sio15-17: lecture 4 plate tectonicsgabi/sio15/pdfs/lecture04-presentation.pdf• folders outside of...

SIO15-17: 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


Lithosphere

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

Asthenosphere

•  warm, soft, ductile

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


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


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)


•  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


a little bit like this

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

•  ductile mantle (video 4a)•  viscosity: resistance to flow (Lecture 2 notes)


•  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


slab pull ridge push

slab pull more dominant

slab pull


•  22 deaths Nicaragua/Costa Rica•  flooding in Honduras•  will strengthen to hurricane•  landfall on LA/MS Sat night•  10-20 in rainfall predicted•  LA declared state of emergency


http://www.nhc.noaa.gov/


•  Maria struck 9/20•  “16” deaths initially•  now 19/15 direct/indirect

unofficial numbers:(10/3)78 fatalities! $91 B damage(second-costliest)

15% have electricity andcell phone coveragelong lines for gaslong trips for food


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


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


•  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


Length of day does not change!


•  magnetic anomalies•  ocean deep drilling

Dietmar Mueller, SIO

no oceanic lith. older than 200 Mio yrs -> subduction zones

Fig. 4.14


•  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

sio15-17: lecture 4 plate tectonicsgabi/sio15/pdfs/lecture04-presentation.pdf• folders outside of...

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