Powerpoint Presentation Physical Geology, 10e

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ISNS 4359 Earthquakes and Volcanoes (aka shake
and bake)
Lecture 4 Plate Tectonics
Fall 2005
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Plate Tectonics

• Tectonic cycle
• Melted asthenosphere flows upward as magma
• Cools to form new ocean floor (lithosphere)
• New oceanic lithosphere (slab) diverges from zone
  of formation atop asthenosphere (seafloor
  spreading)
• When slab of oceanic lithosphere collides with
  another slab, older, colder, denser slab subducts
  under younger, hotter, less dense slab
• Subducted slab is reabsorbed into the mantle
• Cycle takes as long as 250 million years, or more

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Plate Tectonics

• Lithosphere of Earth is broken into plates
• Study of movement and interaction of plates
• Plate Tectonics
• Zones of plate-edge interactions are responsible
  for most earthquakes, volcanoes and mountains
• Divergence zones
• Plates pull apart during seafloor spreading
• Transform faults
• Plates slide past one another
• Convergence zones
• Plates collide with one another

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Plate Tectonics
Lithosphere of Earth is broken into plates
separated by divergence zones, transform faults,
convergence zones
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Development of the Plate Tectonics Concept

• 1620 Francis Bacon noted parallelism of Atlantic
  coastlines of Africa and South America
• Late 1800s Eduard Suess suggests ancient
  supercontinent Gondwanaland (South America,
  Africa, Antarctica, Australia, India and New
  Zealand)
• 1915 Alfred Wegeners book supports theory of
  continental drift all the continents had once
  been supercontinent Pangaea, and had since
  drifted apart
• Theory of continental drift was rejected (well,
  largely so in the northern hemisphere, less so in
  southern) because mechanism for movement of
  continents could not, at the time, be visualized

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Development of the Plate Tectonics Concept

• 20th century study of ocean floors provided
  wealth of new data and breakthroughs in
  understanding
• Lithosphere moves laterally
• Continents are set within oceanic crust and ride
  along plates
• Theory of plate tectonics was developed and
  widely accepted

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Magnetization of Volcanic Rocks

• Magnetic patterns of ocean floor first observed
  in mid 20th century very important to theory of
  plate tectonics
• Why does the ocean floor have a magnetic pattern?
• When lava cools to below 550oC (Curie point),
  atoms in iron-bearing minerals line up in
  direction (polarity) of Earths magnetic field
• Polarity of Earths magnetic field can be either
  to the north or to the south and depends on time
  in Earths history

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Magnetization of Volcanic Rocks

• Successive lava flows stack up one on top of
  another, each lava flow recording the Earths
  polarity at the time at which it formed
• Each lava flow can also be dated using
  radioactive elements in the rock to give its age

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Magnetization of Volcanic Rocks

• Magnetic patterns of ocean floor
• What does magnetic polarity of lava flows tell
  us?
• Plotting the polarity of different lava flows
  against their ages gives us a record of the
  Earths polarity at different times in the past
• Timing of polarity reversals (north to south
  south to north) seems random
• Reversals probably caused by changes in the flow
  of iron-rich liquid in the Earths outer core

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Earths Magnetic Field

• Earths magnetic field acts like giant bar
  magnet, with north end near the North Pole and
  south end near the South Pole
• Magnetic field axis is now tilted 11o from
  vertical (tilt has varied with time) so that
  magnetic poles do not coincide with geographic
  poles (but are always near each other)
• Inclination of magnetic lines can also be used to
  determine at what latitude the rock formed
• Magnetic field is caused by dynamo in outer core
  
• Movements of iron-rich fluid create electric
  currents that generate magnetic field

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Magnetization Patterns on the Seafloors

• Atlantic Ocean floor is striped by parallel bands
  of magnetized rock with alternating polarities
• Stripes are parallel to mid ocean ridges, and
  pattern of stripes is symmetrical across mid
  ocean ridges (pattern on one side of ridge has
  mirror opposite on other side)
• Pattern of alternating polarity stripes is same
  as pattern of length of time between successive
  reversals of Earths magnetic field

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Magnetization Patterns on the Seafloors

• Magma is injected into the ocean ridges to cool
  and form new rock imprinted with the Earths
  magnetic field
• Seafloor is then pulled away from ocean ridge
  like two large conveyor belts going in opposite
  directions seafloor spreading

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Other Evidence of Plate Tectonics

• Earthquake epicenters outline plate boundaries
• Map of earthquake epicenters around the world
  shows not random pattern, but lines of earthquake
  activity that define the edges of the tectonic
  plates

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Other Evidence of Plate Tectonics

• Oceanic mountain ranges and deep trenches
• Ocean bottom is mostly about 3.7 km deep, with
  two areas of exception
• Continuous mountain ranges extend more than
  65,000 km along the ocean floors
• Volcanic mountains that form at spreading
  centers, where plates pull apart and magma rises
  to fill the gaps
• Narrow trenches extend to depths of more than 11
  km
• Tops of subducting plates turning downward to
  enter the mantle

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Other Evidence of Plate Tectonics

• Deep earthquakes
• Most earthquakes occur at depths less than 25 km
• Next to deep-ocean trenches, earthquakes occur
  along inclined planes to depths up to 700 km
• These earthquakes are occurring in subducting
  plates

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Other Evidence of Plate Tectonics

• Ages from ocean basins
• The oldest rocks on ocean floor are about 200
  million years old (less than 5 of Earths 4.5
  billion year age)
• Ocean basins are young features continually
  being formed (at mid ocean ridges) and destroyed
  (at subduction zones)
• Hot spots in the mantle cause volcanoes on the
  plate above, which form in a line as the plate
  moves over the hot spot in the mantle, getting
  older in the direction of plate movement
• Sediment on the seafloor is very thin at mid
  ocean ridges (where seafloor is very young) and
  thicker near trenches (where seafloor is oldest)

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Other Evidence of Plate Tectonics

• Systematic increases in seafloor depth
• Ocean floor depths increase systematically with
  seafloor age, moving away from the mid ocean
  ridges
• As oceanic crust gets older, it cools and becomes
  denser, therefore sinking a little lower into the
  mantle
• Weight of sediments on plate also cause it to
  sink a little into mantle

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Other Evidence of Plate Tectonics

• The Fit of the Continents
• If continents on either side of the Atlantic used
  to be adjacent, their outlines should match up
• Outlines of continents at the 1,800 m water depth
  line match up very well
• 1,800 m water depth line marks boundary between
  lower-density continental rocks and
  higher-density oceanic rocks

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Other Evidence of Plate Tectonics

• Changing Positions of the Continents
• 220 million years ago, supercontinent Pangaea
  covered 40 of Earth (60 was Panthalassa,
  massive ocean)

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Other Evidence of Plate Tectonics

• Changing Positions of the Continents
• 180 million years ago Pangaea had broken up into
  Laurasia and Gondwanaland
• 135 million years ago north Atlantic Ocean was
  opening India was moving toward Asia
• 65 million years ago south Atlantic Ocean was
  opening Africa and Europe had collided
• Present India has collided with Asia Eurasia
  and North America are separate Australia and
  Antarctica are far apart

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The Grand Unifying Theory

• Tectonic cycle
• Rising hot rock in the mantle melts and rises to
  surface as liquid magma
• Buildup of magma causes overlying lithosphere to
  uplift and fracture fractured lithosphere is
  then pulled outward and downward by gravity,
  aided by convection in mantle
• Asthenosphere melts and rises to fill fractures,
  creating new oceanic lithosphere
• New oceanic lithosphere becomes colder and denser
  as it gets older and farther from the ridge where
  it formed
• Eventually oceanic lithosphere collides with
  another plate whichever is colder and denser
  will be forced underneath and pulled back down
  into the mantle

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The Grand Unifying Theory
Tectonic cycle
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Plate Tectonics and Earthquakes

• Most earthquakes can be explained by plate
  tectonics
• Divergent plate boundaries
• Divergent motion and high temperatures cause
  rocks to fail easily in tension
• Earthquakes are small and generally
  non-threatening
• Transform plate boundaries
• Plates slide past each other in horizontal
  movement, retarded at irregularities in plate
  boundaries
• Energy required to move plates is released as
  large earthquakes
• Convergent plate boundaries
• Great amounts of energy are required to pull a
  plate back into the mantle or push continents
  together
• Largest earthquakes are generated at convergent
  boundaries

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Plate Tectonics and Earthquakes

• Examine example of Pacific plate
• Created at spreading centers on eastern and
  southern edges, producing small earthquakes
• Slides past other plates on transform faults
  (Queen Charlotte fault, Canada San Andreas
  fault, California Alpine fault, New Zealand),
  generating large earthquakes
• Subducts along northern and western edges,
  generating enormous earthquakes

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Spreading Centers and Earthquakes

• Iceland
• Volcanic island fed by hot spot along the
  mid-Atlantic ridge
• Swarms of moderate earthquakes too small to
  destroy buildings or kill people

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Spreading Centers and Earthquakes

• Red Sea and Gulf of Aden
• Young spreading center and new ocean basin
• Hot upper mantle under Africa melts and uplifts
  crust, which gravity then pulls apart and
  downward, creating pull-apart basins (or rift
  valleys)
• As down-dropped pull-apart basins widen, become
  flooded by ocean to form new arm
• At south end of Red Sea, three pull-apart basins
  meet at triple junction
• Spreading has split Arabian plate from Africa
• East African Rift Valley may someday split
  Somali plate from African plate

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Convergent Zones and Earthquakes

• Largest earthquakes
• Three types of convergence
• Ocean-ocean older, denser oceanic plate is
  subducted
• Ocean-continent oceanic plate is subducted
• Continent-continent both plates are too buoyant
  to be subducted continental upheaval results

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Subduction Zones

• Sites of great earthquakes
• Shallow earthquakes
• Compressive movements of overriding plate and
  subducting plate
• Pull-apart movements where subducting plate bends
  downward
• Most damaging earthquakes
• Inclined plane of deep earthquakes, defining
  descending slab of oceanic lithosphere
• Rigid interior of slab can stay cold enough to
  generate earthquakes down to depths of 700 km
• Most seismic energy is dissipated before reaching
  surface

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Seismic Gap Method

• If some segments of a fault have moved recently,
  it is reasonable to expect that unmoved portions
  will move next, to fill the gaps
• Yields expectations, not guarantees
• Segments may move in two or more earthquakes
  before adjacent unmoved segments move once

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Subduction Zones

• Tokyo, Japan, 1923 one of worlds most deadly
  disasters (probably about 144,000 people killed)
• Series of earthquakes, with principal one worst
  of year globally
• Tsunami 11 m high hit city
• Fires raced through city for 2½ days, destroying
  71 of Tokyo and all of Yokohama
• 38,000 people were killed by fire, crowded into a
  park that was consumed by fire from three sides

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Continent-Continent Collisions

• Collision of India into Asia
• India has moved 2,000 km north into Asia from
  initial contact
• Pre-collision, Indian and Asian crusts were 35 km
  thick
• Now crust under area of Tibetan plateau is 70 km
  thick and highest-standing continental area on
  Earth
• India continues to move 5 cm/year into Asia,
  along a 2,000 km front, affecting India,
  Pakistan, Afghanistan, Tibetan Plateau, eastern
  Russia, Mongolia and China with great
  earthquakes, and pushing parts of China to the
  east and southeastern Asia farther to the
  southeast

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Continent-Continent Collisions

• Shaanxi province, China, 1556
• Deadliest earthquake in history about 830,000
  people killed
• Soft soil made caves practical homes for many
• Shaking caused ground and caves to collapse
• Tangshan, China, 1976
• Deadliest earthquake in recent history more than
  240,000 people killed (though on the web,
  estimates are up to 655,000)
• Dense mining city of 2 million, with most
  buildings of mud-brick built under lenient
  building codes
• 93 of residential buildings collapsed

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Transform Faults and Earthquakes

• Horizontal movements cause major earthquakes
• Turkey, 1999
• Segment of North Anatolian fault ruptured for 120
  km in magnitude 7.4 earthquake near Izmit,
  followed weeks later by rupture to the east in
  magnitude 7.1 earthquake
• Residential buildings on soft ground, adding sand
  to concrete resulted in buildings collapsing
  during shaking

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Transform Faults and Earthquakes

• Turkey, 1999
• Turkey is pushed westward along the North
  Anatolian fault, which runs for 1,400 km along
  the Black Sea
• Since 1939, the North Anatolian fault has
  ruptured in 11 earthquakes, from east end of
  fault to west
• Unique, semi-regular pattern
• Next event? Probably to west of Izmit, closer to
  Istanbul
• Probably within next 30 years

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The Arabian Plate

• Continent-continent collision earthquakes
• Spreading in Red Sea and Gulf of Aden pushes
  Arabian plate into Eurasia, uplifting mountains
  and creating large earthquakes

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The Arabian Plate

• Continent-continent collision earthquakes
• Armenia, 1988
• Magnitude 6.9 earthquake followed minutes later
  by magnitude 5.9 aftershock
• 25,000 people killed, 31,000 injured and 500,000
  homeless
• Comparison of similar-sized earthquakes
• 1988 Armenian earthquake killed 25,000 of 700,000
  residents
• 1989 Loma Prieta earthquake killed 25 of 1.5
  million residents
• Earthquakes dont kill people, buildings do

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The Arabian Plate

• Transform fault earthquakes
• On western side, Arabian plate slides past
  African plate at edge of Mediterranean Sea along
  Dead Sea fault zone
• Runs through Holy Land
• Steps in fault zone have created pull-apart
  basins that hold Dead Sea, Sea of Galilee
• Dead Sea fault zone generates magnitude 6-7
  earthquakes about every 100-200 years