Lecture Outlines Natural Disasters, 5th edition

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Plate Tectonics and Earthquakes
Natural Disasters, 5th edition, Chapter 3
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Plate Tectonics

1. The Discovery of Plate Tectonics
2. The Mosaic of Plates
3. Rates and History of Motion
4. Mantle Convection

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I. Discovery of Plate Tectonics

1. Continental Drift
2. Seafloor Spreading

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A. Continental Drift

1. Continental drift large-scale movements of
   continents

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A. Continental Drift (continued)

• Support
• Puzzle fit
• Suess (1900)-Gondwana
• Wegner (1915)-Pangea

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A. Continental Drift (continued)

1. Similar rock ages
2. Similar geologic structures
3. Fossil Evidence
4. Climate Evidence

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B. Seafloor Spreading

1. Convection currents move plates around
2. Mantle source
3. Post-WWII Mid-Atlantic Ridge
4. Hess Dietz (1960s) propose new and recycled
   seafloor

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II. The Mosaic of Plates

1. Lithospheric Plates
2. Divergent Boundaries
3. Convergent Boundaries
4. Transform-Fault Boundaries

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A. Lithospheric Plates

• Mosaic of large moving plates
• Geologic activities occur at plate boundaries
• Earthquakes
• Volcanoes
• Rifts
• Folding
• Faulting

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B. Divergent Boundaries

1. Narrow rifts
2. Continental plate separation
3. Oceanic plate separation-spreading centers

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Divergent Boundaries Oceanic Plate Separation
Mid- Atlantic Ridge
North American Plate
Eurasian Plate
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Divergent Boundaries Oceanic Plate Separation
Volcanoes and earthquakes concentrate.
Mid- Atlantic Ridge
North American Plate
Eurasian Plate
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Divergent Boundaries Continental Plate Separation
East African Rift Valley
Somali Subplate
African Plate
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Divergent Boundaries Continental Plate Separation
Parallel valleys volcanoes and earthquakes.
East African Rift Valley
Somali Subplate
African Plate
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C. Convergent Boundaries

1. Conservation of Earths surface area
2. Ocean-ocean convergence
3. Ocean-continent convergence
4. Continent-continent convergence

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Convergent Boundaries Ocean-Ocean Convergence
Mariana Islands
Marianas Trench
Philippine Plate
Pacific Plate
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Convergent Boundaries Ocean-Ocean Convergence
Deep-sea trench volcanic island arc.
Mariana Islands
Marianas Trench
Philippine Plate
Pacific Plate
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Convergent Boundaries Ocean-Continent Convergence
Andes Mountains
Peru-Chile Trench
South American Plate
Nazca Plate
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Convergent Boundaries Ocean-Continent Convergence
A volcanic belt of mountains forms.
Andes Mountains
Peru-Chile Trench
South American Plate
Nazca Plate
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Convergent Boundaries Continent-Continent
Convergence
Tibetan Plateau
Himalaya
Main thrust fault
Indian-Australian Plate
Eurasian Plate
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Convergent Boundaries Continent-Continent
Convergence
Crust crumbles, creating high mountains and a
wide plateau.
Tibetan Plateau
Himalaya
Main thrust fault
Indian-Australian Plate
Eurasian Plate
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D. Transform-Fault Boundaries

1. Plates slide past one another
2. Fracture with relative displacement

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Transform-Fault Boundaries Mid-Ocean Ridge
Transform Fault
Eurasian Plate
North American Plate
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Transform-Fault Boundaries Mid-Ocean Ridge
Transform Fault
Spreading centers offset.
Eurasian Plate
North American Plate
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Transform-Fault Boundaries Continental Transform
Fault
Pacific Plate
North American Plate
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Transform-Fault Boundaries Continental Transform
Fault
Offset continental crust.
Pacific Plate
North American Plate
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As plates move past each other...
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As plates move past each other...
San Francisco
creek beds are offset
San Andreas
fault
Los Angeles
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III. Rates and History of Motion

1. Seafloor Magnetic Tape Recorder
2. Geodesy

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A. Seafloor Magnetic Tape Recorder

• Magnetic reversals
• Switching strength to the south
• Preserved in lava
• Age can be dated
• Magnetic chrons- ½ million years
• Magnetic subchrons 200 000 years

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Magnetic anomalies also in volcanic lava.
Earths magnetic field reverses direction.
Normal
Layers remember.
Reversed
Older layers preserve their direction.
Scientists constructed a magnetic time scale.
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Subchrons
5.0 Ma
Present
4.0
3.0
2.0
1.0
Gilbert reversed chron
Gauss normal chron
Matuyama reversed chron
Brunhes normal chron
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Mid-ocean ridge
Million years ago (Ma)
4.0
3.0
2.0
Ocean crust today
5.0
3.3
2.5
0.7
0
0.7
2.5
3.3
5.0 million years old
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A. Seafloor Magnetic Tape Recorder (continued)

• Magnetic anomaly
• Normal-positive anomaly
• Reverse-negative anomaly
• Seafloor ages
• a. Speed distance / time

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Magnetic mapping can measure the rate of seafloor
spreading An oceanic survey over the Reykjanes
Ridge, part of the Mid-Atlantic Ridge southwest
of Iceland, showed an oscillating pattern of
magnetic field strength. This figure illustrates
how scientists worked out the explanation of this
pattern.
Mid-Atlantic Ridge
High intensity
Low intensity
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Magnetic mapping can measure the rate of seafloor
spreading An oceanic survey over the Reykjanes
Ridge, part of the Mid-Atlantic Ridge southwest
of Iceland, showed an oscillating pattern of
magnetic field strength. This figure illustrates
how scientists worked out the explanation of this
pattern.
A sensitive magnetometer records magnetic
anomalies,
Mid-Atlantic Ridge
High intensity
Low intensity
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Magnetic mapping can measure the rate of seafloor
spreading An oceanic survey over the Reykjanes
Ridge, part of the Mid-Atlantic Ridge southwest
of Iceland, showed an oscillating pattern of
magnetic field strength. This figure illustrates
how scientists worked out the explanation of this
pattern.
A sensitive magnetometer records magnetic
anomalies,
Iceland
Mid-Atlantic Ridge
Mid-Atlantic Ridge
High intensity
Low intensity
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Magnetic mapping can measure the rate of seafloor
spreading An oceanic survey over the Reykjanes
Ridge, part of the Mid-Atlantic Ridge southwest
of Iceland, showed an oscillating pattern of
magnetic field strength. This figure illustrates
how scientists worked out the explanation of this
pattern.
A sensitive magnetometer records magnetic
anomalies,
alternating bands of high and low magnetism.
Iceland
Mid-Atlantic Ridge
Mid-Atlantic Ridge
High intensity
Symmetrical bands on both sides. Why?
Low intensity
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B. Geodesy

• Astronomical Positioning
• Position with respect to fixed stars
• 100 m error
• Global Positioning
• 24 Earth-orbiting satellites
• Measure plate movement

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IV. Mantle Convection

1. Driving Forces
2. Plate Recycling
3. Convection Currents

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A. Driving Forces

1. Mantle convection
2. Gravitational pull

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B. Plate Recyling

1. New lithosphere-ridges
2. Old lithosphere-subduction
3. Recycling within upper and lower mantle-seismic
   waves

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Whole-mantle convection
Mantle
Upper mantle
Outer core
Inner core
700 km
Lower mantle
2900 km Outer core
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Whole-mantle convection
Upper mantle
700 km
Lower mantle
2900 km Outer core
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Stratified convection
Boundary near 700 km separates the two
convection systems.
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C. Convection Currents

• Movement of lithospheric plates
• Mantle plumes
• Hot spots
• Deep mantle

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Plate Tectonics

1. The Discovery of Plate Tectonics
2. The Mosaic of Plates
3. Rates and History of Motion
4. Mantle Convection