Plate Tectonics and Seafloor Spreading

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Plate Tectonics and Seafloor Spreading
OEAS-306
January 15, 2009

• Outline
• How remote sensing using seismic waves reveals
  the structure of the Earths interior.
• The chemical and physical classification of the
  Earths interior.
• Isostatic Equilibrium and Buoyancy.
• Theory of Continental Drift
• Seafloor Spreading
• -- Divergent, Convergent and Transform
  Boundaries
• Evidence for Plate Tectonics
• Homework 1

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Ocean Facts
1) 97 of water on earth is in the Oceans (1.37
billion km3) 2) 70 of earths surface is covered
by water 3) Average ocean depth is 3,796
m 4) Average temperature of ocean is 3.9 C
(39F) 5) Deepest spot in the ocean is 11,022
m. 6) Average age of oceanic crust is 65 million
years old. (Earth is thought to be 4.6 billion
years old).
Goal of this lecture
Understand the dominant processes that shape the
ocean basins
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Most of knowledge of the earths interior comes
from remote sensing of seismic waves
Earthquakes create several types of seismic waves
1) Body Waves
2) Surface Waves

• P-waves (primary waves)

a) Love Waves
Travel through solid AND liquid
b) S-waves (shear waves)
b) Rayleigh Waves
Travel through solid ONLY
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Different types of waves travel at different
speeds
Seismograph
Minutes
0
10
30
40
50
20
P
S
Surface waves
Mantle
P
S
Core
Body waves
Focus
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If the density of the Earths interior increased
with depth, body waves would travel along curved
paths
If the Earths interior had a constant density,
body waves would travel along a straight path.
Body waves travel faster through denser material.
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Remember that S-waves can only travel through a
solid material
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Density differences on Earths interior also lead
to a P-wave shadow zone
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Studying propagation of seismic waves revealed
the composition of the Earth
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Summary of Earths Composition
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Increasing Pressure and Temperature with Depth
result in the physical transition from
lithosphere to asthenosphere (which is plastic or
deformable).
As a result lithospheric crust floats over the
deformable asthenosphere.
Isostatic Equilibrium
Principle of buoyancy object will sink until is
displaces a volume of water equal to the weight
of the object.
Continental crust is thicker, so it displaces a
greater volume of underlying asthenosphere.
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Idea that lithospheric plates float is
essential to the theory of Plate Tectonics
Alfred Wegner first proposed the idea of
Continental Drift in 1912.
The primary evidence for his theory has the
matching shoreline patterns of many of the
continents
Wegner suggested that the continents were flung
toward the equator by centrifugal force of the
spinning earth.
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Major Lithospheric Plates
The major lithospheric plates, showing their
directions of relative movement and the location
of the principal hot spots. Most of the million
or so earthquakes and volcanic events each year
occur along plate boundaries.
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Convection in the mantle provides the driving
force for continental drift
Seafloor spreading was an idea proposed in 1960
to explain the features of the ocean floor. It
explained the development of the seafloor at the
Mid-Atlantic Ridge. Convection currents in the
mantle were proposed as the force that caused the
ocean to grow and the continents to move.
The heat that drives the plates is generated by
the decay of radioactive elements within Earth
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Divergent Plate Boundaries Where Ocean Basins
are formed

• Divergent plate boundaries Boundaries between
  plates moving apart, further classified as
• Divergent oceanic crust for example, the
  Mid-Atlantic Ridge
• Divergent continental crust for example, the
  Rift Valley of East Africa.
• (Figure to the Right)
• (a) As the lithosphere began to crack, a rift
  formed beneath the continent, and molten basalt
  from the asthenosphere began to rise.
• (b) As the rift continued to open, the two new
  continents were separated by a growing ocean
  basin. Volcanoes and earthquakes occur along the
  active rift area, which is the mid-ocean ridge.
  The East African Rift Valley currently resembles
  this stage.
• (c) A new ocean basin (shown in green) of the
  Atlantic. resembles this stage.

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Convergent Plate Boundaries Where Oceanic
Crust is Destroyed

• Convergent Plate Boundaries - Regions where
  plates are pushing together can be further
  classified as
• Oceanic crust toward continental crust - for
  example, the west coast of South America.
• Oceanic crust toward oceanic crust - occurring in
  the northern Pacific.
• Continental crust toward continental crust one
  example is the Himalayas.

(Above Right) A cross section through the west
coast of South America, showing the convergence
of a continental plate and an oceanic plate. The
subducting oceanic plate becomes more dense as it
descends, its downward slide propelled by
gravity. At a depth of about 80 kilometers (50
miles), heat drives water and other volatile
components from the subducted sediments into the
overlying mantle, lowering its melting point.
Masses of the melted material, rich in water and
carbon dioxide, rise to power Andean volcanoes.
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Other Convergent Plate Boundaries
Continental and Continental Crust
Oceanic and Oceanic CrustIsland Arc Systems
Example Himalayas
Example Marianas Island
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Transform Boundaries
Divergent seafloor spreading typically does not
follow a smooth curve. This results in transform
fault boundaries where two plates slide past one
another.
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Evidence Supporting the Theory of Plate
Tectonics The Global Distribution of Earthquakes
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Evidence Supporting the Theory of Seafloor
Spreading Paleomagnetism
1) Earths magnetic field is thought to be driven
by the interaction between the rotation of the
earth and convective circulation of the iron rich
material in the outer core.
2) As a result the magnetic north pole is titled
11 from the geographic north pole.
3) The magnetic north pole wanders considerable
over long time scales, with the magnetic polarity
reversing every few hundred thousand years.
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Paleomagnetism (cont.)

• Upwelling magma that forms the basaltic oceanic
  crust, contains iron-bearing magnetite.
• When these materials cool past the Currie point
  at approximately 600 C, they lock their
  orientation to the Earths prevailing magnetic
  field.

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• As the seafloor spreads of hundreds of thousands
  of year, the Earths magnetic reversals are
  recorded in the ocean crust.
• This information can be measured using a
  magnotometer.
• Surveys with magnotometers revealed symmetric
  patterns of stronger and weaker magnetism on
  either side of the mid-Atlantic ridge.

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Summary of Lecture 1

• Some seismic wavesenergy associated with
  earthquakescan pass through Earth. Analysis of
  how these waves are changed, and the time
  required for their passage, has told researchers
  much about conditions inside Earth.
• Earth is composed of concentric spherical layers,
  with the least dense layer on the outside and the
  most dense as the core. The lithosphere, the
  outermost solid shell that includes the crust,
  floats on the hot, deformable asthenosphere. The
  mantle is the largest of the layers.
• Large regions of Earths continents are held
  above sea level by isostatic equilibrium, a
  process analogous to a ship floating in water.
• Plate motion is driven by slow convection
  (heat-generated) currents flowing in the mantle.
  Most of the heat that drives the plates is
  generated by the decay of radioactive elements
  within Earth.
• Plate tectonics theory suggests that Earths
  surface is not a static arrangement of continents
  and ocean, but a dynamic mosaic of jostling
  segments called lithospheric plates. The plates
  have collided, moved apart, and slipped past one
  another since Earths crust first solidified.
• The confirmation of plate tectonics rests on
  diverse scientific studies from many disciplines.
  Among the most convincing is the study of
  paleomagnetism, the orientation of Earths
  magnetic field frozen into rock as it solidifies.
• Most of the large-scale features seen at Earths
  surface may be explained by the interactions of
  plate tectonics. Plate tectonics also explains
  why our ancient planet has surprisingly young
  seafloors, the oldest of which is only as old as
  the dinosaurs that is, about 1/23 of the age of
  Earth.