The Oceans and Atmosphere
The Oceans and Atmosphere
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The Oceans and Atmosphere
The Oceans and Atmosphere
The shoreline - a dynamic interface
The shoreline is a dynamic interface (boundary) between air, land, and the ocean
The shoreline is constantly being modified by waves
Today the coastal zone is experiencing intense human activity
Waves
Characteristics of Waves
Generated by wind
Parts of a wave
Crest – top of the wave
Trough – low area between waves
Measurements of a wave
Wave height – the distance between a trough and a crest
Wavelength – the horizontal distance between crests
Wave period – the time interval between the passage of two successive crests
Wave parts and the movement of water particles
Particle Movement with Wave Passage
Height, length, and period of a wave depend on
Wind speed
Length of time wind has blown
Fetch – the distance that the wind has traveled across open water
Types of waves
Wave of oscillation
Wave energy moves forward, not the water itself
Occur in the open sea in deep water
Wave of translation
Begins to form in shallower water when the water-depth is about one-half the wavelength and the wave begins to “feel bottom”
As the speed and length of the wave diminish, the wave grows higher
The steep wave front collapses and the wave breaks along the shore
Turbulent water advances up the shore and forms surf
Changes that occur when a wave moves onto shore
Wave erosion
Breaking waves exert a great force
Wave erosion is caused by
Wave impact and pressure
Abrasion by rock fragments
Cliff undercut by wave erosion along the Oregon coast
Wave refraction and longshore transport
Wave refraction
Bending of a wave
Causes waves to arrive nearly parallel to the shore
Consequences of wave refraction
Wave energy is concentrated against the sides and ends of headlands
If the shoreline remains stable, the result of shoreline erosion and deposition is to eventually produce a straighter coast
Refraction of waves
Wave refraction and longshore transport
Wave refraction
Moving sand along the beach
Oblique waves also produce longshore currents
Currents in the surf zone
Flow parallel to the coast
Easily moves fine suspended sand and rolls larger sand and gravel along the bottom
Movement of sand by longshore current
Shoreline features
Features vary depending on several factors including
The rocks along the shore
Currents
Wave intensity
Whether the coast is stable, sinking, or rising
Features caused by wave erosion
Wave-cut cliffs
Wave-cut platform
Features associated with headlands
Sea arch
Sea stack
Wave Erosion
Anacapa Island
Sea stack
Spit
Features related to beach drift and longshore currents
Baymouth bar – a sand bar (spit) that completely crosses a bay
Tombolo – a ridge of sand that connects an island to the mainland or another island
Tombolo
Massachusetts coast Depositional features
Shoreline features
Barrier islands
Mainly along the Atlantic and Gulf coasts
Low ridges of sand that parallel the coast 3 to 30 kilometers offshore
Probably form in several ways
Barrier islands along the Texas coast
Shoreline erosion problems
Shoreline erosion is influenced by several local factors including
Proximity to sediment-laden rivers
Degree of tectonic activity
Topography and composition of the land
Prevailing wind and weather patterns
Configuration of the coastline and near-shore areas
Three basic responses to erosion problems
1. Building structures
Jetties
Usually built in pairs to develop and maintain harbors
Extend into the ocean at the entrances to rivers and harbors
Jetties prevent deposition in channels
Three basic responses to erosion problems
1. Building structures
Groins
Built to maintain or widen beaches
Constructed at a right angle to the beach to trap sand
Three basic responses to erosion problems
1. Building structures
Breakwater
Barrier built offshore and parallel to the coast
Protects boats from the force of large breaking waves
Three basic responses to erosion problems
1. Building structures
Seawall
Barrier parallel to shore and close to the beach to protect property
Stops waves form reaching the beach areas behind the wall
Often the building of structures is not an effective means of protection
Three basic responses to erosion problems
2. Beach nourishment
The addition of large quantities of sand to the beach system
Only an economically viable long-range solution in a few areas
3. Abandonment and relocation of buildings away from the beach
An Underfed Beach
After Beach Nourishment
Contrasting the Atlantic and Pacific Coasts
Shoreline erosion problems are different along the opposite coasts
Atlantic and Gulf Coasts
Broad, gently sloping coastal plains
Tectonically quiet regions
Contrasting the Atlantic and Pacific Coasts
Atlantic and Gulf Coasts
Development occurs mainly on the barrier islands (also called barrier beaches or coastal barriers)
Barrier islands face the open ocean
They receive the full force of storms
Contrasting the Atlantic and Pacific Coasts
Pacific Coast
Relatively narrow beaches backed by steep cliffs and mountain ranges
A major problem is a significant narrowing of many beaches
Shoreline erosion varies considerably from one year to the next largely because of the sporadic occurrence of storms
Emergent and submergent coasts
Emergent coasts
Develop because of uplift of an area or a drop in sea level
Features of an emergent coast
Wave-cut cliffs
Wave-cut platforms
Submergent coast
Caused by subsidence of land adjacent to the sea or a rise in sea level
Features of a submergent coast
Highly irregular shoreline
Estuaries – drowned river mouths
Chesapeake Bay is a submergent coastline
Tides
Daily changes in the elevation of the ocean surface
Causes of tides
Tidal bulges are caused by the gravitational forces of the Moon, and to a lesser extent the Sun
Tides are caused by gravitational forces of the Moon
Spring and neap tides
Spring tides
Occur during new and full moons
Gravitational forces of the Moon and Sun are added together
Especially high and low tides
Large daily tidal range
The Moon and Earth during spring tides
Spring and neap tides
Neap tides
Occur during the first and third quarters of the Moon
Gravitational forces of the Moon and Sun are offset
Daily tidal range is least
The Moon and Earth during neap tides
Other factors that influence tides
Shape of the coastline
Configuration of the ocean basin
Tidal currents
Horizontal flow of water accompanying the rise an fall of the tide
Tidal currents
Types of tidal currents
Flood current – advances into the coastal zone as the tide rises
Ebb current – seaward-moving water as the tide falls
Areas affected by the tidal currents are called tidal flats
Occasionally form tidal deltas
Tidal flats and a tidal delta
The Ocean Floor
The vast world ocean
Earth is often referred to as the water planet
71% of Earth’s surface is represented by oceans
Continents and islands comprise the remaining 29%
Northern Hemisphere is called the land hemisphere, and the Southern Hemisphere the water hemisphere
Distribution of land and water
Distribution of land and water
Extra-Terrestrial Water
Mapping the ocean floor
Depth was originally measured by lowering weighted lines overboard (sounding)
Echo sounder (also referred to as sonar)
Sound navigation and ranging
Invented in the 1920s
Primary instrument for measuring depth
Reflects sound from ocean floor
Multibeam sonar
Employs an array of sound sources and listening devices
Obtains a profile of a narrow strip of seafloor
Echo sounders (A) and multibeam sonar (B)
Three major topographic units of the ocean floor
Continental margins
Deep-ocean basins
Mid-ocean ridges
Major topographic divisions of the North Atlantic Ocean
Passive Continental margins
Found along most coastal area that surround the Atlantic Ocean
Not associated with plate boundaries
Experience little volcanism and few earthquakes
Features comprising a passive continental margin
Continental shelf
Flooded extension of the continent
Varies greatly in width
Gently sloping
Contain important mineral deposits
Some areas are mantled by extensive glacial deposits
Features comprising a passive continental margin
Continental slope
Marks the seaward edge of the continental shelf
Relatively steep structure
Boundary between continental crust and oceanic crust
Features comprising a passive continental margin
Continental rise
Found in regions where trenches are absent
Continental slope merges into a more gradual incline – the continental rise
Thick accumulation of sediment
At the base of the continental slope turbidity currents deposit sediment that forms deep-sea fans
Submarine canyons and turbidity currents
Submarine canyons
Deep, steep-sided valleys cut into the continental slope
Some are extensions of river valleys
Most appear to have been eroded by turbidity currents
Turbidity currents
Downslope movements of dense, sediment-laden water
Deposits are called turbidites
Turbidites are layered and exhibit graded bedding (decrease in sediment grain size from bottom to top)
Submarine canyons are eroded by turbidity currents
Active Continental margins
Continental slope descends abruptly into a deep-ocean trench
Located primarily around the Pacific Ocean
Accumulations of deformed sediment and scraps of ocean crust form accretionary wedges
An active continental margin
Features of the deep-ocean basin
Deep-ocean trench
Long, relatively narrow features
Deepest parts of ocean
Most are located in the Pacific Ocean
Sites where moving lithospheric plates plunge into the mantle
Associated with volcanic activity
The world’s major oceanic trenches
Abyssal plains
Likely the most level places on Earth
Sites of thick accumulations of sediment
found in all oceans
Seamounts
Isolated volcanic peaks
Many form near oceanic ridges
May emerge as an island
May sink and form flat-topped seamounts called guyots
Mid-ocean ridges
Characterized by elevated topography, extensive faulting, and volcanic activity
Interconnected ridge system is the longest topographic feature on Earth’s surface
Along the axis of some segments are deep down-faulted structures called rift valleys
Consist of layer upon layer of basaltic rocks that have been faulted and uplifted
Coral reefs and atolls
Coral reefs
Constructed primarily from skeletal remains and secretions of corals and certain algae
Confined largely to the warm, clear waters of the Pacific and Indian Oceans
Examples of Coral
Atolls
Coral islands – a continuous ring of coral reef surrounding a central lagoon
Form on the flanks of a sinking volcanic island (hypothesis proposed by Charles Darwin)
Formation of a coral atoll
Atolls in the Pacific
Key Terms Chapter 12
Salinity
Tides (ebb, flood, spring, neap)
Tidal delta
Waves, surf
Longshore currents, transport
Beach drift, beach erosion
Wave-cut cliff
Spit
Baymouth bar
Tombolo
Barrier island
Emergent coastline, elevated wave-cut terrace
Submergent coastline, estuary
Continental shelf, slope, rise
Abyssal plain
Mid-ocean ridge
Reef
Greenhouse effect
Ozone layer
Coriolis effect
Source : http://www2.bakersfieldcollege.edu/moldershaw/Ch%2012%20Web%20Notes.doc
Web site link: http://www2.bakersfieldcollege.edu/moldershaw/
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