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Ch11PowerPoint.pdf

Essentials of Geology, 3rd edition, by Stephen MarshakEssentials of Geology, 3rd edition, by Stephen Marshak Chapter 18: Amazing Ice: Glaciers and Ice AgesChapter 18: Amazing Ice: Glaciers and Ice Ages

Oceanography � The oceans are responsible for our presence.

� Blue ocean covers 70.8% of the planet. Oceans…

� Serve as the basis for life on Earth.

� Regulate climate.

� Cycle mass and energy.

�Atmosphere.

�Hydrosphere.

�Lithosphere.

�Biosphere.

� 60% of humans live near

coasts.

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Undersea Landscapes � Oceans exist because of differences in lithosphere.

� Continental lithosphere “floats higher” on the mantle.

� Oceanic lithosphere “floats deeper” in the mantle.

� Ocean basins collect water because they are lower.

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Undersea Landscapes � The world’s ocean dominates the globe.

� Tectonic processes constantly configure oceans.

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Undersea Landscapes � On the present configuration of tectonic plates…

� Most continental crust is in the northern hemisphere.

� Most oceanic crust is in the southern hemisphere.

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Undersea Landscapes � The seafloor exhibits highly varied bathymetry.

� Continental shelf – Shallow (0 to 500 m), gently sloping

(0.3o).

� Continental slope – Descends from 500 m to 4 km at ~ 2o.

� Continental rise – Transition zone from 4 to 4.5 km.

� Abyssal plain – Flat, low-relief bottom below 4.5 km.

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Undersea Landscapes � Submarine canyons crosscut continental shelves.

� Associated with large rivers.

� Erosion carved canyons during sea-level lowstands.

� Submerged canyons funnel sediments to deeper water.

� Submarine fans grow where canyons empty onto the rise.

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Undersea Landscapes � Continental margins are called passive or active.

� A passive continental margin…

� Is located far from a tectonic plate boundary.

� Develops a broad shelf of sediment overlying sialic crust.

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Undersea Landscapes � Continental margins are called passive or active.

� An active continental margin…

� Is immediately adjacent to a tectonic plate boundary.

� Characterized by a thin, narrow continental shelf.

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Ocean Water Composition � Normal marine salinity, reflecting dissolved ions, is…

� 3.5% (35 grams per liter; 35,000 parts per million)

� Dissolved ions derive from chemical weathering of rocks.

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Ocean Water Composition � Salt remains behind during freezing or evaporation.

� Desiccation yields evaporite mineral salts.

� Halite (NaCl).

� Gypsum (CaSO4 . 2H2O).

� Sylvite (KCl).

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Ocean Water Composition � Surface salinity can vary above and below 3.5%.

� Higher salinity from evaporation and formation of sea ice.

� Lower salinity from rainfall, glacial melt, and river input.

� Salinity becomes more uniform with depth.

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� Currents ceaselessly move ocean water in 3-D.

� Surface currents (upper 100 m) due to wind shear.

� Currents spiral by Coriolis deflection into large gyres.

Oceanic Currents

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Downwelling Upwelling

Vertical Oceanic Currents � Currents also transport ocean water vertically.

� Downwelling – Surface waters are drawn downward.

� Upwelling – Deep waters are pushed upward.

� Wind perpendicular to shore drives vertical flow.

� Onshore – Water piling up along coast drives downwelling.

� Offshore – Upwelling replaces water moved away.

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Vertical Oceanic Currents � Thermohaline contrast also drives vertical currents.

� Temperature – Colder water is denser and sinks.

� Salinity – More saline water is denser and sinks.

� Polar water is both colder and saltier.

� Deep-ocean waters are replenished from the poles.

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Oceanic Currents � Sinking polar water is replaced by surface flow.

� This process carries warm water up from the tropics.

� These surface currents warm northern oceans.

� This system forms a global “conveyor belt.”

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Tides � Sea level rises and falls twice daily.

� High tide – Maximum tidal flooding.

� Low tide – Maximum tidal withdrawal.

� Tidal reach – The range between high and low tides.

� The intertidal zone lies between tides.

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Tides � Tides are caused by a tide-generating force, due to...

� The gravitational pull of the Moon and the Sun.

� Centrifugal forces from the rotation of the Earth, Moon, and

Sun.

� The orbiting moon creates the strongest tidal effects.

� The sublunar bulge follows the moon’s orbit.

� A smaller bulge occurs on the opposite side of Earth.

� The bulges make high tides; between bulges are low tides.

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Tides � Lunar and solar tidal effects interact.

� Positive alignment yields enhanced “spring” tides.

� Negative alignment results in dampened “neap” tides.

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Tides � The magnitude and timing of tides varies widely.

� This reflects a complex interplay of factors:

� Relative orientation of the Sun and Moon.

� The orbital properties of the moon.

� Basin geometry.

� Air pressure.

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Waves � Oceanic waves develop via friction of wind on water.

� Gentle wind creates small waves.

� Gales make giant waves.

� Waves translate the uppermost part of the water.

� Wave height, length, and period depend on wind speed, wind duration, and distance of travel (fetch).

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Waves � Definitions:

� Crest – Top of a wave.

� Trough – Low between

crests.

� Wavelength – Distance

between wave crests.

� Depth of influence (wave base) is ½ the wavelength.

� Above wave base, water

moves in a circular motion.

� Below wave base, water is

not affected by wind waves.

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Waves � As waves approach shore, wave base hits bottom.

� Friction slows wave motion near the sea floor.

� Near the surface, waves continue moving fast.

� The wave oversteepens and crashes onto the beach.

� This zone features a diverse array of environments.

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Waves � Waves that crash onto the beach are called breakers.

� Wave energy is dissipated by turbulence.

� This creates frothy white water in the surf zone.

� A surge of water (swash) rushes up the beach face.

� Gravity pulls the backwash down the slope of the beach.

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Wave Refraction � On an irregular shoreline, water depth varies.

� As waves drag on bottom, they are forced to bend.

� This process, (wave refraction), has consequences.

� Wave attack is concentrated against headlands.

� Wave attack is dissipated in embayments.

� This process tends to straighten an irregular shore.

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Longshore Currents � Sediment is transported along the shore.

� Oblique waves push sediment sideways up the beach.

� Gravity then pulls this sediment straight downslope.

� This zig-zag pattern moves

sediment in one direction.

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Rip Currents � Rip currents develop when wave attack is straight on.

� Water piles up on the beach and must return seaward.

�A rip current develops perpendicular to the beach.

�Rip currents are often strong; people drown fighting them.

� Rip currents dissipate away from the surf zone.

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Rip Currents � Swimmers can easily survive rip currents.

� If caught in a rip current, don’t panic.

� Don’t swim against the current; swim parallel to shore.

� Or, ride the current out; waves will carry you back.

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Coastal Landforms � A variety of landforms are found along coastlines.

� Broad, sandy beaches.

� Lush, swampy wetlands.

� Drowned river valleys.

� Steep cliffs.

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Beaches � Gently sloping shorelines made of sediment.

� They are dynamic settings subject to constant change.

� Common as vacation destinations.

� Most people have experienced beaches.

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Beaches � Beaches are dominated by sand.

� Gravel beaches reflect energetic surf and a rock supply.

� Muds are absent.

� Turbulent surf suspends and removes finer sediments.

� Muds are transported to lower-energy environments.

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Beaches � Beach sediment compositions reflect geology.

� Quartz common.

� Carbonates in tropics.

� Resistant minerals.

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Beaches � Beaches develop distinctive cross-sectional profiles.

� Profiles change seasonally with energy regime.

� Summer – Broad, sandy beach.

� Winter – Narrow, gravel beach.

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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 of the beach.

�Beyond the reach of normal high tides.

�Often exhibit berms (stepped terraces of storm sediment).

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Beaches � Longshore currents move sediment along beaches.

� This process, beach drift, moves tons of sand daily.

� Beach drift builds sandbars and sand spits.

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� Barrier islands are elongate, linear sandbars.

� They form where sand is plentiful.

� They create a protected backwater area called a lagoon.

� Barrier islands are common places for development.

� They change constantly.

Barrier Islands

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� Form in intertidal zones lacking strong waves.

� Common behind barrier islands or in estuaries.

� Consist of thinly laminated sand and mud.

� Ancient tidal flat sediments are well studied.

Tidal Flats

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� Along rocky coasts, bedrock rises out of the sea.

� Wave action is often powerful along rocky coasts.

� Concentrated wave energy acts to erode rock.

� Rocky coasts develop unique landforms.

Rocky Coasts

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� Wave-cut notches – Waves erode an overhang.

� The 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

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� Rocky headlands are preferentially eroded.

� Refracted waves focus energy to the sides of a headland.

� Erosional attack from both sides creates a sea arch.

� Collapse of the sea arch leaves remnant sea stacks.

Rocky Coasts

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� Coastal wetlands cover large areas of shore regions.

� They develop in places protected from waves and currents.

� Wetlands fuel high biological productivities.

� Vegetative characteristics are governed by climate.

�Temperate - Vegetated by trees, grasses, or mosses.

�Tropical – Mangrove-dominated.

Organic Coasts

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� Coral reefs grow in tropical marine settings.

� They create large structures of cemented skeletons.

� Among the most biologically

productive ecosystems.

Reefs

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� Coral reefs modify sediment accumulation.

� Reefs alter wave and current energy.

� They protect leeward environments.

� Abundant debris is shed to adjacent environments.

� Size / geometry defines patch, fringing, or atoll reefs.

Reefs

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� Coral atolls are reefs formed on a subsiding volcano.

� Reef is established when volcano is active.

� After extinction, volcano erodes and subsides.

� Coral reef can easily keep pace with subsidence.

� Reef continues long after volcano is below sea level.

Reefs

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� River valleys flooded by marine water are estuaries.

� Characterized by mixing of fresh and salt water.

� Modern estuaries reflect glaciation.

�Rivers cut canyons during low sea level.

�Sea-level rise flooded these canyons.

Estuaries

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� Flooded U-shaped valleys carved by glaciers.

� Form spectacular bedrock bounded troughs.

� Notable examples found in…

�Norway.

�British Columbia.

�New Zealand.

Fjords

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Coastal Variability � Plate tectonic setting governs the style of coastline.

� Passive margin – Broad, low-lying coastal plains typical.

� Active margin – Uplifted, rocky coasts dominate.

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Coastal Variability � Eustatic sea-level changes affect coasts worldwide.

� Inflation / deflation of mid-ocean ridge volumes.

� Glaciation / deglaciation traps or releases water.

�During the last ice age, sea level

was lower exposing more land.

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Coastal Variability � Emergent coasts experience relative sea-level fall.

� Uplift due to tectonic processes.

� Eustatic sea-level drop.

� Emergent coasts characterized by…

� River incision, cliffs, wave-cut notches, and platforms.

� Terraces representing former sea-level positions.

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Coastal Variability � Submergent coasts experience relative sea-level rise.

� Subsidence of a passive margin (i.e. deltaic sediments).

� Eustatic sea-level rise.

� Submergent coasts characterized by…

� Flooded river or glacial valleys create estuaries and fjords.

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Coastal Variability � Shoreline character is linked to sediment supply.

� Balance between accumulation and erosion.

�Accretionary coasts – Net sediment accumulation.

�Erosional coasts – Sediment removed faster than supplied.

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Coastal Variability � Climate is a strong influence on shoreline character.

� Harsh weather accentuates erosion.

� Calm weather favors sediment accumulation.

� Tropics – Carbonate sediment, mangroves, and reefs.

� Temperate – Salt marshes.

� Arctic – Barren, lichen-covered coast.

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Coastal Problems

� Contemporary sea level changes.

� Sea level is presently rising.

� Rates of sea level rise may

increase from ice-cap melting.

� People living in low-lying

coastlines may be in jeopardy.

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Coastal Problems � Beach destruction.

� Storms (especially hurricanes) radically alter shorelines.

� Human development in coastal settings often affected.

� Construction in coastal settings is increasingly regulated.

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Coastal Problems � Artificial barriers are built to reduce beach erosion.

� Groins, jetties, and breakwaters arrest sediment transport.

� Usually this produces unintended consequences.

�Sediment deposition is enhanced in one place.

�Sediment erosion is accelerated in another.

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