Plate Tectonics and Earthquakes

Natural Disasters, 5th edition, Chapter 3

Plate Tectonics

I. The Discovery of Plate Tectonics

II. The Mosaic of Plates

III. Rates and History of Motion

IV. Mantle Convection

I. Discovery of Plate Tectonics

A. Continental Drift

B. Seafloor Spreading

A. Continental Drift

1. Continental drift: large-scale movements of continents

A. Continental Drift (continued)

2. Supporta. Puzzle fit

1. Suess (1900)-Gondwana

2. Wegner (1915)-Pangea

A. Continental Drift (continued)

b. Similar rock ages

c. Similar geologic structures

d. Fossil Evidence

e. Climate Evidence

B. Seafloor Spreading

1. Convection currents move plates around

2. Mantle source3. Post-WWII: Mid-

Atlantic Ridge4. Hess & Dietz

(1960’s) propose new and recycled seafloor

II. The Mosaic of Plates

A. Lithospheric Plates

B. Divergent Boundaries

C. Convergent Boundaries

D. Transform-Fault Boundaries

A. Lithospheric Plates

1. Mosaic of large moving plates

2. Geologic activities occur at plate boundaries

a. Earthquakesb. Volcanoesc. Riftsd. Foldinge. Faulting

B. Divergent Boundaries

1. Narrow rifts2. Continental plate

separation

3. Oceanic plate separation-spreading centers

Divergent BoundariesOceanic Plate Separation

Mid-AtlanticRidge

North Am

erican

PlateNort

h Americ

an

Plate

EurasianPlate

EurasianPlate

Divergent BoundariesOceanic Plate Separation

Mid-AtlanticRidge

North Am

erican

PlateNort

h Americ

an

Plate

EurasianPlate

EurasianPlate

Volcanoes and earthquakes concentrate.

Divergent BoundariesContinental Plate Separation

East AfricanRift Valley

Somali Subplate

Somali SubplateAfric

an Plate

African Pl

ate

Divergent BoundariesContinental Plate Separation

East AfricanRift Valley

Somali Subplate

Somali SubplateAfric

an Plate

African Pl

ate

Parallel valleys; volcanoes and earthquakes.

C. Convergent Boundaries

1. Conservation of Earth’s surface area

2. Ocean-ocean convergence

3. Ocean-continent convergence

4. Continent-continent convergence

Convergent BoundariesOcean-Ocean Convergence

Mariana Islands Marianas Trench

Pacific PlatePacific PlatePhili

ppine

PlatePhili

ppine

Plate

Convergent BoundariesOcean-Ocean Convergence

Mariana Islands Marianas Trench

Pacific PlatePacific PlatePhili

ppine

PlatePhili

ppine

Plate

Deep-sea trench; volcanic island arc.

Convergent BoundariesOcean-Continent Convergence

Nazca Plate

Nazca Plate

AndesMountains

SouthAmericanPlate

SouthAmericanPlate

Peru-Chile Trench

Convergent BoundariesOcean-Continent Convergence

Nazca Plate

Nazca Plate

AndesMountains

SouthAmericanPlate

SouthAmericanPlate

Peru-Chile Trench

A volcanic belt ofmountains forms.

Convergent BoundariesContinent-Continent Convergence

Himalaya

Mainthrustfault

TibetanPlateau

Indian-Australian Plat

eIndian-Aust

ralian Plate

EurasianPlateEurasianPlate

Convergent BoundariesContinent-Continent Convergence

Himalaya

Mainthrustfault

TibetanPlateau

Indian-Australian Plat

eIndian-Aust

ralian Plate

EurasianPlateEurasianPlate

Crust crumbles, creating highmountains and a wide plateau.

D. Transform-Fault Boundaries

1. Plates slide past one another

2. Fracture with relative displacement

Transform-Fault BoundariesMid-Ocean Ridge Transform Fault

North American P

lateNorth Am

erican Plate

Eurasian Plate

Transform-Fault BoundariesMid-Ocean Ridge Transform Fault

North American P

lateNorth Am

erican Plate

Eurasian Plate

Spreading centers offset.

Transform-Fault BoundariesContinental Transform Fault

North American Plate

North American PlatePacific

PlatePacific

Plate

Transform-Fault BoundariesContinental Transform Fault

North American Plate

North American PlatePacific

PlatePacific

Plate

Offset continental crust.

As platesmove past each other...

As platesmove past each other...

…creek beds are offset

As platesmove past each other...

…creek beds are offset

SanFrancisco

Los Angeles

San Andreas fault

III. Rates and History of Motion

A. Seafloor Magnetic Tape Recorder

B. Geodesy

A. Seafloor Magnetic Tape Recorder

1. Magnetic reversalsa. Switching strength to

the south

b. Preserved in lava

c. Age can be dated

d. Magnetic chrons- ½ million years

e. Magnetic subchrons – 200 000 years

Magnetic anomalies also in volcanic lava.

Magnetic anomalies also in volcanic lava.

Normal

Reversed

Magnetic anomalies also in volcanic lava.

Normal

Reversed

Earth’s magnetic fieldreverses direction.

Layers “remember”.

Older layers preservetheir direction.

Scientists constructed a magnetic time scale.

Gilbertreversed chron

Gaussnormal chron

Matuyamareversed chron

Brunhesnormal chron

5.0 Ma Present4.0 3.0 2.0 1.0

Subchrons

Mid-ocean ridge

4.0

3.0

2.0

Oceancrust today

Million years ago (Ma)

5.0

million

years ol

d

3.32.5

0.70 0.7

2.53.3

5.0

A. Seafloor Magnetic Tape Recorder (continued)

2. Magnetic anomalya. Normal-positive

anomaly

b. Reverse-negative anomaly

3. Seafloor agesa. Speed = distance /

time

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 RidgeMid-Atlantic RidgeHigh

intensity

Low intensity

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 RidgeMid-Atlantic RidgeHigh

intensity

Low intensity

A sensitive magnetometerrecords magnetic anomalies,…

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 RidgeMid-Atlantic RidgeHigh

intensity

Low intensity

A sensitive magnetometerrecords magnetic anomalies,…

Iceland

Mid-AtlanticRidge

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 RidgeMid-Atlantic RidgeHigh

intensity

Low intensity

A sensitive magnetometerrecords magnetic anomalies,…

Iceland

Mid-AtlanticRidge

…alternating bands of highand low magnetism.

Symmetrical bands on both sides. Why?

B. Geodesy

1. Astronomical Positioning

a. Position with respect to fixed stars

b. 100 m error

2. Global Positioninga. 24 Earth-orbiting

satellitesb. Measure plate

movement

IV. Mantle Convection

A. Driving Forces

B. Plate Recycling

C. Convection Currents

A. Driving Forces

1. Mantle convection

2. Gravitational pull

B. Plate Recyling

1. New lithosphere-ridges

2. Old lithosphere-subduction

3. Recycling within upper and lower mantle-seismic waves

Whole-mantle convection

Uppermantle

Lowermantle

700 km

2900 kmOuter core

Mantle

Outer core

Inner core

Whole-mantle convection

Uppermantle

Lowermantle

700 km

2900 kmOuter core

Stratified convection

Boundary near700 km separatesthe two convectionsystems.

C. Convection Currents

1. Movement of lithospheric plates

2. Mantle plumesa. Hot spots

b. Deep mantle

Plate Tectonics

I. The Discovery of Plate Tectonics

II. The Mosaic of Plates

III. Rates and History of Motion

IV. Mantle Convection