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 FloorThe 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