unit 6: the sea floor · mid-oceanic ridges & hydrothermal vents warm-water vents are below...
TRANSCRIPT
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Unit 6: The Sea Floor
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Turn to Page 62 in Introduction to the World’s Oceans
• Study the bathymetric chart
• What features do you see?
• How do the Atlantic and Pacific Oceans differ?
• What do you think accounts for those differences?
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Ocean Bathymetry: What do you see?
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What is Bathymetry?
• Bathymetry is from the Greek: bathos = deep, and metry = measure
• is the study of ocean contours
• A bathymetric chart shows what the ocean floor would look like if all the water drained out
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Bathymetry continued
• More is known about the surface of Venus, Mars, and the dark side of the moon than the ocean floor
• Only 5% of the oceans have been mapped with the same precision as the moon
• there are areas the size of Kansas where no ship soundings have been made and even well surveyed areas are based on widely separated survey tracks.
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Early bathymetric studies were performed with a sounding line or lead line to study the depth of the ocean floor
• Sailors measured depth in fathoms (6 ft) by stretching the rope between two outstretched arms
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Precision Depth Recorder (PDR) developed in the 1950s used a focused beam to
measure depth to 1 m in accuracy
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Multi-beam echo sounders and side-scan sonar allows mapping of swaths up to 60 km
wide
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Bottom Coverage & Data Density by Survey Method Leadline Single Beam Multibeam
1-2 K soundings per survey
500 - 750 K soundings per survey
400,000 1,000,000 K soundings per survey
Image courtesy of NOAA & UNH
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National Geographic Video
http://video.nationalgeographic.com/video/player/news/environment-news/us-ocean-floor-mapping-vin.html
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Satellite Altimetry measures differences in sea surface elevation A sea surface anomaly is caused by the different gravitational pull of ocean mounts and trenches
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Satellite Bathymetry continued
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Sea Floor Bathymetry as Measured by Satellite over 4.5 year period
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NOAA Satellite Bathymetry animation
• http://www.youtube.com/watch?v=w1TZObXGUo4
• http://www.youtube.com/watch?v=JkeglSqQMyQ&feature=related
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USS San Francisco
• In 2005 the nuclear attack sub USS San Francisco collided with an uncharted sea mount at flank (maximum) speed
• The sub was sailing at a depth of 500 ft. in the South Pacific about 350 south of Guam
• The sub was almost lost
• They repaired it by cutting the bow off the retiring USS Honolulu and installing it on the San Francisco.
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Why Study Bathymetry?
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Ocean-Floor Topography Varies with Location Hypsographic Curve
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Continental Margins – the submerged outer edge of a continent Deep Ocean Basin – the deep seafloor beyond the continental margin Mid-Ocean Ridges – The submarine mountain range in the middle
What are the three classifications of ocean floor?
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Deepest not in the middle, but off to either side of the
shallower mid-ocean ridge
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Mid-Atlantic Ridge
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Continental Margins
Passive margins Active margins
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passive margin active margin
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Continental Margins
Continental Shelf - shallow, submerged edge of the continent. Shelf break - abrupt transition from continental shelf to the continental slope. Continental Slope - transition between the continental shelf & the deep-ocean floor. Continental Rise - accumulated sediment found at the base of the continental slope.
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Continental Shelves Are Seaward
Extensions of the Continents
The features of a passive continental margin:
(a) Vertical
exaggeration 50:1
(b) No vertical
exaggeration.
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Continental Margins – Submarine
Canyons
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California’s canyons
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turbidity currents
Submarine Canyons Form at the Junction between Continental Shelf & Continental Slope
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How do canyons form? river erosion during previous glaciation periods turbidity currents: rapid movement of a mass of sediment down a
slope ~ avalanche often caused by earthquakes
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A mid-ocean ridge is a mountainous chain of young, basaltic rock at an active spreading center of an ocean.
Topology of Deep-Ocean Basins Differs from That of the Continental Margin
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Hydrothermal Vents hydrothermal vents: spring of hot, mineral- and gas-rich seawater
near the spreading centers of mid-ocean ridges
In 1977, Robert Ballard & J. Grassle discovered new life forms in Alvin, at
3000 m near the Galápagos Islands
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Hydrothermal vents are sites where superheated water containing dissolved minerals & gases escapes through fissures, or vents.
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Mid-Oceanic ridges & Hydrothermal Vents
Warm-water vents are below 30⁰ C and emit clear water. White smokers emit water between 30 ⁰ and 350 ⁰. Water is white because of light-colored dissolved minerals. Black smokers are above 350⁰ C (662 ⁰F) and emit black water because of presence of dark metal sulfides. Question: Why don’t they emit steam instead of water?
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Seamounts are volcanic projections from the ocean floor that do not rise above sea level. Flat-topped seamounts eroded by wave action are called guyots. Abyssal hills are small, extinct volcanoes or rock intrusions near the oceanic ridges.
Volcanic Seamounts & Guyots
Project Above the Seabed
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Trenches are depressions in the ocean floor caused by the subduction of a converging ocean plate.
Deep-Ocean Basins
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Island Arcs, chains of volcanic islands & seamounts, are usually found parallel to the edges of ocean trenches.
Deep-Ocean Basins
(left) As two oceanic plates converge, an island arc is formed by volcanic activity.
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Trenches & Island Arcs Form
in Subduction Zones
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Video Marianas Trench
http://www.youtube.com/watch?v=BYjyGfRp3F4
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Coral atolls form when volcanic islands sink leaving behind the coral reef surrounding a lagoon where the volcano used to be.
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Atolls
Fanning Island
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The Abyss • abyssal plains: flat, featureless expanses of seafloor
• covered w/ sediment between continental margins &
oceanic ridges
• 3700-5500 m
• Why is the abyssal plain so flat? It is flatter than any
similar plain on land
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Sea Floor Spreading Evidence
• In the 1950s,
scientists discovered
parallel patterns of
magnetic stripes on
the sea floor.
• This provides
evidence of seafloor
spreading and a
record of magnetic
pole reversals.
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Global Distribution of Earthquakes
Global distribution of earthquakes also provides evidence for theory of plate tectonics and seafloor movement.
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Age of the Seafloor
• Late 1960s deep-sea drilling began on
the Glomar Explorer
• Radiometric dating of ocean rocks to
determine seafloor age
• Oldest seafloor is about 200 million
years old compared to 3.5 billion years
old for oldest rocks on land.
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Age of Seafloor
Symmetric pattern of age distribution - youngest at the ridges and oldest at the subduction zones
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© 2011 Pearson
Education, Inc.
Plate Tectonics Theory
• Lithosphere – tectonic plates that
float on ductile asthenosphere
• Large scale geologic features occur
at plate boundaries
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Plate Tectonic Processes
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Global Plate Boundaries
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• Slab Pull Theory: Older dense plates at the subduction zones sink into the underlying asthenosphere providing most of the driving force for plate motion.
Driving Forces of Plate Motion
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© 2011 Pearson
Education, Inc.
Types of Plate Boundaries
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Divergent Boundary Features
• Plates move apart at
the mid-ocean ridge
forming a rift valley
• New ocean floor is
created
• Shallow focus
earthquakes happen
here
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Generation of a Divergent Boundary
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Convergent Boundary Features • Plates move toward each other
• Oceanic crust destroyed
• Trenches and volcanic arcs are found here
• Deep focus earthquakes
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Three Types of Convergent Boundaries
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Oceanic – Continental Convergence
• Ocean plate is
subducted
• Continental arcs
generated
• Explosive andesitic
volcanic eruptions
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Oceanic – Oceanic Convergence
• Oceanic crust is destroyed
• Friction of plates melts sediments and crust
• Molten magma erupts in form of island arcs
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Continental –Continental Convergence
• No subduction
• Tall mountains
uplifted –
Himalayans
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© 2011 Pearson
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Transform Boundary Features
• Oceanic Transform Fault – ocean floor only
• Continental Transform Fault – cuts across
continent
• All transform faults
occur between
mid-ocean ridge
segments.
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Global Hot Spots • Scattered around the earth are
approximately 40 fixed areas of volcanic activity known as hot spots
• Hot spots remain stationary as the plates move and are useful for tracing plate movements
• As ocean plate moves over a hot spot, a chain of volcanos is formed.
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© 2011 Pearson
Education, Inc.
Global Hotspot Locations
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Hawaiian Islands Hot Spot