Download Ocean and Coastal Processes Ocean Basins Ocean Basins Tides

Chapter 20
Ocean Basins and Plates
Ocean and
Coastal Processes
Ocean Terms
• Tide
• Wave
– Height
– Length
– Period
– Base
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•
•
•
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.
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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
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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).
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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
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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
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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
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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).
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Submarine Canyons
Ocean Basins and Plates
Deep Sea P Sensor
Deep-sea pressure sensors
(Germany)
47
|48
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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
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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
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