CONTINENTAL MARGINS AND THE SEAFLOOR

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CONTINENTAL MARGINS AND THE SEAFLOOR

• Chapter 4

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• Geology in the oceans is like flying in a balloon
  over the continents on a cloudy day and
  periodically dropping a bucket over and pulling
  things up.
• Imagine what you would think if you only got one
  sample from Vermont and one from Florida.
• The surface of Venus has been mapped in more
  detail than the bottom of the ocean.

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Techniques

• 1) Drilling
• deep cores they can be gt1km long, and collected
  in water depths gt 5 km
• Short cores, generally lt 10 m of sediment on the
  bottom
• 2) Dredging, the "bucket" analogy
• 3) submersibles and ROV- submarines manned and
  unmanned- can make observations and collect
  samples (Box 4.1)

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• 4) echo sounding- detailed bottom depths- similar
  to "fish locators"- measures the travel time for
  sound waves through water.(1500 m/s)(fig. 4.2)

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• 5) Seismic measurements- identical in theory to
  echo sounders, but they are higher energy so the
  sound penetrates into the sediment
• 6) Satellite altimetry measures sea level
  variations(fig. 4.4)

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II. PROFILES OF THE OCEAN FLOOR

• A. Atlantic OceanIf you could drive on the ocean
  floor from North America to Europe, you would see
  the following major features (fig. 4.5)

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Continental shelf

• extends from shoreline to tens to hundreds of
  kilometers offshore. Depths range from 0 m to
  about 130 m at the edge(fig. 4.10)

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• (2) Continental slope- extends from edge of shelf
  (130 m water depth) to depth of about 2000 to
  3000 m. It is fairly narrow, around 50 km wide.
• On average the slope is about 4 degrees, or about
  70 m vertical for every 1000 m horizontal.
  Cutting across the slope are submarine canyons
  deep valleys eroded into the slope(fig. 4.16-17)

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• (3) Continental rise- a place where the slope
  gradually becomes less steep, this areas grades
  into very flat places called abyssal plains.
• They are the flattest places on earth. Occur at
  depths ranging from 4000 to 6000 m.

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• (4) Abyssal hill- Water depth gradually becomes
  less deep as we move up the Mid-Atlantic ridge.
  (fig. 4.21-4.22)
• Go up the side of the ridge until reach a steep
  cliff. this cliff marks the edge of the rift
  valley- a place where the plates are separating.
  
• All of these features are found in a mirror image
  on the other side of the rift valley.

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B. Pacific Ocean

• Now imagine driving from South America to
  Australia. Some of the same features, but
  different scales, and there are some new
  features.
• (1) Continental shelf and slope- these are still
  present, but they are much more narrow.
• The base of the slope is much deeper than the
  Atlantic (8000 m) and there is no rise present.

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• (2) Trench- long narrow and deep depressions on
  the seafloor.(fig 4.28-4.29)
• These are the deepest parts of the ocean, they
  are generally narrow and reach depths down to
  11,000 m
• The deepest is the Mariana's Trench in the
  western Pacific.

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• (3) East Pacific Rise- This is similar to the
  mid-Atlantic ridge- it is a place where seafloor
  spreading occurs.
• There are some differences, it is broader, not as
  deep and the central rift valley is not as large.
• As you continue the journey, you would cross
  another trench (the Tonga trench) and then cross
  the continental margin of Australia.
• It would look very similar to the Atlantic
  margin.

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• From these descriptions, it would appear that
  there are two types
• the Atlantic type with a spreading center in the
  middle and no deep trenches, and the Pacific type
  that has trenches surrounding it.
• Look at figures 4.5 to see the differences in
  these basins. Realize that the vertical
  exaggeration is very great- in other words, none
  of the places are as steep as these figures
  imply.

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III. CONTINENTAL MARGINS

• There are two types of continental margins-
  active and passive.
• Active margins have trenches on the ocean side
  and mountains on the continent side.
• They are continental margins that also mark plate
  boundaries. (fig 3.14)
• An example is the west coast of South America.

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• Passive margins are the edge of the continent,
  but do not also mark the edge of a plate.
• An example is the east coast of North America.

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• Continental margins are shaped by sediment
  deposition.
• Much sediment builds up on the continental shelf-
  sediment that is eroded from the continents.
• This sediment will not move over the shelf edge
  until disturbance (such as an earthquake) shakes
  the sediment loose
• Sediment and water mixture flows downhill in a
  dense slurry of water and sediment called a
  turbidity current (fig. 4.18-4.19).

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• There is a special kind of sedimentary deposit
  that is called a turbidities that forms from
  turbidity currents
• The best studied example of a turbidity current
  happened after an earthquake on the Grand Banks
  of Newfoundland.

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• The Turbidities covered nearly 100,000 km2- This
  is about the size of Florida.
• It was 1 m thick. The total volume was 100 km3
• This large amount of sediment easily erodes the
  bottom of the seafloor.
• It is one of the processes that form submarine
  canyons.

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Mid-Ocean Ridge- (represent divergent boundaries)

• Mountain range that stick up above the
  surrounding abyssal plains.
• It is the largest single feature on the earth-
  80,000 km long, 1,500 to 2,500 km wide and 2-3 km
  above the abyssal plains.
• In certain places it is characterized by a rift
  valley- A feature that is 1-2 km deep and up to
  10 km wide- approximately like the Grand Canyon

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• There are many earthquakes along the rift valley,
  but they are shallow- generally lt 20 km deep
• Another important feature of rift valleys is they
  are the location of numerous hot springs.
• Water circulates into the crust, it is heated by
  the magma chambers present at these places, the
  water rises to the surface of the crust and is
  expelled to the ocean.

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• These places, along with rivers, are what control
  the chemistry of seawater.
• They are also the first places where organisms
  were found that do not rely on energy from the
  sun to survive.
• Look at at Box 4.2 for description of the
  hydrothermal systems at the ridges.

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Seamounts and Guyots

• Seamounts- Conical mountains formed of basalt and
  thought to have once been volcanoes-
• Other evidence is that now there are a few
  seamounts that are volcanoes

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• Guyots- These are round, flat-topped structures
  that are less than 1000 m above the surrounding
  seafloor.
• They often are capped by coral, and generally are
  made of volcanic rock- basalt
• Where the water is warm enough, coral reefs form
  around the edges of seamounts.
• When the volcanoes stops growing, it begins to
  subside (i.e. sink) back into the ocean.

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• The rate which the volcano sinks is about the
  same rate that coral can grow upward.
• the coral reef usually forms a ring several km in
  diameter surrounding a shallow lagoon.
• These places are called Atolls.
• Look at Fig. 12.27 to see the origin of Atolls

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