Unit 4 Marine Geology

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Unit 4 Marine Geology

• Chs 11, 12, 13

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Earths Internal Structure

• Inner core - solid particles composed mostly of
  mixtures or alloys of iron
• Outer core - liquid outer core iron rich,
  produces the earths magnetic field.

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• Mantle - most of it is thought to be a solid but
  it flows almost like a liquid but slower
• Crust - extremely thin and rigid with the
  composition varying from ocean floor and
  continents

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Isostatic equilibrium

• The balance between the weight of the crust and
  the force of buoyancy provided by the mantle.
• This helped to explain earthquakes and
  continental movement prior to the theory of plate
  tectonics.

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Evidence for Continental Drift

• As early as 1600s it was noted that the
  continents along the Atlantic coast seemed to fit
  together like a puzzle.
• Over time more evidence accumulated to show that
  this was true
• Coal deposits and geologic formations match up on
  both sides of the Atlantic.
• Fossils were also similar on both coasts.

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Alfred Wegener

• In 1912 Alfred Wegener, combined all of this data
  and proposed the theory of continental drift.
• He proposed that all of the continents were once
  joined together into a super continent he
  called Pangaea.

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• This hypothesis was not widely accepted because
  Wegener could not provide the mechanism for
  movement. (sea-floor spreading)
• In the late 1950s 60s scientists finally put
  all the evidence together. They came up with the
  theory of plate tectonics.

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Theory of Plate Tectonics

• Theory states that the continents float on the
  Earths molten interior, gradually moving over
  millions of years
• Crust is created where the sea floor expands, and
  is destroyed on the opposite side.

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Evidence of Seafloor Spreading

• In 1968 the Glomar Challenger, obtained actual
  samples of sea floor rock.
• It was found that the rocks closer to the
  Mid-ocean ridge were more thin than the rocks
  found near the continents.
• Using radiometric dating they determined the
  rocks closer to the MOR were younger too.

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Mid-Ocean Ridge
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• At the mid-ocean ridges, huge pieces of oceanic
  crust separate, forming a crack in the crust
  called a rift.
• At this separation, molten material rises up to
  form a ridge. This process is called sea-floor
  spreading.
• Occasionally these volcanoes along the MOR reach
  the surface and form islands like Iceland and the
  Azores.

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How does Sea floor spreading occur?

• The lithosphere is made up of the upper mantle
  and the crust.
• The lithosphere is broken up into plates called
  lithospheric plates, which can either contain
  continental crust, oceanic crust, or both.
• These plates float on the mantle and because of
  convection in the asthenosphere they are able to
  move.

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Magnetic Anomolies

• Scientists also discovered using magnetometers, a
  symmetrical pattern in the polar orientation of
  magnetite on the ocean floor.
• The particles in magnetite on the ocean floor
  align itself with the polar orientation of the
  Earth.

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• Geologists found a pattern of magnetic bands or
  stripes that run parallel to the mid-ocean
  ridge in regular intervals.

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Types of Plate Boundaries

• Convergent Boundary
• Moving together
• Cont. - cont. forms mtn. chain
• Ex. Himalayas, Atlas Mountains
• Oceanic - Cont or Oceanic - Oceanic forms
  subduction zones
• Ex. Peru/Chili Trench

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• Subduction zone
• Trenches are formed where the crust is sub-ducted
  back into the mantle.
• Subduction zones can occur between oceanic
  crusts, or a continental crust and an oceanic
  crust.

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• When two continental crusts collide, neither one
  subducts, they collide and fold up into mountains.

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• Divergent Boundary
• Moving apart
• Ex. Mid-ocean ridge
• Iceland is one of two ares where the Mid- Ocean
  Ridge is exposed at the surface.

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More boundaries

• Transform boundary
• Two plates slide past each other.
• Most well known in US is the San Andreas fault in
  CA
• Hot spot
• Small melting areas that form magma plumes
• Hawaiian islands

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Sea Floor Structures

• Abyssal plains - flat, featureless region
• Abyssal hill - domes of sediment
• Seamount - peaks of volcanic material
• Guyot - eroded seamount
• Island - seamounts extending out of water
• Trenches - subduction zones

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Geological History of the Earth

• About 200 mya all continents were connected
  forming the super continent Pangaea.
• Pangaea was surrounded by Panthalassa, a single
  ocean, which later evolved into the Pacific Ocean.

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Geological History of the Earth

• From that point on Pangaea continued to break up
  into the continents we know today.
• The Atlantic ocean has been steadily growing and
  the Pacific ocean has been shrinking.

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Sediment and the Coastline
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Sediment origins

• Classified into 4 origin categories
• Lithogenous sediments (also called terrigenous)
• Comes from the land
• Resulting from erosion, landslides, and volcanic
  eruptions
• Most common mineral quartz

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• Biogenous sediment
• Comes from organisms (from shells and hard
  skeletons
• Overtime biogenous sediments accumulate in layers
  which have the potential to turn into crude oil
  and natural gas
• Hydrogenous sediments
• Result from chemical reactions with sea water
• The reaction causes minerals to come out of the
  solution and form particles that settle on the
  bottom
• The source of the dissolved minerals include
  submerged rock ad sediments, new crust formation,
  and hydrothermal vent water.

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• Cosmogenous sediment
• Come from outer space
• Cosmic dust continually settles out of the
  atmosphere
• Least abundant of the sediments

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Sediment Sizes

• Sediments are classified by grain size ranging
  from boulders (largest) to clay (smallest)
• Current tends to distribute sediment according to
  particle size.
• Smaller sediment travels farther

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Wentworth Scale
Classification Grain diameter Example
Clay Smaller than 0.004 mm Talc or fine powder
Silt 0.004 mm0.0625 mm Powder
Sand 0.0625 mm-2 mm Sugar crystals
Granule 2 mm-4 mm Aquarium gravel
Pebble 4 mm-64 mm Grape
Cobble 64 mm-256 mm Cobblestone
Boulder 256 mm and larger A brick or larger
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Hjulstroms diagram
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Coastal Classification

• Active coasts are close to plate collisions that
  result in volcanic activity and earthquakes

• Passive coasts are far from active plate
  boundaries

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• Primary coasts are formed by nonmarine forces.
• Land-based erosion
• Sedimentation
• Volcanic activity
• Tectonic activity

• Secondary coasts are formed by marine action.
• Wave-erosion
• Deposition by sea water
• Deposition by marine life

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Sandy Coastal Beaches

• Longshore drift is the tendency for material to
  move along the coastline due to longshore
  currents.
• As waves approach the shoreline at angles wave
  refraction causes the wave to bend them parallel
  to shore
• These currents move sand along the coast
• On both US coasts longshore currents tend to move
  material south

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Beach Dynamics

• Most sand comes from erosion
• Typically from a long distance away (for example
  sand along our coast comes from the Appalachian
  Mountains)
• Some sand like on the volcanic islands of Hawaii
  come from the island itself

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• The motion of the sediment along the coast must
  stay in balance.
• Humans have caused an imbalance
• Jetties are long structures built perpendicular
  to the shore to keep sand from flowing into
  shipping channels.
• This inadvertently causes sand to be pulled away
  from the back side of the jetty causing erosion

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Sand features

• Barrier islands are large sediment deposits
  that form between the ocean and the shoreline.
• Spit a length of accumulated sand attached to
  land at one end, pointing in the direction of the
  longshore drift
• Tombolos - are spits that extend between 2
  islands or an island to the mainland.

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Dune Succession

• Primary dune
• Dune that forms closest to the shoreline
• Winds blow sand off the beach where it is trapped
  by sea oats and other vegetation
• Secondary dune
• Mature dune, vegetated with woody shrubs and
  eventually a maritime forest develops
• Between successive dunes is a swale. This is a
  low area where breezes are deflected and the
  result is furnace-like temperatures

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• On each dune is a windward side and a leeward
  side. The windward side (the side the wind is
  blowing from) is a gradual slope while the
  leeward side is steep and is called a berm.
• The berm prevents land breezes from blowing sand
  back into the ocean.

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• Dunes that are located on the continental shelf
  that are caused by breaking waves are called sand
  bars.