18
Coastal Variability
• Plate tectonic setting governs coastline style • Passive margin – broad low lying • Active margin –
Chapter 18
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Chapter 18Chapter 18
Coastal Variability
• Global sea-level changes effect coasts worldwide• Inflation/deflation of mid ocean ridges • Glaciation/deglaciation traps or releases water
• Pleistocene glaciations exposed all continental shelves
Chapter 18
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Chapter 18Chapter 18
Coastal Variability
• Emergent coasts experience relative sea-level fall• Via uplift due to tectonic processes• Via sea-level drop (drop to global climate change)
• Emergent coasts characterized by…• River incision, cliffs, wave cut notches• Terraces representing former sea level positions
Chapter 18
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Chapter 18Chapter 18
Coastal Variability
• Submergent coasts experience relative sea-level rise• Subsidence of passive margin (deltaic sediment) • Global sea-level rise
• Submergent coasts characterized by flooded river or glacial valley that create estuaries and fjords
Chapter 18
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Chapter 18Chapter 18
Coastal Variability
• Shoreline character is linked to• Balance between
• Accretionary coasts – • Erosional coasts –
Chapter 18
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Chapter 18Chapter 18
Coastal Variability
• Climate is a strong influence on• Harsh weather enhances• Calm weather favors• Tropics – • Temperate – • Arctic –
Chapter 18
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Chapter 18Chapter 18
Coastal Problems
• Sea-level change.• Sea level is• Rate of
• People living in
Chapter 18
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Chapter 18Chapter 18
Coastal Problems
• Beach• Storms (e.g. hurricanes)• Human development in coastal settings• Construction in coastal settings is
Chapter 18
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Chapter 18Chapter 18
Mitigating Coastal Problems
• Artificial barriers built to• Groins, jetties, breakwaters• Usually produces
• Deposition is enhanced• BUT, erosion is accelerated
Chapter 18
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Chapter 18Chapter 18
• Approach to combat erosion
• Seawalls may hasten erosion.• Wave energy is• Erosion enhanced• Seawalls can
Chapter 18
Mitigating Coastal Problems