SIO15-18: Lecture 4 Plate Tectonics SIO15-18: Lecture 4 Plate - - PDF document

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SIO15-18: Lecture 4 Plate Tectonics SIO15-18: Lecture 4 Plate - - PDF document

SIO15-18: Lecture 4 Plate Tectonics SIO15-18: Lecture 4 Plate Tectonics https://igppweb.ucsd.edu/~gabi/sio15 SIO15-18: Lecture 4 Plate Tectonics Monday before class folders outside of lecture hall submit by first letter of


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SIO15-18: Lecture 4 Plate Tectonics SIO15-18: Lecture 4 Plate Tectonics

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SIO15-18: Lecture 4 Plate Tectonics

https://igppweb.ucsd.edu/~gabi/sio15

SIO15-18: Lecture 4 Plate Tectonics

  • Monday before class
  • folders outside of lecture hall
  • submit by first letter of FAMILY NAME
  • email submissions not accepted!
  • Q&A blog on class website!!

HOMEWORK PAGE

  • emailed questions on weekends may not

be answered

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SIO15-18: Lecture 4 Plate Tectonics SIO15-18: Lecture 4 Plate Tectonics

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SIO15-18: Lecture 4 Plate Tectonics SIO15-18: Lecture 4 Plate Tectonics

Lithosphere

  • cool and strong but brittle
  • zone of Eqs/volcanoes

Asthenosphere

  • warm, soft, ductile

Image: S. Marshak “Earth, Portrait of a Planet”

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SIO15-18: Lecture 4 Plate Tectonics

Why do continents have roots? What keeps the lithosphere afloat?

SIO15-18: Lecture 4 Plate Tectonics

2 counteracting forces

  • gravity
  • buoyancy

isostatic equilibrium

  • forces are balanced
  • body floats

Image: S. Marshak “Earth, Portrait of a Planet”

rigid lithosphere floats on soft asthenosphere asthenosphere reacts to imbalance (flows)

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SIO15-18: Lecture 4 Plate Tectonics

Which of the previously 9 planets in our solar system is now a dwarf planet?

POPQUIZ #1

YOUR FULL NAME AND STUDENT ID# WRITE DOWN ONLY 1 WORD!

SIO15-18: Lecture 4 Plate Tectonics

  • lithosphere is broken up

into 12 major plates

  • move about (a few cm per year)
  • driven by mantle convection
  • divergent
  • convergent
  • transform

Image: P. Abbott “Natural Disasters”

Mendocino Triple Junction

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SIO15-18: Lecture 4 Plate Tectonics

a little bit like this

Image: S. Marshak “Earth, Portrait of a Planet”

  • ductile mantle (video 4a)
  • viscosity: resistance to flow

(Lecture 2 notes)

SIO15-18: Lecture 4 Plate Tectonics

slab pull

ridge push

slab pull more dominant

slab pull

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SIO15-18: Lecture 4 Plate Tectonics

  • lith. formed at MORs
  • lith. consumed in subduction zones
  • same rate
  • plates move a few cm/yr
  • earthquakes
  • volcanism
  • mountain building and other features on Earth’s surface

SIO15-18: Lecture 4 Plate Tectonics

How do we know that plates move sideways?

  • Alfred Wegener, 1915
  • continents like jigsaw puzzle
  • fossil records match across oceans
  • geologic units match across oceans
  • apparent polar wander curves

don’t match

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SIO15-18: Lecture 4 Plate Tectonics

Shape: like that of bar magnet (dipole)

  • magnetic north 2000 at:

81.5ºN, 111.4ºW Origin: currents in metallic liquid outer core

  • > magnetic dynamo

Time Evolution: changing field; reversals Observables: strength, declination, inclination

SIO15-18: Lecture 4 Plate Tectonics

  • rock freezing from magma record current field
  • Earth’s field/rock position change over time

(record of magn. field)

Image: S. Marshak “Earth, Portrait of a Planet”

Curie Temperature (~ 500ºC)

above: domains align with mag. field below: domains frozen

Source: Wikipedia

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SIO15-18: Lecture 4 Plate Tectonics

  • freezing rock conserve current magnetic field
  • measure inclination and declination in layers of geol. profile
  • determine age of layers (fossils, radioactive dating)
  • for one location, construct polar wander curve

Image: S. Marshak “Earth, Portrait of a Planet”

SIO15-18: Lecture 4 Plate Tectonics

continental drift (Alfred Wegener 1915) sea floor observations (Harry Hess 1950s)

What Makes Continents Drift?

  • sea floor deepens away from mid-ocean ridges (MORs)
  • sediments thicken away from MORs
  • heat flow is greater at MORs than elsewhere
  • dredging (ocean rock different from continental rocks)
  • earthquakes along MORs -> seafloor cracking

map by Heezen and Tharp

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SIO15-18: Lecture 4 Plate Tectonics

  • mapping of magnetic anomalies (1960s)
  • ship tows magnetometer
  • records magnetic anomalies

(Earth field+magnetized rock)

  • new lava cools below Curie T
  • current magnetic N frozen

into rock

  • rock moves away from ridge
  • symmetric patterns
  • Fig. 4.13

SIO15-18: Lecture 4 Plate Tectonics

  • magnetic anomalies
  • ocean deep drilling

Dietmar Mueller, SIO

no oceanic lith. older than 200 Mio yrs

  • > subduction zones
  • Fig. 4.14
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SIO15-18: Lecture 4 Plate Tectonics

  • New lithosphere at MOR pushes plates apart
  • thickens, cools, gets dense
  • loses buoyancy
  • more likely to subduct
  • sinking slabs pull rest of lithosphere behind it