chapter 4 continental margins and ocean basins

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1 Chapter 4 Chapter 4 Continental Margins and Ocean Basins Continental Margins and Ocean Basins Bathymetry of Sea Floor Bathymetry of Sea Floor Continental Margins and Ocean Continental Margins and Ocean Basins Basins Submarine Canyons Submarine Canyons Hydrothermal Vents Hydrothermal Vents Trenches Trenches

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Chapter 4 Continental Margins and Ocean Basins. Bathymetry of Sea Floor Continental Margins and Ocean Basins Submarine Canyons Hydrothermal Vents Trenches. Bathymetry: a map of the ocean floor Early bathymetric studies were often performed using a weighted - PowerPoint PPT Presentation

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Page 1: Chapter 4  Continental Margins and Ocean Basins

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Chapter 4 Chapter 4 Continental Margins and Ocean Continental Margins and Ocean

BasinsBasins• Bathymetry of Sea FloorBathymetry of Sea Floor

• Continental Margins and Ocean Continental Margins and Ocean BasinsBasins

• Submarine CanyonsSubmarine Canyons

• Hydrothermal VentsHydrothermal Vents

• TrenchesTrenches

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• Echo sounding Echo sounding

• Multibeam SystemsMultibeam Systems

• Satellite AltimetrySatellite Altimetry

Bathymetry: a map of the ocean floorBathymetry: a map of the ocean floorEarly bathymetric studies were often performed using a weighted line to measure the depth of the ocean floor.

Echo Sounders Bounce Sound off the Seabed

Vw ~= 1500 m/s (Pres, Temp, Salinity)

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Echo sounders sense the contour of the seafloor by beaming sound waves to the bottom and measuring the time required for the sound waves to bounce back to the ship.

If the round-trip travel time and wave velocity are known, distance to the bottom can be calculated. This technique was first used on a large scale by the German research vessel Meteor in the 1920s.

Review:Review:

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LIDAR: Light Imaging Detection And Ranging

Vw ~= 1500 m/s (Pres, Temp, Salinity)

Va ~= 344 m/s (Pres, Temp, other)

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Multi-beam echo sounder

Using satellite measurements to map the ocean floor

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Gravitational Anomalies:Gravitational Anomalies:Bathymetry of ocean floor-ridges, shelvesBathymetry of ocean floor-ridges, shelves

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Topography of Ocean Floors

Cross section of the Cross section of the Atlantic ocean Atlantic ocean basin and the basin and the continental United continental United States, showing the States, showing the range of elevations. range of elevations. Ocean depth is Ocean depth is clearly clearly greatergreater than than the average height the average height of the continent, of the continent, but general range but general range of contours is of contours is similar.similar.

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Continental margin

Deep-ocean basin Continental margin

Submarine canyon profile (cut through continental shelf)Sediment

Continental shelfContinental slope

Continental rise

Oceanic ridge Sediment

Continental crust (granitic)

Continental crust (granitic)

Oceanic crust (basaltic)

Oceanic crust (basaltic)

Asthenosphere

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An active margin

Narrow continental shelf

Peru–Chile Trench

A passive margin

Plate boundary

Andes Mountains

South AmericaBroad continental shelf

Plate boundary

Pacific Ocean South American Plate

Deep basin

Atlantic Ocean

Nazca PlatePlate movement

African Plate

Plate movement

Plate movement

Subduction zone (deep and shallow earthquakes)

Mid-Atlantic Ridge (spreading centers, shallow earthquakes)

• Atlantic = Passive Margin

little/no geologic activity

• Pacific = Active Margin

geologic activity

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The ocean floor can be classified as(a) Continental Margins – the submerged outer edge of a continent(b) Ocean Basin – the deep seafloor beyond the continental margin

There are two types of continental Margins

(a) passive or trailing margins: margin of continent that moves away from spreading center – Atlantic-style margins (also Artic Ocean, Antarctica and Indian Ocean). Very little volcanic or earthquake activity is associated with passive margins.

(b) active or leading margins: plate boundary located along a continental margin – ocean trenches where there is subduction of oceanic lithosphere – narrow, steep, with volcanic mountains (West Coast of the Americas). Active margins are the site of volcanic and earthquake activity.

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Continental Margins

• Region where continental crust meets oceanic crust

• Continental Shelf

• Shelf Break

• Continental Slope

• Continental Rise

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Continental ShelvesContinental Shelves

• Gently sloping (~0.5 degrees) Gently sloping (~0.5 degrees)

• Depositional environmentsDepositional environments

• Average width 65 km (40 miles)Average width 65 km (40 miles)

• Average depth 130 m (430 feet)Average depth 130 m (430 feet)

• Narrow along Active marginsNarrow along Active margins

• Wide along Passive marginsWide along Passive margins

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Continental Shelves:

broad shallow extension of the continents (~75km wide)

Regions of deposition (rivers, glaciers, scrapped marine deposits, calcium carbonate)

Large bedform features, reworked by tides, storms, waves

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Anatomy of a passive marginAnatomy of a passive margin

deep

sediments accumulate here, thickness varies

(~ 140 m)

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Continental MarginsContinental Margins

• Shelf Break Shelf Break Edge of the continental shelfEdge of the continental shelfChange in slope Change in slope

• Continental SlopeContinental SlopeExtends from break to ocean basinExtends from break to ocean basinSteep (3 – 6 degrees) Steep (3 – 6 degrees) As high as 25 degreesAs high as 25 degreesLittle/no depositionLittle/no deposition

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Continental Slopes:

continental crust thins into oceanic crust

steep (~20km, 1-25 degrees), 5deg Pacific, 3deg Atlantic

extend to depths between 1500-4000 m

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Submarine Canyons Form at the Junction between Continental Shelf and Continental Slope

These are features of some continental margins. They cut into the continental shelf and slope, often terminating on the deep-sea floor in a fan-shaped wedge of sediment.

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Submarine CanyonSubmarine Canyon

• Steep V shape channel, incised in the Steep V shape channel, incised in the continental slope (and shelf)continental slope (and shelf)

• Created byCreated by

Rivers during the last low stand (some)Rivers during the last low stand (some)Turbidity currentsTurbidity currents

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Turbidity CurrentsTurbidity Currents• Fast moving avalanches of mud and Fast moving avalanches of mud and

sand scour slopessand scour slopes

• Form turbidite depositsForm turbidite deposits

• 90 km/hr (56 mi/hr)90 km/hr (56 mi/hr)

Turbidite bedTurbidite bed

ancient deposit, exposed ancient deposit, exposed to erosion, graded to erosion, graded deposits: largest deposits: largest particles at bottomparticles at bottom

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Continental MarginsContinental Margins

• Continental RiseContinental Rise

Base of the continental slopeBase of the continental slope

slope 0.5 – 1 degree slope 0.5 – 1 degree

Depositional environmentDepositional environment

Formed by:Formed by:

Turbidity currentsTurbidity currents

Underwater landslidesUnderwater landslides

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Continental Rise:

Fan like deposit where the continental slope intersects the abyssal plains

Formed by turbidity currents

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Features of the Sea floor

Oceanic RidgesHydrothermal VentsAbyssal Plains and Abyssal HillsSeamounts and GuyotsTrenches and Island Arcs

SeafloorSeafloor: 4000 – 6000 m water depth, 30% of the : 4000 – 6000 m water depth, 30% of the Earth’s surfaceEarth’s surface

Abyssal Plain: Abyssal Plain: vast, flat plain extending from the vast, flat plain extending from the base of the continental slope.base of the continental slope.

Ocean Basins: Ocean Basins: sections of the abyssal plain separated sections of the abyssal plain separated by continental margins, ridges, and rises.by continental margins, ridges, and rises.

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Basins Basins sections of the abyssal plain sections of the abyssal plain separated by continental margins, separated by continental margins, ridges, and rises.ridges, and rises.

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Sea floor featuresSea floor features

Abyssal hillsAbyssal hills are small, extinct are small, extinct volcanoes or rock intrusions near the volcanoes or rock intrusions near the oceanic ridges.oceanic ridges.

Flat-topped seamounts eroded Flat-topped seamounts eroded by wave action are called by wave action are called guyots.guyots.

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Seamounts Seamounts are volcanic projections from the ocean floor that do not are volcanic projections from the ocean floor that do not rise above sea level. Flat-topped seamounts eroded by wave action rise above sea level. Flat-topped seamounts eroded by wave action are called are called guyotsguyots

Abyssal hills Abyssal hills are flat areas of sediment-covered ocean floor found are flat areas of sediment-covered ocean floor found between the continental margins and oceanic ridges. between the continental margins and oceanic ridges. Abyssal hillsAbyssal hills are small, extinct volcanoes or rock intrusions near the oceanic are small, extinct volcanoes or rock intrusions near the oceanic ridges.ridges.

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Hydrothermal vents Hydrothermal vents are sites where superheated water are sites where superheated water containing dissolved minerals and gases escapes through fissures, containing dissolved minerals and gases escapes through fissures, or vents. Cool water (blue arrows) is heated as it descends toward or vents. Cool water (blue arrows) is heated as it descends toward the hot magma chamber, leaching sulfur, iron, copper, zinc, and the hot magma chamber, leaching sulfur, iron, copper, zinc, and other materials from the surrounding rocks. The heated water (red other materials from the surrounding rocks. The heated water (red arrows) returning to the surface carries these elements upward, arrows) returning to the surface carries these elements upward, discharging them at hydrothermal springs on the seafloor.discharging them at hydrothermal springs on the seafloor.http://www.divediscover.whoi.edu/vents/index.htmlhttp://www.divediscover.whoi.edu/vents/index.html

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Mid-Ocean Ridges and RisesMid-Ocean Ridges and Rises

An An oceanic ridgeoceanic ridge is a mountainous chain of young, basaltic is a mountainous chain of young, basaltic rock at an active spreading center of an ocean.rock at an active spreading center of an ocean.

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Trenches Trenches are arc-shaped depressions in the ocean floor caused by the are arc-shaped depressions in the ocean floor caused by the subduction of a converging ocean plate. subduction of a converging ocean plate. Most trenches are around the edges of the active Pacific. Trenches are the Most trenches are around the edges of the active Pacific. Trenches are the deepest places in Earth’s crust, 3 to 6 kilometers (1.9 to 3.7 miles) deeper than deepest places in Earth’s crust, 3 to 6 kilometers (1.9 to 3.7 miles) deeper than the adjacent basin floor. The ocean’s greatest depth is the Mariana Trench the adjacent basin floor. The ocean’s greatest depth is the Mariana Trench where the depth reaches 11,022 meters (36,163 miles) below sea level.where the depth reaches 11,022 meters (36,163 miles) below sea level.

Major ocean trenchesMajor ocean trenches

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Same scale for comparison

‘real’ scale – no exaggeration

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Chapter 4 - SummaryChapter 4 - Summary

• Bathymetric devices used to study seabed features Bathymetric devices used to study seabed features include multibeam echo sounder systems and include multibeam echo sounder systems and satellites that use sensitive radar for altimetry. satellites that use sensitive radar for altimetry.

• Seafloor features result from a combination of Seafloor features result from a combination of tectonic activity and the processes of erosion and tectonic activity and the processes of erosion and deposition.deposition.

• Near shore, the features of the ocean floor are Near shore, the features of the ocean floor are similar to those of the adjacent continents because similar to those of the adjacent continents because they share the same granitic basement. The they share the same granitic basement. The transition to basalt marks the edge of the continent transition to basalt marks the edge of the continent and divides ocean floors into two major provinces, and divides ocean floors into two major provinces, The submerged outer edge of a continent is called The submerged outer edge of a continent is called the continental margin. The deep-sea floor beyond the continental margin. The deep-sea floor beyond the the continental margincontinental margin is called the is called the ocean basinocean basin

• Features of the continental margins include Features of the continental margins include continental shelves, continental slopes, submarine continental shelves, continental slopes, submarine canyons, and continental rises.canyons, and continental rises.

• Features of the deep-ocean basins include oceanic Features of the deep-ocean basins include oceanic ridges, hydrothermal vents, abyssal plains and hills, ridges, hydrothermal vents, abyssal plains and hills, seamounts, guyots, trenches, and island arcs.seamounts, guyots, trenches, and island arcs.