The Sea Floor

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• The Sea Floor

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Figure 2.01
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• The Ocean Basins
• Distribution of Oceans 61 of the Northern
  Hemisphere is ocean while 80 of the Southern
  Hemisphere is ocean.
• Four large basins exist all are connected
  allowing seawater, materials and organisms to
  move from one ocean to another sometimes called
  the world ocean.
• Pacific deepest and largest ocean
• Atlantic larger than Indian, but equal in depth
• Indian
• Arctic smallest and shallowest

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Figure 2.02
The Southern Ocean refers to the body
of water that surrounds Antarctica.
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Figure 2.03
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• Review of the Structure of the Earth
• Core innermost layer, composed mostly of iron.
  Pressure is 106 times that of the surface.
  Temperature is over 4000 Celsius. Swirling
  motions of the outer core produce earths
  magnetic field.
• Mantle layer just beyond the core, thought to
  be solid and near the melting point of rock. It
  slowly flows like a liquid.
• Crust outermost layer, extremely thin - a rigid
  skin floating on the mantle

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Figure 2.04
The land masses on opposite side of the
Atlantic have coastlines and geological features
that fit together like pieces of a puzzle.
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• Theory of Plate Tectonics
• Alfred Wegner, a German geophysicist, first
  proposed the theory of continental drift in 1912
• He suggested that all of the continents had once
  been joined in a supercontinent called Pangaea.
• Pangaea began to break up into our current
  continents about 180 million years ago.

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Oceanic Crust - (Basalt) Continental Crust - (Granite)
Density 3.0 g/cc Density 2.7 g/cc
5 km thick Geologically young lt200 million years old 20 to 50 km thick Can be 3.8 billion years
Dark in color Rich in Fe and Mg Light in color Rich in Na, K, Ca and Al
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Figure 2.05
Major features of the sea floor.
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• SEA FLOOR FEATURES
• Mid-Ocean Ridge a continuous chain of submarine
  volcanic mountains that encircle our planet.
• Sea-floor rock near the ridge is young. Rock
  gets older as one moves away from center.
• Bands of magnetic stripes that run parallel to
  the mid-ocean ridge represent zones in which the
  rocks on the sea floor alternate between normal
  and reversed magnetization.
• Bands are symmetric on either side of ridge.
• Magnetic anomalies is name given to these
  stripes.
• Importance sea floor must have cooled from
  molten material at different times.

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Figure 2.09
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• Sea-floor spreading is the name given to the
• process during which the sea floor moves away
• from the mid-ocean ridges to create new sea
  floor.
• Mid-ocean ridge forms the edges of many tectonic
  plates
• It is a continuous chain of submarine volcanic
  mountains that encircles the globe, the largest
  geological feature on earth.
• Mid-Atlantic Ridge runs down the center of the
  Atlantic Ocean

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• How does sea-floor spreading work?
• As pieces of oceanic crust separate at the
  mid-ocean ridges, they create a rift. This
  releases some pressure on the mantle and causes
  it to melt.
• The liquid mantle material rises up through the
  rift.
• Ascending mantle material pushes up the oceanic
  crust around the rift to form the mid-ocean
  ridge.
• When this molten material reaches the surface, it
  cools and solidifies to form new oceanic crust.

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Figure 2.07
Distribution of earthquakes and volcanoes.
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Figure 2.06
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• Magnetic Reversals
• Magnetic reversals occur about every 700,000
  years
• It takes about 5000 years for the field to
  reverse.
• Cause is thought to be the movement of material
  in earths molten outer core.
• When rocks cool, the magnetic particles within
  them align themselves with the current pole.
• Stripes appear that are symmetrical around the
  ridge.
• Called magnetic anomalies showed that the sea
  floor cooled at different times

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Figure 2.08
Normal (dark) and reversed (light) magnetism. stri
pes form bands running parallel to the
mid-ocean ridge.
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Figure 2.10
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Figure 2.11
Formation of a trench by the collision of an
oceanic and continental plate. Earthquakes are
produced as the Nazca Plate descends into the
mantle. Lighter material from the plate rises as
it melts to create the Andes Mountains.
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Figure 2.12
Here two oceanic plates meet and form another
type of trench. Earthquakes are produced by the
descending plate and the volcanoes have produced
the Aleutian Islands.
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• Facts about Trenches
• They are curved because they follow the
• curvature of the earth.
• 2. Island arcs (Aleutian and Mariana) which
• are really volcanic chains also look curved.
• When two continental plates collide..
• Both are light in density and they dont subduct,
  but buckle and produce mountain chains, e.g.
  Himalayas formed when India collided with Asia.

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Figure 2.13
Mt. Veniaminof, an active volcano on the Alaska
Peninsula, part of the Aleutian Island chain
that formed from behind the Aleutian Trench.
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Figure 2.15
Current theory says that plates move mainly
because of slab pull, ( old, cold and dense
lithosphere sinks into the mantle and pulls the
rest of the plate behind it.)
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• Traditional theory said that the movement
• of plates is driven by large scale convection
  currents
• in the asthenosphere and lower mantle which are
  driven
• in turn by heat from the earths core.
• Even though slab pull theory is currently
  accepted
• Convection may still play a role in plate motion.
• http//www.youtube.com/watch?vp0dWF_3PYh4

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Figure 2.16
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• Marine Sediments
• Lithogenous sediment, derived from the physical
  and chemical breakdown of rocks which are found
  mostly on continents.
• Biogenous Sediment, consisting of skeletons and
  shells of marine organisms.
• 1. calcareous ooze sediment composed of CaCO3
• 2. siliceous ooze made of SiO2

Microfossils are important because they tell us
what organisms lived in the ocean in the past and
give clues to ocean temperatures. Ocean
temperatures are dependent upon the earths
climate and ocean currents.
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Figure 2.17
Fossil Shell of a
foraminiferan.
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• Climate of the earth
• 1. Can be determined by C-14 dating
• 2. Water temperature is found by the ratio of Mg
  to Ca or of different isotopes of O in
  microfossils.
• 3. The ratio of Sr to Ca in coral skeletons
  records past
• ocean temperatures.
• 4. Ice cores from Greenland and Antartica
  preserve records of past temperatures as well as
  samples of our ancient atmosphere from tiny air
  bubbles trapped in the ice.

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Figure 2.18
History of the earths climate over the past
half-million years as Determined by
foraminiferan microfossils. Red line average
sea surface temps. (Mg to Ca ratios) Blue and
white bands major glacial periods (oxygen
isotopes)
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Figure 2.19
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• Continental Margins
• -boundaries between continental crust and oceanic
  crust
• -sediment from the continents settles to the
  bottom after reaching the sea and accumulates on
  the margins
• -sediment may be 6 miles thick!
• Continental margins consist of a shallow, gently
  sloping shelf , a steeper slope and a gently
  sloping region the continental rise at the base
  of the slope.
• Varies from 1 km on the Pacific coast of S.A. to
  750 km on the Arctic coast of Siberia.
• - Ends in the shelf break where it abruptly
  gets steeper.

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• Continental Slope
• - This is the closest thing to the edge of the
  continent, beginning at the shelf break and
  descending downward to the deep sea floor.
• Continental Rise
• A thick layer of sediment piled up on the sea
  floor.
• - The deep sea fan is like a river delta and
  is caused by sediment moving down a submarine
  canyon and accumulating at the canyons base.

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• Continental Shelf
• This is the shallowest part of the margin, making
  up 8 of the oceans surface and is the richest
  part of the ocean!
• It is composed of the continental crust and is
  really part of the continent thats under water.
• Submarine canyons are canyons formed by the
  erosion by rivers and glaciers that are now under
  water.

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Figure 2.22
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• Active and Passive Margins
• Active margins are trenches with intense
  geological
• activity, such as earthquakes and volcanoes.
• Characterized by rocky shorelines, narrow
  continental shelves and steep continental slopes
  and lacking a well developed continental rise. (
  the west coast of N.A.)
• Passive margins have flat coastal plains, wide
  shelves and gradual continental slopes. Sediment
  accumulates at the base of the slope and they
  have a thick continental rise.

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Figure 2.20
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Figure 2.24
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Figure 2.23
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• Deep-Ocean Basins
• The sea floor is almost flat the abyssal plain.
• It is dotted with submarine volcanoes called
  seamounts and volcanic islands.
• Guyots are flat topped seamounts and are common
  in the Pacific. The abyssal plain also has
  plateaus, rises and other features.
• The Central Rift Valley is the depression at the
  center of the mid-ocean ridge. It is extremely
  hot and dotted by hydrothermal vents or deep-sea
  hot springs.

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Figure 2.25
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• Hot water dissolves a variety of minerals mostly
  sulfides.
• Black Smokers are one type of mineral deposit
  found at hydrothermal vents chimney like
  structures that build up around a vent as
  minerals solidify.

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Figure 2.21
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Figure 2.26
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Figure 2.27
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Text Art 2.01
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Text Art 2.02
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TABLES
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Table 2.01
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Table 2.02