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Chapter 20 Coastlines and Ocean Basins, and Continental Shelves, Slopes, and Rises


Coastlines The land-sea interface (+/- 50m of sea level) Coastal processes are the interaction of the climate system Tectonics Oceanic systems (deep ocean currents) Terrestrial (stream) systems Waves, tides, and sediment delivery at river mouths are key processes that control the nature of the coastline Is it rocky? A sandy beach? A bay suitable for a port? A swamp? A reef good for fishing and diving?

Ocean Basins: 

Ocean Basins The deep sea (3.5-8 km below sea level) Morphology Mid-ocean ridges (divergent plate boundaries) Trenches (convergent plate boundaries) Transform faults (transform plate boundaries) Seamounts and Mid-ocean islands (hotspots). Rocks Thin layers of pelagic (deep sea sediment) deposits Overlay basalts & gabbros of oceanic crust created at spreading centers.

Shelves, Slopes, and Rises: 

Shelves, Slopes, and Rises Transition from deep-sea to coast (50-300 m, 300- 3.5 km, and deeper, respectively) Bathymetry changes from almost flat close to shore steep on slope gradation to abbysal plain on rise Rocks terrestrial sediments (delivered by river systems) Reefs Other carbonate-dominated sediments

Lecture Outline: 

Lecture Outline Shorelines & coastal processes Continental margins (shelf, slope & rise) The deep ocean basins Sea-level change as an indicator of global climate

Coastal Processes: 

Coastal Processes Sediment Delivery Provided by rivers A lot = deltas A little = sandy or muddy beaches Not much = rocky shorelines Waves, Longshore Currents, and Rip Tides Sculpt the shoreline Sort fine vs. coarse (clay vs. silt. vs. sand) delivered by river systems Transport fines out to sea (building shelf), leaving coarse materials near shore (beaches) Balance between sediment delivery and waves/currents controls: Erosion (rocky shorelines of California): supply less than transport capacity Deposition (growing deltas of central Gulf of Mexico): supply exceeds transport capacity Equilibrium (stable beaches of western Gulf of Mexico): balance between sediment supply and transport capacity Storms are abrupt events that dramatically change coasts through both erosion and deposition – temporary disruptions of sediment supply/transport equation


Straight sandy beach: supply = transport Pea Island, North Carolina


Rocky coastline: supply < transport Mount Desert Island, Maine


Wave effects – “sea stacks” Erosion or deposition? Supply >, =, or < transport? Port Campbell, Australia


southern Florida Reef and beach: transport </= supply REEF BEACH LAGOON


Waves Shape Describing a wave wave length wave height wave period Depends on wind Velocity Fetch Duration Motion Refraction Wave crests change direction as they approach shoreline Bend toward shoreline Breakers Top of wave outruns bottom as seafloor shallows, creating a breaker Creates surf zone Effect on shorelines depends on angle seafloor morphology (shape) composition of coastline (eroding rocks or sandy beaches)


Spits and barrier islands: Supply = transport Cape Cod, Massachusetts


Breakers refracting toward an eroding beach


wave motion near shore – a “head-on collision”

The Surf Zone: aargh, ‘tis a swashbuckler I be …: 

The Surf Zone: aargh, ‘tis a swashbuckler I be … The surf is the area near shore where waves advance & retreat Waves break (collapse) as they approach the shore The swash is the part of the wave that washes up onto the shore The backwash is the part of the wave that retreats off the shore I have no idea where the buckles come in, except that pirates liked to wear buckles on their boots (“swashbuckles”?).


wave motion near shore – coming in at an angle


wave motion near shore: forming headlands

Coastal Currents: 

Coastal Currents Longshore currents (parallel to shore) caused by wave approach at an angle to the shore zig-zag path of swash and backwash occurs at the shore Rip currents (perpendicular to shore) caused by buildup of water moving parallel to shore in longshore currents often follow small creek drainages perpendicular to shore


Longshore & offshore transport


Tides Twice daily advance & retreat of shoreline flood tide ebb tide Mainly of Gravitational origin Solar tides daily Lunar tides spring neap A few inches to a few meters, depending on … Earth-Moon-Sun alignment shoreline configuration tidal surges with storms


Tides: Lunar effects …


Spring tides – large twice-daily tide changes, large tidal currents.


Neap tides – modest twice daily tidal changes, small tidal currents

Hurricanes – Abruptly modify coastlines: 

Hurricanes – Abruptly modify coastlines Seasonal Due to warming of surface waters Latitudinal effect due to solar radiation (greater in low latitudes) Coriolis effect (due to Earth’s rotation) Think of motion of Earth relative to atmosphere Low latitudes, earth moves fast in east-to-west direction Rising air mass develops counter-clockwise rotation in northern hemisphere (earth beneath air to south of eye of storm moves west, making rising air appear to move east relative to surface) Storm track usually rotates toward poles (clockwise in northern hemisphere). As rising air move pole-ward, earth beneath it moves westward, creating clockwise storm path in northern hemisphere. Safford-Simpson Scale Related to wind speed Damage Wind Flooding Storm Surge


Hurricane Katrina, August 2005 Storm Surge Leeward Windward


storm surge effects New Orleans, Louisiana


storm surge effects Hurricane Katrina – August 2005



Continental Margins: 

Continental Margins Types of continental margin passive (former divergent plate boundaries) East coast U.S. U.S. Gulf of Mexico Red Sea active (modern convergent or divergent plate boundaries) U.S. west coast South America west coast Components of continental margins shorelines shelves, slopes, and rises


Submarine canyons Coastal plain Continental shelf Continental slope Continental rise Continental crust Oceanic crust Mantle Deep-sea fan Abyssal plain Passive Continental Margins


Continental crust Oceanic crust Mantle Accretionary wedge Offshore trench Continental volcanic belt Active Continental Margins: Convergent

The Deep Ocean Basins: 

The Deep Ocean Basins mid-ocean ridges transform faults abyssal plains deep sea trenches island arcs oceanic islands, seamounts, and guyots


Example of an ocean basin: the Atlantic profile


Example of an ocean basin: the Pacific profile


Abyssal Plain Sedimentation: a thin layer of muddy “oozes” derived from slow settling of the skeletons of plankton (silica and calcite) – depends on latitude & depth


Ridges & Transform Faults – the divergent boundary transform faults rift valley


The Atlantic Mid-Ocean Ridge

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