Plate Tectonics

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Plate Tectonics
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I. Continental Drift

• Suggested by Alfred Wegener
• Hypothesis-- Continents had once been one large
  land mass that had separated and moved apart.
• Called this supercontinent Pangea, meaning all
  Earth

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• A. Evidence for Continental Drift
• Shape- continent shapes fit together like a
  puzzle.
• Fossils- same species on separate continents.

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Rock Evidence - rock types on edges of
continents match.
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Glacial Features Evidence - best explained by
joined continents
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Glacial Features Evidence
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Climate Evidence

• Tropical salt bed deposits found in pole
  regions, glacial features in tropics, etc.

The fossilized coral reef in Isle Lamotte,
Vermont, shows that the area was once in the
tropics.
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Evidence for Continental Drift

• Shape- continent shapes fit together like a
  puzzle
• Fossils- same species on separate continents
• Rocks- rock types on edges of continents match
• Glacial Features- best explained by joined
  continents
• Climate- tropical salt bed deposits found in pole
  regions, glacial features in tropics

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Trouble With Continental Drift
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Holmes Solution
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B. New Evidence from Ocean Floor

• Wegeners theory rejected by geologists
• Revived by evidence from study of magnetic
  anomalies on ocean floor on either side of
  Mid-Atlantic Ridge
• Modern theory explains movement of the continents
  and changes in the Earths crust

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Map of Ocean Ridges
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Magnetic Record on Ocean Floor
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II. Modern Theory of Plate Tectonics

• Plates- rigid blocks of Earths crust and upper
  mantle (lithosphere) move over the surface
  spreading apart and bumping into each other
• 2. Powered by convection currents in nearly
  molten layer of mantle (asthenosphere)

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Major Tectonic Plates
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III. Plate Boundaries
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A. Diverging Boundaries

• where 2 plates are pulling apart by tension
  forces
• mid ocean ridge has central crack called a rift
  zone
• at times the ridge opens to release basaltic
  magma forming new oceanic crust
• moves 2.5 cm per year
• quiet volcanic activity

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Ocean Ridges and Rifts are associated with
diverging boundaries
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The Sinai Peninsula is a rift zone
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B. Transforming Boundaries

1. boundaries at which plates move past one another
   in opposite directions or in same direction but
   at different rates
2. high area for earthquakes but low volcanic
   activity
3. San Andreas Fault is example

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Transformation-- plates moving beside each other
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Earthquakes are common along transform boundaries
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Earthquakes are common along transform
boundaries.
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C. Converging Boundaries

• Plates move toward each other from compressive
  forces
• subduction is one plate diving beneath another
• collision of two continental plates can produce
  mountains (Himalayas)

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Convergence -- two plates moving towards each
other
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Collisions- Mountains can be formed when
continental plates converge
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Subduction

• denser plate dives under another
• descending plate melts deep sea trenches on the
  ocean floor
• melting magma is less dense, rises forming a
  chain of volcanoes

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Volcanoes and earthquakes are associated with
subduction.
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Deformation of the Crust
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Forces that cause rocks to break

• a. tension- stretching or pulling apart
• b. compression- forces that push against a body
  from opposite sides and squeezes it into folds
• c. shearing- forces that push against a body from
  different sides, producing twisting and tearing

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Folds

• bends in rock layers caused by compression
• form alternating ridges (anticlines) or troughs
  (synclines)

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Anticline

• Ante up
• Folds Upward

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Syncline

• Syn Sink
• Bends downward

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Monocline

• Monoclines
• Simple fold with only one bend

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Folded Mountains

• Formed from compression forces.
• Appalachian Mountains (Green Mts. are part of
  these)
• Alps

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Fractures

• breaks in rocks
• called faults when movement
• occurs along
• break.
• energy
• released as seismic waves.

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Normal Fault

• Normal Fault hanging wall is below the foot
  wall.
• Caused by
• tension.

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Reverse Fault

• Reverse Fault Hanging wall is above the foot
  wall.
• Caused by compression.

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Thrust Fault

• Type of reverse fault
• Almost horizontal (normal reverse faults are more
  vertical)
• Mixes Up the order of the layer older rocks
  above younger rocks

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Lateral Faults

• Shearing causes side to side movement
• Example is the San Andreas Fault

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Fault Block Mountains

• A series of normal faults in one area causes a
  series of mountains and valleys known as fault
  block mountains.

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Plutonic Mountains

• Plutonic mountain are formed from vertical forces
  pushing magma upward against overlying rock.
• Plutons molten rock built up under the surface
  that causes the surface to bulge

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Batholiths

• Batholiths are plutons exposed by erosion.
• Cadillac Mountain, Acadia National Park, ME
• Half Dome, Yosemite National Park, CA

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Floating Crust

• Isotasy balance of two forces
• Top Force mass of crust and gravity pushing
  crust into the mantle
• Bottom force crust is less dense than mantle and
  buoyancy pushes crust up

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How does isostasy affect Mountain Ranges?

• Mountains contain more mass
• Result is they sink deeper into the mantle
• Erosion reduces the mass of the mountains
• Buoyancy takes over and pushes mountain back up

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Volcanoes
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Types of Volcanoes
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• Shield Volcanoes

• Low, dome-shaped
• mostly basalt
• quiet flowing
• Hawaiian Islands, Iceland are examples

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Shield Volcano Haleakala, Hawaii
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Cinder Cones

• Violent eruptions blow out bits of lava that
  harden in the air.
• Accumulated cinders form cones.
• Gas-rich basaltic lava.

• Small, short-lived

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Puu Lilinue on Mauna Kea in Hawaii.
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Strato- or Composite Volcanoes

• Cone-shaped structures made of alternating layers
  of lava and ash.

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Mt. Hood, Oregon
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Composite Cone Mount Shasta, California
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Mt. St. Helens, Oregon
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Calderas

• Large depression in volcano
• Formed from
• violent eruption of composite volcano or
• collapse of shield volcano after magma chamber
  empties

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Domes

• small with steep sides and dome-shaped mass
  within crater
• formed from slower moving, more viscous lava

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Island-Arc Volcanoes

• trenches with new rocks being formed from quiet
  flowing basaltic lava
• island arcs formed at subduction zones

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Hot spots

• areas in mantle where radioactivity causes more
  heat that melts rocks as plate moves over
• cause volcanic island chains such as Hawaii
• also cause of Yellowstone thermal activity

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The Hawaiian Islands were formed over a hot spot
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Factors Affecting Eruptions

• Lava Viscosity
• resistance to flow
• High temp. more viscous
• Increases with silica content.
• Rhyolitic more silica, less fluid, more violent
  flow.
• Basaltic less silica, more fluid, quieter flow.

• Dissolved Gases
• Mostly water vapor CO2
• Held in magma by pressure, released as pressure
  decreases
• Provides force to eject material, sometimes
  violently.
• More gas more violent eruption.

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Environmental Hazards of Volcanoes

• Pollution
• Lava Flows
• Falling Ejecta
• Ash Falls
• (Building Collapse,
• Crop Destruction)

• Mudflows
• Direct Damage
• Floods
• Blast
• Pyroclastic Flow
• Toxic Gas

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Pyroclastic Flow or Nuee-Ardente (French Fiery
Cloud)
http//www.youtube.com/watch?vXLmbnnh5OLs

• Associated w/ Composite Cones
• Hot gases, glowing ash, rock fragments
• Race downhill _at_ 200 km/hr

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Earthquakes and Volcanoes
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Earthquakes
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Epicenter and Focus

• Focus-
• actual point on the fault where movement occurs
  and vibrations begin
• usually deep beneath the surface

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• 2. Epicenter
• where the effects of the seismic activity are
  first felt on the surface directly above the
  focus
• point of greatest damage

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Measuring Earthquakes

• Intensity- the damage an earthquake causes at the
  surface -can be either physical or geological-
  measured with Mercalli scale
• Magnitude- the strength of an earthquake from the
  seismograms- Richter scale
• Seismographs- instruments that record tremors
  traveling through the Earth

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Seismograph
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Richter Magnitudes Effects  lt 3.5 Generally
not felt, but recorded.  3.5-5.4 Often
felt, but rarely causes damage.  lt6.0 At
most slight damage to well-designed
buildings 6.1-6.9 Can be destructive
in areas up to about 100 kilometers across
where people live.  7.0-7.9 Major
earthquake. Can cause serious damage over
larger areas.  gt8 Great earthquake. Can cause
serious damage in areas several hundred
kilometers across.  
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Earthquake Waves

• P-waves- primary or "pressure waves-travel
  forward- the 1st vibrations felt- compression
  waves

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S-waves- secondary or "shaking waves- move
forward but vibrate at right angles to the
direction of movement- shear waves
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Locating an Epicenter

• Distance from seismic stations is calculated by
  difference in arrival times of P-waves and
  S-waves
• Epicenter can be located by triangulating data
  from at least three stations

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Earthquakes and Plate Tectonics

• Most earthquakes (and volcanoes) occur along
  plate boundaries

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A Week of Earthquakes
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How Do We Know Theres a Core?
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When waves strike the interface between two
substance of different densities, they can be
reflected or refracted.
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The End