geology lecture 17

Chapter 18Chapter 18

Oceans 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..

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Oceans• Humans have explored the ocean for centuries.• Before 1800s, sea floors was unknown

• HMS Challenger (1872-1876) 1st oceanography study• Dredged sea floor rocks• Measured seawater, depth, currents• Analyzed water• Collected biota

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Oceans• Our knowledge of oceans has expanded greatly:

• Oceanography – study of ocean water and its movement• Marine geology – study of ocean floor rocks/seds• Marine biology – study of ocean life

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Oceanography• Instruments have greatly expanded our knowledge.

• Submarine exploration – Alvin (WHOI)• Bathymetric mapping – sonar/radar• Ocean drilling- recovers time archived below the seafloor

• Major drilling projects (DSDP, ODP, IODP)

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Oceanography• Seismic reflection profiling.

• Sound waves pass through water and sediment• Some waves bounce off each subsurface layer• Travel time of reflected waves captured by geophones• Used to make images of ocean floor geology

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The Global Oceans• Ocean covers 70.8% of Earth’s surface

• Largest reservoir in hydrologic cycle

• Oceans…• Serve as basis for life• Regulate climate• Cycle mass and energy

• Linked to atmosphere,• lithosphere, biosphere

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Global Landscape

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

• Ocean basins collect

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Global Ocean Landscape • The world ocean is

• Tectonic processes

• Biotic activity:

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Global Ocean Landscape• Present configuration of tectonic plates…

• Most continental crust in• Most oceanic crust in

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Undersea Landscapes• Sea floor bathymetry

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

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Undersea Landscapes• Sea floor reflects

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

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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

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

• Dissolved ions derive from• Ions mostly

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Ocean Water Composition• Surface salinity can vary.

• Higher salinity • Lower salinity

• Salinity becomes

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Ocean Water Temperature

• Ocean surface T varies• Water buffers• Water T approaches• Ocean bottom water is

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Oceanic Currents• Currents continuously

• Surface currents (upper 100 m) - due to• Current motion creates

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The Coriolis Effect• Earth rotation deflects• Acts as an• Coriolis deflection sense depends upon…

• Merry-go-round analogy:

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The Coriolis Effect• Surface winds & currents are

• North hemisphere:• S-moving winds/currents deflected• N-moving winds/currents deflected

• South hemisphere.• N-moving winds/currents deflected• S-moving winds/currents deflected

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Downwelling Upwelling

Vertical Oceanic Currents• Currents also transport

• Downwelling – • Upwelling –

• Wind perpendicular to shore• Onshore – • Offshore –

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Vertical Oceanic Currents• Thermohaline contrast also

• Temp – cold water• Salinity – More saline water

• Polar water is both• Deep ocean waters are

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Oceanic Currents

• Sinking polar water is• This process carries• These surface currents

• System forms a

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Tides• Sea level rises and falls

• High tide – • Low tide – • Tidal reach – • Intertidal zone

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Tides• Caused by:

• Gravitational pull of• Centrifugal forces from

• Orbiting moon creates strongest tidal effects.• Sublunar bulge follows• Smaller bulge occurs• Bulges = ; low tides =

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Tides

• Lunar & solar tidal effects• Positive alignment yields• Negative alignment results in

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

• Gentle wind ; gales

• Waves move upper part• Wave height, length, and period depend on

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Waves• Wave anatomy:

• Crest – • Trough – • Wavelength –

• Depth of influence (wave base) is ½ the wavelength. • Above wave base, water• Below wave base, water

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Waves• As waves approach shore,

• Friction• Near surface, waves• Wave

• This zone features

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

• Creates• Water surge (swash)• Gravity pulls

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Wave Refraction• Irregular shoreline • As waves drag on bottom, • This process, wave refraction, has consequences:

• Wave attack concentrated• Wave attack is dissipated• Tend to

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Longshore Currents

• Sediment is transported along shore. • Oblique waves push• Gravity then pulls• Zig-zag pattern

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Rip Currents• Develop when wave flow is

• Water piles up on beach, • Rip current develops• Rip currents are

• Rip currents dissipate away from

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Coastal Landforms• A variety of landforms

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Beaches• Dominated by• Gravel beaches reflect• Muds

• Turbulent surf• Muds transported to

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Beaches

• Beach sediment composition reflects• Quartz• Carbonates• Resistant

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Beaches• Beaches develop distinctive• Profiles change

• Summer – • Winter –

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Beaches• Distinct zones exist along a beach profile.

• Foreshore or intertidal – • Beach face – • Backshore –

• Beyond reach of• Often exhibit

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Beaches

• Longshore currents move• This process, beach• Beach drift builds

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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• Form where• Protected backwater area called• Common places for• They are

Barrier Islands

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• Form in intertidal zones• Common behind• Thinly laminated• Ancient tidal flat sediments

Tidal Flats

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

Rocky Coasts

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• Wave-cut notches – • Cliff• Over time, cliff

• An erosional remnant of• Often exposed at

Rocky Coasts

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• Rocky headlands are• Refracted waves focus• Erosion from both sides• Collapse of the

Rocky Coasts

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• Wetlands cover• Develop in places protected from• Fuel high• Vegetation governed by

• Temperate -• Tropical –

Wetlands

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• River valleys flooded by• Mixed• Modern estuaries are

• Rivers carved canyons during• Sea-level rise

Estuaries

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• Flooded• Form spectacular• Notable examples found in…

• Norway.• British Columbia.• New Zealand.

Fjords

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• Coral reefs grow in• Large structures of• Most biologically productive

Reefs

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• Coral reefs• Reefs alter• Protect• Abundant debris is shed to adjacent environments.

Reefs

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• Coral atolls reefs formed on• Reef is established when• After extinction, • Reef can easily keep pace• Reef continues long after

Reefs

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Coastal Variability

• Plate tectonic setting governs• Passive margin – • Active margin –

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Coastal Variability• Global sea-level changes effect

• Inflation/deflation of• Glaciation/deglaciation

• Pleistocene glaciations

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Coastal Variability• Emergent coasts experience relative sea-level

• Via uplift due to• Via sea-level

• Emergent coasts characterized by…• River• Terraces representing

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Coastal Variability

• Submergent coasts experience relative sea-level• Subsidence of• Global sea-level

• Submergent coasts characterized by

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Coastal Variability

• Shoreline character is linked to• Balance between

• Accretionary coasts – • Erosional coasts –

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Coastal Variability• Climate is a strong influence on

• Harsh weather enhances• Calm weather favors• Tropics – • Temperate – • Arctic –

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Coastal Problems

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

• People living in

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Coastal Problems

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

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Mitigating Coastal Problems• Artificial barriers built to

• Groins, jetties, breakwaters• Usually produces

• Deposition is enhanced• BUT, erosion is accelerated

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• Approach to combat erosion

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

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Mitigating Coastal Problems

geology lecture 17

Documents

global oceans ocean

world ocean

global landscape oceans

continental margins

knowledge of oceans

continental crust

oceans humans

hydrologic cycle oceans