The tectonic system

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The tectonic system

1. Internal structure of Earth
2. Plates and plate boundaries
3. Evidence for movement of continents
4. The Earths magnetism
5. Earthquakes and the Earths interior
6. Direct measurement of plate motion

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A. Internal structure of the Earth

1. By physical properties
2. By chemical composition

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Divisions of the Earth by physical properties

• Atmosphere
• Hydrosphere
• Lithosphere
• Asthenosphere
• Mesosphere
• Outer core
• Inner core

100 km
250 km

• Mesosphere - solid rock

2900 km

• Outer core - liquid, metallic

5200 km

• Inner core - solid, metallic

6370 km
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Divisions of the Earth by chemical composition

• Atmosphere
• Hydrosphere
• Crust
• Mantle
• Core

5-70 km

• Mantle (oxygen, silicon, magnesium, iron)

2900 km

• Core - iron, nickel

6370 km
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Divisions of the Earth by chemical composition

• Continental crust less dense rock with more
  silicon, aluminum
• Oceanic crust more dense rock with more iron,
  magnesium

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B. Plates and plate boundaries

• Lithosphere is divided into plates.
• Plates are in relative motion at speeds of a few
  cm per year
• There are 3 types of plate boundary

1. Spreading centres
2. Subduction zones
3. Transform faults

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Plates and plate boundaries

• Map of principal plates

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1. Spreading centres

• Mid-Atlantic ridge

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1. Spreading centres

• Pillow lavas from the ocean floor

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1. Spreading centres

• Iceland

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1. Spreading centres

• Cross-section of a spreading centre

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1. Spreading centres summary

• Occur beneath the oceans
• Marked by a mid-ocean ridge several thousand km
  wide, rising 2 or 3 km above surrounding ocean
  floor
• Site of submarine volcanoes and earthquake
  activity
• New lithosphere formed by ocean-floor spreading
• Plates move apart (a few centimetres per year)

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2. Subduction zones

• Deep trenches around the Pacific Ocean

PS 1.12
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2. Subduction zones

• Subduction zone volcanoes (Mount St. Helens -
  before)

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2. Subduction zones

• Subduction zone volcanoes (Mount St. Helens
  after)

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2. Subduction zones

• Where subduction occurs close to a continental
  margin, there is often a mountain belt (orogen)
• Rocks within orogen are crumpled (deformed)

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2. Subduction zones summary

• Subduction zone or convergent plate boundary
• Deep ocean trench (up to 11 km deep)
• Benioff zone of deep earthquakes
• Melting in mantle produces magma
• Volcanic arc
• One plate moves under another (a few centimetres
  per year)
• Orogens (mountain belts) form where subduction
  zones affect continental crust

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3. Transform faults

• Transform faults (transcurrent plate boundaries)

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3. Transform faults

• San Andreas Fault

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3. Transform faults

• Dextral or right-lateral transform fault

PS 1.17
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3. Transform faults

• Right-lateral transform fault

Asthenosphere
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3. Transform faults

• Left-lateral transform fault

Asthenosphere
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3. Transform faults

• Many small transform faults occur along the
  mid-ocean ridges
• Larger transform faults cut continental crust
• Many shallow earthquakes

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C. Evidence for moving continents

• Common sense tells us the Earth is solid
• Until 1960 most scientists also believed
  continents remained fixed
• Lines of evidence supporting moving plates
• Match of geologic structures
• Fossils
• Glaciation and climate
• Paleomagnetism
• Match of continent outlines
• Seismicity
• Direct measurement of plate movement by GPS

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1. Match of continent outlines
Some continents show 'jig-saw' fit
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2. Match of rock units between continents
Very similar rock units are now separated by
oceans
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3. Fossil evidence
Fossils of very similar land animals and plants
are now separated by oceans
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4. Glaciation and climate

• Locations of ice sheets at 350-300 Ma - no sense
  on modern map
• Can be explained if "Gondwanaland" is reassembled

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D. Paleomagnetism

• Before 60's most geophysicists claimed that Earth
  was too rigid to allow continental drift.
• But first measurements of movement came
  geophysics studies of Earth's magnetism.

1. The Earth's magnetic field
2. Remanent magnetization
3. Magnetic reversals and anomalies on the ocean
   floor

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1. Earth's magnetic field

• Earth behaves approximately as if there is a bar
  magnet in the core

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1. Earth's magnetic field

• Field at any place has an inclination (steepness)
  and a declination (direction)
• Inclination indicates distance from pole
• Declination indicates direction to pole

Inclination angle
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2. Remanent magnetism

• Some ancient rocks were (weakly) magnetized when
  formed - "Remanent magnetism"
• "Fossil compass needles"
• If age of rocks is known, remanent magnetism
  indicates the ancient location of the pole

Ma in the diagrams signifies Million years
before present
500 Ma
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2. Remanent magnetism

• Some ancient rocks were (weakly) magnetized when
  formed - "Remanent magnetism"
• "Fossil compass needles"
• If age of rocks is known, remanent magnetism
  indicates the ancient location of the pole
• Pole appears to have wandered through time

Ma in the diagrams signifies Million years
before present
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2. Remanent magnetism

• Some ancient rocks were (weakly) magnetized when
  formed - "Remanent magnetism"
• "Fossil compass needles"
• If age of rocks is known, remanent magnetism
  indicates the ancient location of the pole
• Pole appears to have wandered through time
• Apparent polar wander path (APWP)
• Hence either the pole moved or the continent moved

Ma in the diagrams signifies Million years
before present
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2. Remanent magnetism
Ma in the diagrams signifies Million years
before present

• Different continents show different APWPs
• Hence it must be the continents that moved

North America
Europe
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2. Remanent magnetism

• Other changes are recorded by remanent magnetism
• N. and S. magnetic poles appear to have
  "flipped' through time

Volcano showing magnetized lava flows
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2. Remanent magnetism

• N. and S. magnetic poles appear to have
  "flipped' through time

Gary A Glatzmaier University of California,
Santa Cruz www.es.ucsc.edu/glatz
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2. Remanent magnetism

• Time scale of magnetic reversals

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3. Reversals and ocean-floor anomalies

• Magnetic anomaly
• field slightly stronger or weaker than normal
• Surveys in the oceans show
• Central positive anomaly
• Symmetric pattern

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3. Reversals and ocean-floor anomalies

• Vine-Matthews hypothesis
• Magnetic anomalies result from remanent magnetism
  acquired during spreading of ocean-floor while
  magnetic reversals occurred.

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3. Reversals and ocean-floor anomalies

• Match with reversal history
• Measure rates
• Map age of ocean floor
• New ocean floor is found along mid-ocean ridges

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E. Earthquakes and seismicity

1. Intensity and magnitude
2. Seismic waves
3. Origin of earthquakes
4. Locating earthquakes
5. Earthquakes at plate boundaries
6. Interior of the Earth

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1. Intensity and magnitude
PS 18.18

• Effect of earthquake in Japan

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1. Intensity and magnitude
Ancient seismic detector

• Seismographs seismometers

• Seismograph

Seismometer
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1. Intensity and magnitude

• Intensity Strength of ground shaking at a point.
• Intensity depends on many factors e.g. distance
  from the focus.

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1. Intensity and magnitude

• Magnitude a measure of total energy released
• Charles Richter
• Ground movement at standardized distance
• Log scale
• Modern scale based on Richter's
• Each step on scale multiplies energy by v1000.
• E.g., M 8 releases 1000 times more energy than M
  6.

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1. Intensity and magnitude
PS 18.11
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1. Intensity and magnitude

• http//wwwneic.cr.usgs.gov/neis/bulletin/

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2. Seismic waves

• Body waves and surface waves

Epicentre
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2. Seismic waves

• Body waves Primary or P-waves
• 3-7 km/s in the crust
• Similar to sound waves
• Compression and expansion ('dilation')
• Vibration direction parallel to propagation
• Pass through solid, liquid or gas.

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2. Seismic waves

• Body waves Secondary or S-waves
• 1.5- 5 km/s in the crust
• Shear waves
• Vibration direction perpendicular to propagation
• Solids only

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2. Seismic waves

• Surface waves on land
• Surface waves form when body waves reach the
  surface
• Slower but larger than body waves
• Cause most damage

Rayleigh waves
Love waves
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2. Seismic waves

• Tsunami surface waves on ocean
• Low on open ocean ( 1 m)
• 600 km/hr
• In shallow water, slow down, get higher (gt10 m)
• Devastate coastal communities

Effect of 1929 tsunami on Burin Peninsula,
Newfoundland
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3. Origin of earthquakes

• Most earthquakes originate lt 70 km deep.
• Result from
• Elastic strain
• Brittle fracture (or brittle failure).
• These processes occur in cold rocks, typically
  near Earth's surface

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3. Origin of earthquakes
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4. Locating earthquakes

• Distance of focus is found from interval between
  P and S arrival

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4. Locating earthquakes

• Example
• Station A 1500 km
• Station B 5600 km
• Station C 8600 km

B
C
A
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5. Earthquakes at plate boundaries

• Epicentre is point on Earths surface directly
  above focus
• Map of epicentres Earthquakes are concentrated
  at plate boundaries

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5. Earthquakes at plate boundaries

• All deep (gt 100 km) events are at subduction
  zones.
• Why?
• Only cold rocks display brittle fracture
• In Benioff zone cold rocks are found deep.

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6. Interior of the Earth

• Body waves tell us about Earth's interior
• Reflection
• Refraction

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6. Interior of the Earth

• S-waves cannot pass through liquid

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6. Interior of the Earth
Focus

• Evidence for core
• P waves from major earthquake

142
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6. Interior of the Earth
Focus

• Evidence for core
• S waves from major earthquake

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6. Interior of the Earth

• S-waves are blocked by liquid core

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6. Interior of the Earth

• P-waves are refracted by core
• Acts as lens

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Seismicity summary

• Earthquakes are a major hazard when located close
  to population centres
• Intensity amount of shaking at a point
• Magnitude total energy released at focus
• Seismic waves
• Surface waves
• L-waves, R-waves, Tsunami waves
• Body waves
• P-waves, S-waves
• Arrivals of P and S waves at locations distant
  from the epicentre can be used
• To locate earthquakes
• To recognize plate boundaries
• To identify major features of Earths interior

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F. Direct measurement of plate movement

• Global Positioning System (GPS) is a network of
  satellites, used to provide very accurate
  locations on Earths surface
• By reoccupying sites over a period of years it is
  possible to measure plate movement directly

http//www.unavco.org/pubs_reports/brochures/1998_
UNAVCO/1998_UNAVCO.html
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F. Direct measurement of plate movement

• Global Positioning System (GPS) is a network of
  satellites, used to provide very accurate
  locations on Earths surface
• By reoccupying sites over a period of years it is
  possible to measure plate movement directly

http//www.unavco.org/pubs_reports/brochures/1998_
UNAVCO/1998_UNAVCO.html
69
F. Direct measurement of plate movement

• Global Positioning System (GPS) is a network of
  satellites, used to provide very accurate
  locations on Earths surface
• By reoccupying sites over a period of years it is
  possible to measure plate movement directly

http//www.unavco.org/pubs_reports/brochures/1998_
UNAVCO/1998_UNAVCO.html