Plate Tectonics

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Plate Tectonics Earthquakes

• Mr. Sam Vinson
• http//www2.volstate.edu/SVinson/
• http//www.dlese.org/dds/index.jsp

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Summary of Topics

• Basic Concepts of Plate Tectonics
• History of Plate Tectonics
• Alfred Wegeners Continental Drift
• Harry Hess Sea Floor Spreading
• Earthquakes
• Seismic Wave Properties
• Seismograph and locating an Epicenter
• Intensity Magnitude
• The Structure of the Earth

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Presentation philosophy and organization

• Deliver necessary background and explanation
  while providing many ideas for active student
  learning.
• Emphasize
• student activities (bright yellow background)
• web activities (green links)
• writing reading across the curriculum (black
  white quill icon).

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Earths Structure
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• Stress Strain
• Elastic, Plastic
• Ductile or Brittle

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THE EARTHS DYNAMIC CRUST

• Basic concepts of plate tectonics

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Plate tectonics

• Plate tectonic theory-lithosphere is divided into
  rigid plates that move over the asthenosphere.
• Seven major and about 20 minor
• Plate boundaries rub together are geologically
  active. Earthquakes Volcanoes

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Worlds Plates
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3 types of plate boundaries

• Convergent
• A. Compression causes shortening of the rock
  layers through folding faulting.
• Divergent
• B. Tension lengthens and causes faulting.
• Transform
• C. Shear stress results in displacement (with a
  horizontal component) along a plane.

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Applied Stress
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Plate Boundaries
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Convergent boundaries

• Associated with structures of compression
• High frequency of earthquakes
• May be marked by oceanic trenches and subduction
  zones
• Suture zones are formed when continental masses
  are welded together

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North Americas Convergence
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Divergent boundaries

• Associated with tensional structures
• New crust is added at the trailing edges as
  basaltic lava wells up from the mantle
• Many shallow earthquakes associated with rifting
• Characterized by a mid-oceanic ridge split by a
  deep rift

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Cracking Up
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Birth of Divergence
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Filling the Students Plates

• PBS.org has an excellent on line tutorial on the
  moving plates.
• Simple introductive text, interactive Plate
  Tectonics Graphic, an Egg-cellent hands on
  activity.
• http//www.pbs.org/wgbh/aso/tryit/tectonics/

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Plate Dynamics
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What mechanism causes plate tectonics?

• Most theories involve large, rolling convection
  cells in the asthenosphere
• Density Differences
• Mantle Convection
• Try using a Lava Lamp.

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Plate tectonics has its roots in two older ideas

• Continental Drift Hypothesis
• Seafloor Spreading Theory

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Continental Drift

• Main ideas set forth by
• Alfred Wegener (presented in Europe in 1912, U.S.
  1924)

• Introduction to Alfred Wegener and Continental
  Drift http//www.ucmp.berkeley.edu/history/wegener
  .html
• Wrath of Science Obligation to be a Hero
  http//pangaea.org/wegener.htm

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Alfred Wegener

• Argued for a much larger supercontinent which he
  called Pangaea
• Pangaea was surrounded by a universal ocean
  (Panthalassa)

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Continental Drift

• Primary Evidence
• Fit of Continents
• Evidence of Fossils
• Paleoclimatology

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Geographic fit of continents

• See west Africa and eastern South America
• Especially true if the continental shelves are
  included
• Mismatch is minimal with proper alignment

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Worlds largest Jigsaw Puzzle

• Find and print a map of the worlds continents.
  Specifically, show North America, South America,
  Europe Africa.
• Have the students cut the continents out and
  reassemble them to a single landmass. They
  should use arrows to show the plate movements
  from the single landmass to fragmented
  continents.
• Have the students write a descriptive narrative
  explaining the events that have separated the
  landmasses. Require them to use proper
  terminology for the boundaries.

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Reconstruction of Pangaea
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Rocks and sediments

• Paleozoic tillites in all parts of Gondwanaland
• Without Gondwanaland, the center of glaciation
  would have been the ocean
• Boulders in tillites had sources on other
  continents
• Mountain ranges which would be continuous, but
  for Atlantic Ocean
• Evaporite deposits in cold areas

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Gondwanaland Glaciation
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Current Glacial Deposits
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Paleontological evidence

• Tropical plants in Antarctic coal beds
• Coral reef deposits in cold climates
• Glossopteris flora-fossil plants found throughout
  Gondwanaland
• Mesosaurus fauna-aquatic reptiles whose remains
  have been found in many sites in southern
  hemisphere

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Fossil Evidence
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The Flaw of Wegeners Hypothesis

• Many professionals criticized Wegener because he
  had no plausible mechanism which could account
  for the movement of continents
• (???plowing ship or sliding over???)

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Wegener Forgotten

• "Utter, damned rot!" said the president of the
  prestigious American Philosophical Society.
• "If we are to believe this hypothesis, we must
  forget everything we have learned in the last 70
  years and start all over again," said another
  American scientist.
• quotes earlier biographical sketch from,
  http//earthobservatory.nasa.gov/Library/Giants/We
  gener

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Technology developed during World War II led to
an explosion of knowledge concerning the ocean
basins during the post war years.
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Examples of this technology

• Magnetometers
• Devices used in monitoring nuclear tests
• Sonar
• SCUBA and deep-diving submarines

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Paleomagnetism

• Sea Floor Spreading

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EARTH as a MAGNET?

• Earth can be thought of as a giant dipole magnet
• New rock is created at divergent boundaries as
  oceanic crust (igneous rock) moves upward to fill
  the void as crust pulls apart.
• When the molten rock cools, iron bearing minerals
  align themselves with Earths Magnetic field. The
  lava records the direction strength of the
  magnetic field. CURIE POINT

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Earth as a magnet
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Magnetic Patterns

• After WWII we began mapping the magnetic fields
  on the continents and the sea floor.
• The newest continental ocean rock showed
  patterns of todays magnetic field, but older
  rocks did not, they showed different
  orientations!
• If you trace the path the continents have moved
  since Pangaea split, then the orientation of
  Fe-bearing minerals on the continent do show the
  magnetic field to be aligned north to south.

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Magnetic Reversals

• Based on land rock we know In the past the
  magnetic field has completely reversed, it keeps
  its north to south alignment, but the poles of
  the magnet have reversed.
• Todays field is considered normal magnetism
  and when the poles have swapped is called
  reversed magnetism
• If new crust is constantly being formed from lava
  at divergent boundaries in the ocean shouldnt we
  see the reversals there?

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Harry Hess Seafloor Spreading

• Hess proposed the continents do not move across
  the oceanic crust, but rather they move together.
  Like a conveyor belt that originates at the
  divergent boundaries.
• Samples taken confirm that indeed, not only does
  the oceanic crust become older as one moves away
  from the ridge but the ocean floor is striped by
  parallel symmetric magnetic reversals that have
  occurred in the past.

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-Continued...

• Apparently the polarity of Earths magnetic field
  flips from time to time.
• This led Vine and Matthews, in 1963, to suggest
  that all sea floor was formed at the mid-oceanic
  ridges, from which point it spread outward.
• This is the concept of seafloor spreading.

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More evidence of seafloor spreading

• Rocks get progressively older as one moves away
  from the midoceanic ridges.
• Sediments over the rocks are youngest near
  ridges.
• Sediments are thicker toward the continents.
• Seafloor spreading has been measured using lasers
  and moon-based reflectors.

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The Graffiti Conveyor Belt

• Construct a divergent boundary (rift zone).
• Use a cardboard box, coat hanger, corks, two
  pieces of adding machine tape, some spray paint
• Assign two persons to be tectons (Greek for
  builder) and let them move the plates.
• Assign two persons to record the polarity of the
  new rock being formed. 1 normal 1 reverse.
• One person will be the unpredictable, whimsical
  mother nature.

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Hot spots

• Evidence suggests that all the Hawaiian
  Islands the adjoining Emperor Seamounts were
  formed over the same hot spot in the mantle and
  were transported to their present positions by
  moving plates.
• Yellowstone is also over a hot spot and can be
  used as an example as well.
• image taken from http//volcano.und.edu/vwdocs/vw
  lessons/hot_spots/introduction.html

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Measuring the rate of plate movement

• Plates move about as fast as my fingernails
  grow?
• How can we measure plate movement?
• Use the age of the Hawaiian Islands and their
  distance from the hot spot to determine the rate
  of movement.
• The average growth rate for nails is 0.1 mm each
  day (or 1 centimeter in 100 days). The exact rate
  of nail growth depends on numerous factors
  including the age and sex of the individual and
  the time of year. Fingernails generally grow
  faster in young people, in males, and in the
  summer. Taken from www.medicinenet.com

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Measuring the rate of plate movement

• Use the following pages to find printable maps to
  use when determining the rate
• http//www.bsu.edu/csh/geology/dept/faculty/teri/e
  c2b.html
• http//volcano.und.edu/vwdocs/vwlessons/hot_spots/
  introduction.html

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Measuring the rate of plate movement

• Measure the distance between island, divide by
  the difference in years.
• Convert to cm/yr compare to the rate of growth
  for average fingernails.

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Movement will be 3.5 cm
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Paleo-movement
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Seismic evidence

• Most earthquakes have a shallow focus
• Deep-focus earthquakes occur only in a narrow
  zone along the margins of continents or island
  arcs
• These areas are termed Benioff Seismic Zones

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Benioff Seismic Zones

• Dip at an angle of about 45 degrees toward the
  continent
• Begins near the oceanic trench in the subduction
  zone
• Earthquake foci become progressively deeper away
  from the trench

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Deep Quakes
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Earthquakes

• Earth vibrations caused by a sudden release of
  energy beneath the surface, as a result of
  displacement along faults.

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• Stress Strain
• Elastic, Plastic
• Ductile or Brittle

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Earths Structure
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Elastic Rebound Theory
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Seismic

• Seismology-study of earthquakes
• Seismographs-instrument to record that shaking
• Seismogram-the record made by a seismograph
• Seismic waves-wave energy

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Seismographs
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Focus of an Earthquake
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Where are earthquakes occurring?

• Recent earthquakes http//earthquakes.usgs.gov/rec
  enteqsUS/ http//www.iris.edu/
• Seismic Hazard maps for the U.S.
  http//eqhazmaps.usgs.gov/
• Close to Home New Madrid Information
  http//www.hsv.com/genlintr/newmadrd/

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Frequency Distribution

• Shallow lt70km Intermediate between 70 300 km
  deep gt300km
• 90 are less than 100km deep, shallow are the
  most destructive
• 80 of all earthquakes along the ring of fire,
  only 5 intra-plate
• 150,000/year strong enough to be felt 900,000
  recorded by seismograph, but too small to
  catalogs

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P, S, L waves
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Slinky, Seismic Waves
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Shake, Rattle Roll!

• Using the supplied seismographs let the students
  create their own earthquake recording and then
  use the seismogram to locate an earthquake.

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Locating the Epicenter
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Intensity

• Intensity-subjective measure based on damage
  peoples reaction to the quake.
• Measure using the Modified Mercalli Intensity
  Scale
• other factors include distance from epicenter,
  focal depth, population density, local geology,
  duration, building types

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Modified Mercalli Intensity Scale, from FEMA

• I. People do not feel any Earth movement.
• II. A few people might notice movement if they
  are at rest and/or on the upper floors of tall
  buildings.
• III. Many people indoors feel movement. Hanging
  objects swing back and forth. People outdoors
  might not realize that an earthquake is
  occurring.
• IV. Most people indoors feel movement. Hanging
  objects swing. Dishes, windows, and doors rattle.
  The earthquake feels like a heavy truck hitting
  the walls. A few people outdoors may feel
  movement. Parked cars rock.
• V. Almost everyone feels movement. Sleeping
  people are awakened. Doors swing open or close.
  Dishes are broken. Pictures on the wall move.
  Small objects move or are turned over. Trees
  might shake. Liquids might spill out of open
  containers.
• VI. Everyone feels movement. People have trouble
  walking. Objects fall from shelves. Pictures fall
  off walls. Furniture moves. Plaster in walls
  might crack. Trees and bushes shake. Damage is
  slight in poorly built buildings. No structural
  damage.

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Modified Mercalli Intensity Scale, from FEMA

• VII. People have difficulty standing. Drivers
  feel their cars shaking. Some furniture breaks.
  Loose bricks fall from buildings. Damage is
  slight to moderate in well-built buildings
  considerable in poorly built buildings.
• VIII. Drivers have trouble steering. Houses that
  are not bolted down might shift on their
  foundations. Tall structures such as towers and
  chimneys might twist and fall. Well-built
  buildings suffer slight damage. Poorly built
  structures suffer severe damage. Tree branches
  break. Hillsides might crack if the ground is
  wet. Water levels in wells might change.
• IX. Well-built buildings suffer considerable
  damage. Houses that are not bolted down move off
  their foundations. Some underground pipes are
  broken. The ground cracks. Reservoirs suffer
  serious damage.
• X. Most buildings and their foundations are
  destroyed. Some bridges are destroyed. Dams are
  seriously damaged. Large landslides occur. Water
  is thrown on the banks of canals, rivers, lakes.
  The ground cracks in large areas. Railroad tracks
  are bent slightly.
• XI. Most buildings collapse. Some bridges are
  destroyed. Large cracks appear in the ground.
  Underground pipelines are destroyed. Railroad
  tracks are badly bent.
• XII. Almost everything is destroyed. Objects are
  thrown into the air. The ground moves in waves or
  ripples. Large amounts of rock may move.

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Wave travel
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Modified Mercalli Scale in TN

• Accounts of 1811-1812 New Madrid Earthquakes.
• http//www.hsv.com/genlintr/newmadrd/accnt3.htm

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Magnitude

• Quantitative measure of the total amount of
  energy released at its source using the Richter
  magnitude scale. Highest peak recorded
• Largest ever upper 8, gt9 possible, but not likely
  since rocks cant store that much energy
• Base 10 logarithmic scale that converts the
  largest recorded seismic wave to a numerical
  magnitude, (10 fold increase in magnitude). Each
  magnitude increase represents a 30 fold increase
  in the energy released

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Waves Material Changes
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How do we know the Earths outer core is liquid?
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Moho