The%20Sea%20Floor

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The Sea Floor

• Chapter 2

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Continental Drift

• Theory proposed by Alfred Wegner (German
  geophysicist),
• Stated that all the continents were joined
  together at one time in to a supercontinent
  called Pangea.
• Pangea began to break apart 180 million years ago.

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Pangea
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Theory of Plate Tectonics

• Theory that provides the explanation for
  continental drift.
• It is the process involved in the movement of
  large plates on the earths crust.

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Mid Ocean Ridge

• Discovered from sonar images after WWII.
• It is a continuous chain of volcanic mountains
  that encircles the globe like a baseballs seams.
• It is the largest geological feature on Earth!!!
• The formation was made when transformation faults
  were displaced by cracks in the Earths crust.

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The Sea Floor

• The sea floor is geologically distinct from
  continental land masses.
• Geological processes sculpt the shoreline,
  determine water depth, control if the sea bottom
  is muddy, sandy, or rocky.
• Geological processes create new islands and
  undersea mountains for colonization and determine
  specific marine habitats.

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Earth is a water planet.

• Oceans cover 71 of the globe.
• Oceans regulate Earths climate and atmosphere.
• The Northern Hemisphere is 61 covered by oceans.
• The Southern Hemisphere is 80 covered by oceans.

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Ocean Classification

• Oceans are divided into four basins.
• Pacific Basin- deepest and largest
• Atlantic Basin- second largest
• Indian Basin- third largest
• Arctic Basin- smallest and shallowest

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Connected to basins are seas.

• Examples of seas are Mediterranean, Gulf of
  Mexico, South China, Red, Dead, Baltic, etc.
• All oceans are interconnected and compromise a
  single world ocean referred to as Panthalassa.
• The Southern Ocean is a continuous body of water
  that surrounds Antarctica.

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Internal Earth Structure
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The Core

• The innermost portion of Earth.
• The inner core is made of solid iron.
• The outer core is made of liquid iron that has a
  swirling motion.
• This motion is responsible for Earths magnetic
  field.

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The Mantle

• Found outside the core.
• Most is solid and very hot, near the melting
  point of most rocks.
• Some flows very slowly like a liquid.

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The Crust

• Outermost layer made of thin, rigid rock that
  floats on top of the mantle.
• Continental Crust- mostly made of granite and is
  light in color. It is less dense than oceanic
  crust so it floats on the mantle. Approx. 3.8
  million yr. old.
• Oceanic Crust- mostly made of basalt and is
  denser and thinner than continental crust. It
  does not float as high on the mantle. Aprox. 180
  million years old.

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Mid Ocean Ridge
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Classification

• In the Atlantic- Called Mid Atlantic Ridge
• In the Pacific- Called East Pacific Rise

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Trenches

• Deep depressions in the sea floor especially
  common in the Pacific.

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Significance of Mid Ocean Ridge

• Lead to the discovery that from time to time the
  Earths magnetic field has reversed direction.
• This happens about every 700,000 years.
• Reversal takes 5000 years.
• Reversal occurs from the movements of materials
  in the Earths core.

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Magnetic Anomalies

• Geologists found striped patterns of magnetic
  bands in the sea floor.
• Show alternate periods of normal and reversed
  periods of earths magnetic field.

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The Sea Floor

• Created by oceanic crust separated at the
  mid-ocean ridge allowing some mantle to rise
  through the rift.
• When the mantle rises through the rift it cools
  and solidifies to form new oceanic crust.

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Sea Floor Spreading

• Process of making new oceanic crust.
• What do you remember about the oceanic crust?

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Lithosphere

• Litho Rock
• Rock sphere
• Composed of plates of crust and upper mantle.
• These plates can contain continental/oceanic
  crusts or both.
• The lithosphere floats on the denser, more
  plastic athenosphere.

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Plate Movement
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Subduction

• The downward plate movement into the mantle at
  a subduction zone to form a trench.
• This movement causes earthquakes and when some
  molten material rises to the surface it may form
  volcanoes.
• Ex. When oceanic crust subducts under
  continental crust the Andes mountains of South
  America were formed.

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Subduction
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• When two oceanic crusts collide one will subduct
  below the other and form trenches that lead to
  the formation of volcanic island chains called
  island arcs.
• Ex The Aleutian and Marianas Islands

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• When two continental plates collide both tend to
  float and buckle and neither is subducted.
• This forms mountains ranges such as the
  Himalayas.
• What properties of the continental crust make
  this happen?

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Shear Boundary

• When two plates slide past each other causing
  extreme friction.
• This stress can cause earthquakes.
• Ex A shear boundary formed the San Andreas
  Fault in California.

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Shear Boundary
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Convection

• When heat from the mantle swirls the plates.
• This used to be the main hypothesis of plate
  tectonics.
• Now it is believed to be an auxiliary form of
  plate motion.

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Convection
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Geologic History
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Pangea

• 200 million years ago Pangea was surrounded by
  Panthalassa.
• What was Panthalassa?
• Panthalassa is thought to be the ancestor to the
  Pacific ocean.
• Tethys Sea- separated Eurasia from Africa (It is
  the modern Mediterranean Sea.
• Sirius Borealis- is the modern Arctic Ocean.

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180 million years ago. . .

• A new rift formed between North America and
  combined the continents of South America and
  Africa.
• This rift formed the early Mid-Atlantic Ridge and
  the North Atlantic Ocean.
• Pangea broke into two continents
• Laurasia- North America and Eurasia
• Gondwana- South America, Antarctica, India, and
  Australia. A rift split at this time forming the
  Indian Ocean.

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Marine Sediments

• Lithogenous- sediments derived from physical and
  chemical weathering of rocks.
• These are most common.
• Ex red clay on the open ocean floor.

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• Biogenous- made of skeletons, shells of marine
  organisms such as diatoms, radiolarians,
  foraminiferans, and coccolithophorids.
• Composed of calcium carbonate- sometimes referred
  to as calcareous ooze.

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• Diatoms

• Foraminiferans

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• Microfossils- microscopic preserved remains of
  marine organisms that give clues about the type
  of water the organism lived in such as whether it
  was cold or warm.

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Microfossils
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Radiometric Dating

• The use of radioactive isotopes to determine the
  exact age of a fossil.
• Also referred to as carbon dating.

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Climate and Changes in Sea Level
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• Earths climate fluctuated between warm
  interglacial periods and cold ice ages.
• During ice ages the sea level falls because water
  is trapped as glaciers.

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Pleistocene Epoch

• Two million years ago- was the last major period
  of glaciation.
• Sea level has risen over the past 3,000 years due
  to greenhouse gases such as carbon dioxide and
  methane.
• These gases are said to cause global warming
  therefore melting glaciers and subsequently
  causing a rise in sea level.

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Geology of the Sea Floor
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The sea floor is divided into two main regions

• Continental margin- are boundaries between
  continental and oceanic crust.
• They consist of shallow, gently sloping
  continental shelf, a steeper continental slope,
  and a gently sloping continental rise.

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Continental shelf

• Shallowest part of the continental margin.
• Makes up only 8 of the oceans surface area but
  are the most biologically rich portion of the
  ocean.
• When the sea level rises- submarine canyons have
  formed here.
• The shelf ends at the shelf break where the
  slope gets steeper 120-400m.

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Continental Shelf
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Continental Slope

• Begins at the shelf break and descends to deep
  sea floor.
• Submarine canyons cause its depth to reach
  3000-5000m.
• The continental slope channels sediments to the
  sea floor.

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Continental Slope
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Continental Rise

• When sediment from the submarine canyons
  accumulates and deep sea fans (deposits that are
  similar to a river delta) piles on the sea floor.

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Continental Rise
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Active and Passive Margins
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Active Margins

• Continental margins with intense geological
  activity.
• Earthquakes and volcanoes are characteristic
  here.
• Have steep, rocky shorelines, narrow continental
  shelves, steep continental slopes, and lack a
  developed continental rise.

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Active Margin
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Passive Margin

• Inactive geologically.
• Flat, coastal plains with wide continental
  shelves and gradual slopes, leading to a thick
  continental rise.

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Passive Margins
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Deep Ocean Basins

• Most of the deep sea floor lies at a depth of
  3000-5000m (10,000-16,500 ft).
• Abyssal plain- deep sea floor, rises at gentle
  slope toward mid-ocean ridge.
• Relatively flat but can contain submarine
  channels, abyssal hills, plateaus, rises and
  other features.

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Abyssal Plain
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Seamounts

• Comprised of volcanic islands and submarine
  volcanoes.
• Ex Guyots- flat-topped seamounts that are
  common in the Pacific.
• These areas have a great diversity of marine life.

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Trenches

• Areas where plates descend into mantle.
• Are the deepest parts of the ocean.
• The deepest is the Mariana Trench in the Western
  Pacific at 11, 022m (36,153ft).

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Mariana Trench
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Mid-Ocean Ridge and Hydrothermal Vents
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• The Mid-Ocean Ridge contains a gap called the
  central rift valley.
• Formed when fractures and crevices let seawater
  seep downward where it is then heated to a very
  high temperature by the mantle.
• This heated water is forced back through the
  crust via hydrothermal vents or deep sea
  hotsprings.

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Mid-Ocean Ridge
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Hydrothermal Vents
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Deep Sea Hot Springs
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• Water from hydrothermal vents can be warm, 10-20C
  (50-68F) or blistering 350C (660F)!
• This hot water dissolves minerals such as
  sulfides. These minerals cool and form deposits
  around the vents.

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Black Smokers

• One type of deposit. They look like a chimney.
• Made of solidified minerals.
• The smoke the boils from them is a dense cloud of
  mineral particles.

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Black Smokers
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• Deep Sea hot springs are of interest to
  geologists and biologists.
• They are an abundant source of marine life.
• Many marine organisms are found around these
  vents.

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Tube worms around a vent.