© 2012 pearson education, inc. earth science, 13e tarbuck & lutgens
TRANSCRIPT
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Earth Science, 13e
Tarbuck & Lutgens
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The Dynamic OceanEarth Science, 13e
Chapter 15
Stanley C. Hatfield Southwestern Illinois College
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Ocean water movements
Surface circulation • Ocean currents are masses of water that
flow from one place to another • Surface currents develop from friction
between the ocean and the wind that blows across the surface
• Huge, slowly moving gyres
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Ocean water movements
Surface circulation • Five main gyres
• North Pacific Gyre • South Pacific Gyre • North Atlantic Gyre • South Atlantic Gyre • Indian Ocean Gyre
• Related to atmospheric circulation
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Global surface circulation
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Ocean water movements
Surface circulation • Deflected by the Coriolis effect
• To the right in the Northern Hemisphere • To the left in the Southern Hemisphere
• Four main currents generally exist within each gyre
• Importance of surface currents • Climate
• Currents from low latitudes into higher latitudes (warm currents) transfer heat from warmer to cooler areas
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Ocean water movements
Surface circulation • Importance of surface currents
• Climate• Influence of cold currents is most pronounced in the
tropics or during the summer months in the middle latitudes
• Upwelling • The rising of cold water from deeper layers • Most characteristic along west coasts of continents • Brings greater concentrations of dissolved nutrients
to the ocean surface
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Ocean water movements
Deep-ocean circulation • A response to density differences• Factors creating a dense mass of water
• Temperature – cold water is dense• Salinity – density increases with increasing
salinity
• Called thermohaline circulation
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Ocean water movements
Deep-ocean circulation • Most water involved in deep-ocean
currents begins in high latitudes at the surface
• A simplified model of ocean circulation is similar to a conveyor belt that travels from the Atlantic Ocean, through the Indian and Pacific Oceans and back again
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Idealized “conveyor belt” model of ocean circulation
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The coastal zone
The land-sea boundary • Shoreline – contact between land and sea• Shore – area between lowest tidal level
and highest areas affected by storm waves• Coastline – the seaward edge of the coast• Beach – accumulation of sediment along
the landward margin of the ocean
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The coastal zone
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Ocean water movements
Waves • Energy traveling along the interface
between ocean and atmosphere • Derive their energy and motion from wind • Parts
• Crest • Trough
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Ocean water movements
Waves • Measurements of a wave
• Wave height – the distance between a trough and a crest
• Wavelength – the horizontal distance between successive crests (or troughs)
• Wave period – the time interval for one full wave to pass a fixed position
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Characteristics and movement of a wave
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Ocean water movements
Waves • Wave height, length, and period depend on
• Wind speed • Length of time the wind blows • Fetch – the distance that the wind travels
• As the wave travels, the water passes energy along by moving in a circle
• Waveform moves forward • At a depth of about one-half the wavelength,
the movement of water particles becomes negligible (the wave base)
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Changes that occur when a wave moves onto shore
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Wave erosion
Wave erosion • Caused by
• Wave impact and pressure • Breaks down rock material and supplies sand
to beaches• Abrasion – sawing and grinding action of water
armed with rock fragments
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Sand movement on the beach
Beaches are composed of whatever material is available • Some beaches have a significant
biological component • Material does not stay in one place
• Wave energy moves large quantities of sand parallel and perpendicular to the shoreline
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Beaches and shoreline processes
Wave refraction • Bending of a wave• Wave arrives parallel to shore • Results
• Wave energy is concentrated against the sides and ends of headland
• Wave erosion straightens an irregular shoreline
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Wave refraction along an irregular coastline
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Beaches and shoreline processes
Longshore transport • Beach drift – sediment moves in a
zigzag pattern along the beach face • Longshore current
• Current in surf zone • Flows parallel to the shore• Moves substantially more sediment than
beach drift
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Beach drift and longshore currents
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Shoreline features
Erosional features • Wave-cut cliff• Wave-cut platform • Marine terraces• Associated with headlands
• Sea arch• Sea stack
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Sea arch
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A sea stack and a sea arch
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Shoreline features
Depositional features • Spit – a ridge of sand extending from the
land into the mouth of an adjacent bay with an end that often hooks landward
• Baymouth bar – a sand bar that completely crosses a bay
• Tombolo – a ridge of sand that connects an island to the mainland
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Aerial view of a spit and
baymouth bar along the
Massachusetts coastline
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Spit
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Tombolo
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Shoreline features
Depositional features • Barrier islands
• Mainly along the Atlantic and Gulf Coastal Plains
• Parallel the coast• Originate in several ways
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Stabilizing the shore
Shoreline erosion is influenced by the local factors • Proximity to sediment-laden rivers • Degree of tectonic activity • Topography and composition of the
land • Prevailing wind and weather patterns • Configuration of the coastline
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Stabilizing the shore
Responses to erosion problems • Hard stabilization – building structures
• Types of structures • Groins – barriers built at a right angle to the
beach that are designed to trap sand
• Breakwaters – barriers built offshore and parallel to the coast to protect boats from breaking waves
• Seawalls – Armors the coast against the force of breaking waves
• Often these structures are not effective
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Stabilizing the shore
Responses to erosion problems • Alternatives to hard stabilization
• Beach nourishment by adding sand to the beach system
• Relocating buildings away from beach
Erosion problems along U.S. Coasts• Shoreline erosion problems are different
along the opposite coasts
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Miami Beach before beach nourishment
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Miami Beach after beach nourishment
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Stabilizing the shore
Erosion problems along U.S. Coasts• Atlantic and Gulf Coasts
• Development occurs mainly on barrier islands • Face open ocean
• Receive full force of storms
• Development has taken place more rapidly than our understanding of barrier island dynamics
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Stabilizing the shore
Erosion problems along U.S. Coasts• Pacific Coast
• Characterized by relatively narrow beaches backed by steep cliffs and mountain ranges
• Major problem is the narrowing of the beaches • Sediment for beaches is interrupted by dams
and reservoirs
• Rapid erosion occurs along the beaches
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Coastal classification
Shoreline classification is difficult Classification based on changes with
respect to sea level • Emergent coast
• Caused by • Uplift of the land, or
• A drop in sea level
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Coastal classification
Classification based on changes with respect to sea level • Emergent coast
• Features of an emergent coast • Wave-cut cliffs
• Marine terraces
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Coastal classification
Classification based on changes with respect to sea level • Submergent coast
• Caused by• Land adjacent to sea subsides, or
• Sea level rises
• Features of a submergent coast • Highly irregular shoreline
• Estuaries – drowned river mouths
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Major estuaries
along the East Coast of the
United States
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Tides
Changes in elevation of the ocean surface
Caused by the gravitational forces exerted upon Earth by the • Moon, and to a lesser extent by the • Sun
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Idealized tidal bulges on Earth
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Tides
Monthly tidal cycle • Spring tide
• During new and full moons• Gravitational forces added together • Especially high and low tides • Large daily tidal range
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Earth-Moon-Sun positions during the Spring tide
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Earth-Moon-Sun positions during the Neap tide
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Tides
Monthly tidal cycle • Neap tide
• First and third quarters of the Moon • Gravitational forces are offset• Daily tidal range is least
Tidal patterns • Many factors influence the tides
• Shape of the coastline • Configuration of the ocean basin • Water depth
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Tides
Tidal patterns • Main tidal patterns
• Diurnal tidal pattern• A single high and low tide each tidal day
• Occurs along the northern shore of the Gulf of Mexico
• Semidiurnal tidal pattern • Two high and two low tides each tidal day
• Little difference in the high and low water heights
• Common along the Atlantic Coast of the United States
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Tides
Tidal patterns • Main tidal patterns
• Mixed tidal pattern • Two high and two low waters each day
• Large inequality in high water heights, low water heights, or both
• Prevalent along the Pacific Coast of the United States
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Tides
Tidal patterns • Tidal currents
• Horizontal flow accompanying the rise and fall of tides
• Types of tidal currents • Flood current – advances into the coastal
zone
• Ebb current – seaward moving water
• Sometimes tidal deltas are created by tidal currents
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Features associated with tidal currents
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End of Chapter 15