16 oceans coasts_forstudents

Chapter 18Chapter 18

Oceans and CoastsOceans and Coasts

Chapter 18

Outline• Ocean exploration & oceanography

• The globes’ oceans-Lithosphere density and ocean basins-Global ocean landscape

• Submarine landscape-Major zones (shelf to Abyssal plains) – reflection of tectonics-Continental margins, submarine canyons-Ocean water composition, temperature

• Ocean dynamics-Currents, Coriolis effect, tides, waves

• Coastal landforms-beaches, barrier islands, rocky coasts, tidal flats, reefs etc..

Chapter 18

Oceans• Humans have explored the ocean for centuries.• Before 1800s,

• Dredged• Measured• Analyzed• Collected

Chapter 18

Oceans• Our knowledge of oceans has expanded greatly:

• Oceanography – • Marine geology – • Marine biology –

Chapter 18

Oceanography• Instruments have greatly expanded our knowledge.

• Submarine exploration – • Bathymetric mapping – • Ocean drilling

• Major

Chapter 18

Oceanography

• Seismic reflection profiling.• Sound waves• Some waves• Travel time of• Used to make images of

Chapter 18

The Global Oceans• Ocean covers

• Largest reservoir

• Oceans…• Serve as• Regulate• Cycle

• Linked to

Chapter 18

Global Landscape

• Oceans exist because of lithosphere differences. • Continental lithosphere• Oceanic lithosphere

• Ocean basins collect

Chapter 18

Global Ocean Landscape • The world ocean is

• Tectonic processes

• Biotic activity:

Chapter 18

Global Ocean Landscape

• Present configuration of tectonic plates…• Most continental crust in• Most oceanic crust in

Chapter 18

Undersea Landscapes• Sea floor bathymetry

• Continental shelf – • Continental slope – • Abyssal plain –

Chapter 18

Undersea Landscapes• Sea floor reflects

• Continental shelf – • Continental slope – • Abyssal plain –

Chapter 18

Undersea LandscapesContinental margins:• Passive• Active

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Eastern NA: passive

western SA: active

Undersea Landscapes

• Submarine canyons• Associated with• Carved during• Funnel sediments to• Submarine fans grow where

Chapter 18

Ocean Water Composition• Normal marine salinity, reflecting dissolved ions, is• 3.5%

• Dissolved ions derive from rock chemical weathering• Ions mostly cl, Na, SO, Mg, Ca, K

Chapter 18

Ocean Water Composition

• Surface salinity can vary.• Higher salinity evaporation and sea ice formation• Lower salinity rainfall, glacial melt, river input

• Salinity becomes more uniform with depth

Chapter 18

Ocean Water Temperature

• Ocean surface T varies inversely with latitude• Water buffers wide T shifts (moderates climate)• Water T approaches a uniform value with depth• Ocean bottom water is near freezing <4 degree C

Chapter 18

Oceanic Currents• Currents continuously move ocean water in 3d

• Surface currents (upper 100 m) - due to wind shear • Current motion creates large spirals (notice a pattern)

Chapter 18

The Coriolis Effect• Earth rotation deflects prevailing winds and currents• Acts as an “apparent”force on winds/currents• Coriolis deflection sense depends upon…

• Direction of motion• Position relative to equator

• Merry-go-round analogy:

Chapter 18

The Coriolis Effect• Surface winds & currents are both influenced

• North hemisphere:• S-moving winds/currents deflected to W

• N-moving winds/currents deflected to E

• South hemisphere.• N-moving winds/currents deflected to W

• S-moving winds/currents deflected to E

Chapter 18

Downwelling Upwelling

Vertical Oceanic Currents

• Currents also transport ocean water vertically • Downwelling – surface waters drawn downward• Upwelling – deep waters pushed upward

• Wind perpendicular to shore• Onshore – water piling up along coast drives downwelling • Offshore – upwelling replaces water moved away

Chapter 18

Vertical Oceanic Currents• Thermohaline contrast also drives vertical currents

• Temp – cold water is dense> sinks• Salinity – More saline water is dense> sinks

• Polar water is both cold and salty • Deep ocean waters are replenished from poles

Chapter 18

Oceanic Currents

• Sinking polar water is replaced by surface flows• This process carries warm water up from the tropics • These surface currents warm northern oceans

• System forms a global “conveyor belt”

Chapter 18

• Sea level rises and falls twice daily• High tide – max tidal flooding• Low tide – max tidal withdrawal• Tidal reach – range between high and low tides• Intertidal zone lies between tides

Chapter 18

Tides• Caused by:

• Gravitational pull of Moon and sun• Centrifugal forces from Earth Moon and Sun rotations

• Orbiting moon creates strongest tidal effects.• Sublunar bulge follows Moons orbit• Smaller bulge occurs on opposite side of Earth• Bulges = high tides ; low tides = between bulges

Chapter 18

• Lunar & solar tidal effects interact • Positive alignment yields enhance spring tides• Negative alignment results in dampened neap tides

Chapter 18

Waves• Ocean waves develop via friction from wind on water.

• Gentle wind small waves ; gales giant waves

• Waves move upper part of the water• Wave height, length, and period depend on wind speed,

wind duration, and travel distance (fetch)

Chapter 18

Waves• Wave anatomy:

• Crest – wave top• Trough – low between crests• Wavelength – distance between adajcent wave crest

• Depth of influence (wave base) is ½ the wavelength. • Above wave base, water moves in circular motion• Below wave base, water is not affected

Chapter 18

• As waves approach shore, wave base hits bottom • Friction slows wave motion near sea floor• Near surface, waves continue moving fast• Wave over steepens and

• This zone features

Chapter 18

• Waves that crash onto beach breakers• Wave energy dissipated by turbulence

• Creates white water in surf zone• Water surge (swash) rushes up beach face• Gravity pulls backwash back down beach slope

Chapter 18

Wave Refraction

• Irregular shoreline water depth varies• As waves drag on bottom, they are forced to bend• This process, wave refraction, has consequences:

• Wave attack concentrated on headlands • Wave attack is dissipated in embayments• Tend to straighten irregular shores

Chapter 18

Longshore Currents

• Sediment is transported along shore. • Oblique waves push sediment sideways up the beach• Gravity then pulls this sediment straight downshore• Zig-zag pattern moves sediment in one direction

Chapter 18

Rip Currents

• Develop when wave flow is perpendicular to shoreline• Water piles up on beach, must return seaward

• Rip current develops perpendicular to beach• Rip currents are often strong; people can get pulled under

• Rip currents dissipate away from the surf zone

Chapter 18

Coastal Landforms

• A variety of landforms

Chapter 18

Beaches

• Dominated by sand• Gravel beaches reflect energetic surf and a rock supply• Muds absent

• Turbulent surf suspends and removes finer sediment • Muds transported to lower energy environments

Chapter 18

Beaches

• Beach sediment composition reflects• Quartz• Carbonates• Resistant

Chapter 18

Beaches

• Beaches develop distinctive cross sectional profiles• Profiles change seasonally with energy regime

• Summer – broad sandy beach• Winter – narrow gravel beach

Chapter 18

Beaches• Distinct zones exist along a beach profile.

• Foreshore or intertidal – region between high and low tide• Beach face – steep, concave zone formed by wave swash• Backshore – upper part

• Beyond reach of• Often exhibit

Chapter 18

Beaches

• Longshore currents move sediment along beaches• This process, beach drift moves tons of sand daily• Beach drift builds bars and spits

Chapter 18

Beach Drift

This animation illustrates the sawtooth motion that causes sand to gradually migrate along beaches in a process called beach drift, and shows how this can create sand spits in places where the coastline indents landward. For more information, see Section 18.6 Where Land Meets Sea: Coastal Landforms starting on p. 638 and Figure 18.20 in your textbook.

Beach Drift

• Barrier islands are elongate, linear sand bars• Form where sand is plentiful• Protected backwater area called a lagoon• Common places for development• They are ephemeral (temporary)

Barrier Islands

Chapter 18

• Form in intertidal zones lacking strong waves • Common behind barrier islands or in estuaries • Thinly laminated sand and muds• Ancient tidal flat sediments

Tidal Flats

Chapter 18

• Bedrock intersects• Wave action is• Wave energy acts• Develop unique

Rocky Coasts

Chapter 18

• Wave-cut notches – waves erode an overhang • Cliff collapses and process resumes• Over time, cliff retreat is marked by a wave cut bench

• An erosional remnant of former cliffs• Often exposed at low tide

Rocky Coasts

Chapter 18

• Rocky headlands are preferentially eroded• Refracted waves focus energy to headland • Erosion from both sides• Collapse of the

Rocky Coasts

Chapter 18

• Wetlands cover large coastal regions• Develop in places protected from waves and currents• Fuel high biological productivities • Vegetation governed by climate

• Temperate -• Tropical –

Wetlands

Chapter 18

• River valleys flooded by marine water• Mixed fresh and salt water• Modern estuaries are related to glaciation

• Rivers carved canyons during sea level lows• Sea-level rise flooded the canyons

Estuaries

Chapter 18

• Flooded U shaped valleys carved by glaciers• Form spectacular bedrock bounded troughs • Notable examples found in…

• Norway.• British Columbia.• New Zealand.

Fjords

Chapter 18

• Coral reefs grow in tropical marine settings • Large structures of cemented skeletons • Most biologically productive ecosystems

Chapter 18

• Coral reefs modify sediment accumulation • Reefs alter wave and current energy• Protect• Abundant debris is shed to adjacent environments.

Chapter 18

• Coral atolls reefs formed on sudsiding volcano• Reef is established when is active • After extinction, volcano erodes and subsides • Reef can easily keep pace with subsidence • Reef continues long after volcano is below sea level

Chapter 18

Coastal Variability

• Plate tectonic setting governs coastline style • Passive margin – broad low lying • Active margin –

Chapter 18

Coastal Variability

• Global sea-level changes effect coasts worldwide• Inflation/deflation of mid ocean ridges • Glaciation/deglaciation traps or releases water

• Pleistocene glaciations exposed all continental shelves

Chapter 18

Coastal Variability

• Emergent coasts experience relative sea-level fall• Via uplift due to tectonic processes• Via sea-level drop (drop to global climate change)

• Emergent coasts characterized by…• River incision, cliffs, wave cut notches• Terraces representing former sea level positions

Chapter 18

Coastal Variability

• Submergent coasts experience relative sea-level rise• Subsidence of passive margin (deltaic sediment) • Global sea-level rise

• Submergent coasts characterized by flooded river or glacial valley that create estuaries and fjords

Chapter 18

Coastal Variability

• Shoreline character is linked to• Balance between

• Accretionary coasts – • Erosional coasts –

Chapter 18

Coastal Variability

• Climate is a strong influence on• Harsh weather enhances• Calm weather favors• Tropics – • Temperate – • Arctic –

Chapter 18

Coastal Problems

• Sea-level change.• Sea level is• Rate of

• People living in

Chapter 18

Coastal Problems

• Beach• Storms (e.g. hurricanes)• Human development in coastal settings• Construction in coastal settings is

Chapter 18

Mitigating Coastal Problems

• Artificial barriers built to• Groins, jetties, breakwaters• Usually produces

• Deposition is enhanced• BUT, erosion is accelerated

Chapter 18

• Approach to combat erosion

• Seawalls may hasten erosion.• Wave energy is• Erosion enhanced• Seawalls can

Chapter 18

Mitigating Coastal Problems

16 oceans coasts_forstudents

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