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Powerpoint Presentation Physical Geology, 10e


At south end of Red Sea, three pull-apart basins meet at triple junction ... Deadliest earthquake in recent history: more than 240,000 people killed (though ... – PowerPoint PPT presentation

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Title: Powerpoint Presentation Physical Geology, 10e

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

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

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

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

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

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

Magnetization of Volcanic Rocks
  • Magnetic patterns of ocean floor
  • What does magnetic polarity of lava flows tell
  • 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

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

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

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

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

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
  • Tops of subducting plates turning downward to
    enter the mantle

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

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)

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

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

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)

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

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

The Grand Unifying Theory
Tectonic cycle
Plate Tectonics and Earthquakes
  • Most earthquakes can be explained by plate
  • Divergent plate boundaries
  • Divergent motion and high temperatures cause
    rocks to fail easily in tension
  • Earthquakes are small and generally
  • Transform plate boundaries
  • Plates slide past each other in horizontal
    movement, retarded at irregularities in plate
  • 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
  • Largest earthquakes are generated at convergent

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

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

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
  • 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

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

Subduction Zones
  • Sites of great earthquakes
  • Shallow earthquakes
  • Compressive movements of overriding plate and
    subducting plate
  • Pull-apart movements where subducting plate bends
  • 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

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

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

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
  • Now crust under area of Tibetan plateau is 70 km
    thick and highest-standing continental area on
  • 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

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

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

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
  • Probably within next 30 years

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

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
  • Comparison of similar-sized earthquakes
  • 1988 Armenian earthquake killed 25,000 of 700,000
  • 1989 Loma Prieta earthquake killed 25 of 1.5
    million residents
  • Earthquakes dont kill people, buildings do

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
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