Plate Tectonics II

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Plate Tectonics II

• Structure of the Earth

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Quick Review

• Historical Development
• Continental Drift
• Magnetic anomalies
• Seismic Reflection and Refraction

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Three Layers

• Crust
• Mantle
• Core

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How do we know what the Earth's Interior is like?

• Drilling Wells drilled into Earth are mostly in
  the upper 7 km of the crust
• Deepest well Soviet (Russian) well in northern
  Kola Peninsula 20 year effort to drill a 12 km
  hole. Stopped in 1989.
• History 5 years to drill 7 km 9 years to drill
  the next 5 km got stuck at 12 km.
• Target depth is 15 km.
• Costs are more than 100 million.
• Bottom hole temperature is 190 º degrees C
• Current status??

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How do we know what the Earth's Interior is like?

• Deepest US well is next to San Andreas Fault
  (Cajon Pass)
• Had reached 3.5 km in 1988
• Cost was 5 million (1400 per meter)
• Cost overruns and budget cuts suspended drilling
  in 1988
• Other deep holes are planned.

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How do we know what the Earth's Interior is like?

• Volcanic activity Materials are brought up from
  below.
• Xenoliths foreign rock (pieces of the mantle in
  lava) example coarse-grained olivine
  (peridotite) xenoliths in basaltic lava
• Only useful to depth of about 200 km

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How do we know what the Earth's Interior is like?

• High pressure laboratory experiments
• Samples of the solar system (meteorites)
• Study of seismic waves generated by earthquakes
  and nuclear explosions

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Internal Structure of the Earth
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Crust

• 1. Oceanic crust (basaltic 3.0 g cm3)
• a. Approximately 5-12 km thick
• b. Average density of 3.0 g/cm
• c. The upper mantle is the ultimate source for
  the lavas that formed the oceanic crust
• 2. Continental crust (granitic 2.7 g cm3)
• a. thickest crust (average 35 km 20 to 100 km)
• b. floats due to isostasy
• continents float higher on the denser mantle
  than the adjacent oceanic crustal segments

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Crust

• Seismically defined as all of the solid Earth
  above the Mohorovicic discontinuity

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Inner Layers of the Earth

• Andrija Mohorovicic
• born in 1857, was a scientist from Croatia who
  worked in the fields of meteorology and
  seismology
• showed how the seismic waves of earthquakes
  spread through the Earth

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Inner Layers of the Earth

• What is the Mohorovicic Discontinuity?
• The surface of the earth is called the crust,
  which is the uppermost part of the Lithosphere
  (which includes the upper portions of the
  mantle). The "Moho" is the boundary between the
  crust and upper mantle.

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Inner Layers of the Earth

• Mohorovicic discovered the discontinuity in 1909.
  The Moho separates crustal rocks with P-wave
  velocities of 6 to 7 km/s from underlying mantle
  rocks with P-wave velocities greater than 8 km/s

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Divisions of Inner Space

• Seismic Waves
• Generated when rocks are suddenly disturbed they
  break or rupture
• Vibrations spread out in all directions from the
  source of the disturbance they move outward in
  waves that travel at different speeds through
  materials that differ in chemical composition or
  physical properties

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Divisions of Inner Space

• Seismic Waves
• Primary -p-waves
• Compressional or P waves in which particle
  oscillate in the direction parallel to the
  direction of the wave propagation. P-waves are
  the fastest and most abundant therefore easiest
  to detect.
• are the speediest of the three
• travel through the upper crust of the Earth at
  speeds of 4-5 km/sec
• near the base of the crust they speed along at
  6-7 km/sec

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Seismic Waves

• Secondary or s-waves waves
• travel 1-2 km/sec slower than p-waves
• able to penetrate deep into the interior or body
  of the planet
• s-waves cannot propagate through fluids
• p-waves are markedly slowed through fluids

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Seismic Waves
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Seismic Waves

• Reflection of seismic waves. These studies use
  the principle that as P-waves encounter internal
  boundaries in the earth some of the energy is
  reflected back to the surface. The energy source
  is usually man-made (air or water guns, dynamite)
  and is detected by geophones or hydrophones. The
  amount of energy returned is a function of the
  change in physical properties at that layer
  (acoustic impedance).

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Seismic Waves

• Acoustic impedance is the contrast of the density
  x velocity. This determines how much energy will
  be reflected or returned to the surface.
• Function of the velocity and density differences

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Seismic Waves

• Refraction - as the seismic waves propagate
  through the earth the wave energy not reflected
  by at a boundary is refracted or bent. In
  general, where the wave velocity increases with
  depth, the waves are bent upwards.

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Seismic Waves
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Refraction
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Refraction
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Inner Layers of the Earth
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Mohorovicic Discontinuity
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Mantle

• Average density is about 4.5 g/cm3
• 1. Stony composition (4.5 g cm3)
• a. oxygen and silicon predominate accompanied by
  iron and magnesium
• b. the mineral peridotite approximates the kind
  of material inferred for the mantle appropriate
  for the mantle's density similar in composition
  to stony meteorites

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Internal Structure of the Earth
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The Core

• The Gutenberg Discontinuity is the boundary
  between the Core and the Mantle
• It is located 2890 km from the surface of the
  Earth, or 3500 km from the center of the Earth.

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The Core

• 1. Detected by P and S waves shadow zones
• Inferences from Body Waves
• a. The precise boundary of the core was
  determined by the study of earthquake waves
• b. The outer core barrier to s-waves results in
  an s-wave shadow zone on the side of the Earth
  opposite the earthquake - passed through a liquid
  medium
• c. Radius of the core is about 3500 km

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The Core

• d. inner core is solid with a radius of 1220 km
• e. evidence for the existence of a solid inner
  core is derived from the study of hundreds of
  seismograms
• a transition zone approximately 500 km thick
  surrounds the inner core with the same
  composition as the outer core
• Important for Latent Heat of Fusion

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The Core

• 2. Average density 10.7 g/cm3
• The Earth had an overall density of 5.5 g/cm3
• the average density of rocks at the surface is
  lt3.0 g/ cm3
• rocks of the mantle have a density of about 4.5
  g/cm3
• QED the average density of the core is about
  10.7 g/cm3

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The Core

• Composed mainly of Fe and Ni
• Composed of 85 iron with lesser amounts of
  nickel as determined from the study of meteorites
• consist of metallic iron allowed with a small
  percentage of nickel
• abundance in the solar system suggests the
  existence of an iron-nickel core
• additional evidence from the existence of a
  magnetic field produced by an electric current
  flowing through a wire
• 4. Radius 3500 km

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Plate Boundaries

• Three types
• Divergent
• Convergent
• Transform

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Divergent Plate Boundaries

• In certain regions, plates are separating. At
  these divergent plate boundaries , new
  lithospheric crust is being created. In ocean
  basins this process is referred to as sea-floor
  spreading as magma wells up from the
  asthenosphere along mid-oceanic ridges , widening
  the ocean basin.

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Divergent Plate Boundaries

• In continents, along continental rift zones ,
  huge land masses can be separated by new ocean
  basins. Both such regions are referred to as
  constructive plate margins because lithospheric
  formation is occurring. Perhaps the best example
  of this process is along the mid-Atlantic ridge.

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Divergent Boundary
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Divergent Boundary
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Divergent Boundary
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Divergent Boundary
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Divergent Boundary
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Divergent Boundary
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Divergent Boundary
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Divergent Boundary
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Convergent Plate Boundaries

• In other regions, tectonic plates are colliding.
  At these convergent plate boundaries , one plate
  is driven down under its neighbour in a process
  called subduction . Such regions are referred to
  as destructive plate margins because the
  advancing edge of one lithospheric plate is being
  re-absorbed into the mantle .

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Convergent Plate Boundaries

• A very good example of subduction is occurring
  along the western coast of South America,
  immediately west of the famous Andes mountains.

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Convergent boundary
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The Andes
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The Cascades
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Island Arcs
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Continent-Continent
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Continent-Continent
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Transform Plate Boundaries

• The third type of relative plate movement occurs
  when one plate slips laterally past its
  neighbouring plate along a major fault . This
  horizontal motion is usually called transform
  plate movement but is also known as slip-strike
  movement, tear faulting ,shear faulting and
  sometimes transcurrent faulting . The best
  large-scale example of this phenomenon occurs
  along the well-known San Andreas fault in
  California.

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Transform boundary
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San Andreas Fault
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Heat - Driver of Plate Tectonics

• Heat is the measure of internal energy of the
  atoms and molecules - translational and
  rotational motions.
• Temperature is an arbitrary numeric scale
  proportional to the average translation kinetic
  energy. Kinetic energy is the energy associated
  with motion.

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Heat - Driver of Plate Tectonics

• The heat flux is the transfer of energy from high
  to low temperature. Understanding the earth's
  heat flux is important to determine
• How is heat transferred within the earth
• Where does it come from
• What processes produce or release heat
• Is the earth heating up or cooling off?

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Heat - Driver of Plate Tectonics

• The downward gradient in the earth's temperature
  was first discovered first by measuring the
  vertical temperature in caves. Typical gradient
  within the earth is 20 to 30C/km.

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Heat - Driver of Plate Tectonics

• Sources of heat within the earth are considered
  to be
• primordial heat which is heat left over from the
  original accretion of the Earth from planetary
  nebula.
• radioactive decay - less obvious but more
  significant.
• crystallization of inner core.

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Heat - Driver of Plate Tectonics

• Types of heat transfer
• Conduction
• Convection
• Radiation

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Heat - Driver of Plate Tectonics

• Conduction is the heat that is transferred
  through molecular collision. Molecules with
  higher vibrational energy collide with molecules
  with lower vibrational energy causing a transfer
  of energy.

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Heat - Driver of Plate Tectonics

• A good example is the heat that is transferred up
  a spoon handle that sits in a pot of boiling
  water.

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Convection

• Heat transferred by the motion of the material
  itself. Movement of material within the earth
  occurs by density differences. As lower mantle
  heats upper by conduction heat transfer across
  the core mantle boundary, increase temperature
  causes decreased density.

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Heat - Driver of Plate Tectonics

• Convection

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Heat - Driver of Plate Tectonics

• A good example of convection is the rising of air
  masses as the sun heats the surface of the Earth.

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Heat - Driver of Plate Tectonics

• Radiative Heat Transfer - occurs as the internal
  energy at one place is converted into
  electromagnetic radiation which radiates out and
  is absorbed by material at another location. The
  electromagnetic energy is converted by into
  internal energy (Heat).

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Heat - Driver of Plate Tectonics

• Examples would be the radiative warming for the
  sun. Electromagnetic waves produced in the sun
  are absorbed by your skin and converted into
  heat.
• Microwave ovens cook by this principle.

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Rheologic Model of Earth

• Traditional subdivisions of the earth interior
  are based on compositional changes. Crust,
  Mantle, Core. Each of the layers have been
  further subdivided based on changes in
  geophysical properties that represent
  compositonal changes.
• The Rheological model of the earth is based on
  the flow properties-rigid vs plastic.

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Rheologic Model of Earth

• The large-scale features of the outer part of the
  earth show a rigid layer in isostatic equilibrium
  underlain by a weaker layer that deforms by flow

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Structure of the Plates

• The Lithosphere is the strong outer layer that
  deforms elastically. This layer includes the
  crust and upper mantle.
• Asthenosphere is weaker and reacts to stress in a
  fluid manner. The Asthenosphere extends from the
  base of the lithosphere to 700 km.
• The lithosphere- Asthenosphere boundary is more
  like a transition zone, not distinct.

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Structure of the Plates

• What makes the aesthenosphere?
• Partial Melting
• How Much?
• 2 to 3

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• The Asthenosphere represents the location in the
  mantle where the melting point is closely
  approached. The rocks in the asthnosphere are
  not molten. S-waves penetrate this layer but the
  velocity drops considerably. This is partial
  melting. It is estimated that only 1 melting
  occurred to explain the observed decrease in
  Seismic wave velocity.

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Driving Forces

• The large-scale features of the outer part of the
  earth show a rigid layer in isostatic equilibrium
  underlain by a weaker layer that deforms by flow

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Isostacy

• Isostacy equals the response of the outer shell
  of the earth to the imposition and/or removal of
  large loads (i.e., continents). The outer shell
  of the earth cannot support large stresses.
• The Principle of Isostacy is that beneath a
  certain depth (Depth of Compensation) the
  pressures (weights) of overlying column of
  material are equal.

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Isostacy
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Isostacy

• Mid Atlantic Ridge sits higher than older crust
  because it is warmer and less dense.

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Plate Tectonics predicts that the earthquakes
with occur and therefore mark plate boundaries.
Examine the figure below which charts the
epicenters of the earthquakes with magnitude
gt4.0.
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Conveyor idea of the mantle convection. This
idea is that the lithospheric plates are passive,
riding the moving rivers of mantle material.
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The Ridge Push Force is the push from a divergent
margin. The topogrpahic slope resulting from the
isotoatic uplift creates forces that pushes the
plate down the slope away from the spreading
center. Essentially, a horizontal pressure
gradient is created by the differential weights
along the plate with a high weight near the ridge
that pushes toward the ends.
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Slab Pull Force originates as the tendancy of the
colder older curst material to sink into the hot
less dense mantle material beneath it. As the
slab sinks it tends to pull the plate behind it.
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Putting it all together