1 the tectonic system a.internal structure of earth b.plates and plate boundaries c.evidence for...

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The tectonic systemA. Internal structure of Earth

B. Plates and plate boundaries

C. Evidence for movement of continents

D. The Earth’s magnetism

E. Earthquakes and the Earth’s interior

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

Divisions of the Earth by physical properties

~100 km

~250 km

2900 km

5200 km

6370 km

• Inner core - solid, metallic

• Outer core - liquid, metallic

• Mesosphere - solid rock

• Asthenosphere -rock, partly (5-25%) molten

• Lithosphere - solid rock

• Hydrosphere - liquid • Atmosphere - gas

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• Atmosphere• Hydrosphere• Crust• Mantle• Core

Divisions of the Earth by chemical composition

2900 km

6370 km

5-70 km

• Core - iron, nickel

• Mantle (oxygen, silicon, magnesium, iron)

• Crust (oxygen, silicon, aluminum, iron, magnesium)

• Hydrosphere - (Water: oxygen, hydrogen)• Atmosphere - (nitrogen, oxygen )

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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

1. Spreading centres

2. Subduction zones

3. Transform faults

•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

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Plates and plate boundaries

• Map of principal plates

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• Mid-Atlantic ridge


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Pillow lavas from the ocean floor

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• Iceland

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• 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

P&S 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

P&S 1.17

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3. Transform faults

Asthenosphere

Lit

hosp

here

Crust

Right-lateraltransform fault

Shallow earthquakes

• Right-lateral transform fault

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3. Transform faults

Asthenosphere

Lit

hosp

here

Crust

Left-lateraltransform fault

Shallow earthquakes

• Left-lateral transform fault

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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

a) The Earth's magnetic field

b) Remanent magnetization

c) Magnetic reversals and anomalies on the ocean floor

• 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.

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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

500 Ma

“Ma” in the diagrams signifies “Million years before present”

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• Pole appears to have wandered through time

600 Ma500

400

270

225100

0 Ma


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• Pole appears to have wandered through time

• Apparent polar wander path (APWP)

• Hence either the pole moved or the continent moved

600 Ma500

400

270

225100

0 Ma


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• Different continents show different APWPs

• Hence it must be the continents that moved

0 MaNorth America

Europe


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• 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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• 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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• 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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• Vine-Matthews hypothesis• Magnetic anomalies result from remanent magnetism

acquired during spreading of ocean-floor while magnetic reversals occurred.

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• 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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• Effect of earthquake in JapanP&S 18.18

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Seismograph

• Seismographs & seismometers

Ancient seismic detector

Seismometer

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• Intensity: Strength of ground shaking at a point.

• Intensity depends on many factors e.g. distance from the focus.

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• 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

√1000. • E.g., M 8 releases 1000 times more energy

than M 6.

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P&S 18.11

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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 (>10 m)

– Devastate coastal communities

Effect of 1929 tsunami on Burin Peninsula, Newfoundland

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• Most earthquakes originate < 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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• Distance of focus is found from interval between P and S arrival

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Example• Station A

1500 km• Station B

5600 km• Station C

8600 km

5600 km8600 km

1500 km

A

B C

Epicentre

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• Epicentre is point on Earth’s surface directly above focus• Map of epicentres: Earthquakes are concentrated at plate boundaries

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• All deep (> 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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• Body waves tell us about Earth's interior – Reflection– Refraction

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• S-waves cannot pass through liquid

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• Evidence for core

• P waves from major earthquake

2468

10

12

20

105°

142°

Focus

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• Evidence for core:

• S waves from major earthquake

71014

17

20

105°

Focus

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• S-waves are blocked by liquid core

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• 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 Earth’s 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 Earth’s 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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1 the tectonic system a.internal structure of earth b.plates and plate boundaries c.evidence for...

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