SIO15-17: Lecture 4 Plate Tectonics
SIO15-17: Lecture 4 Plate Tectonics
SIO15-17: Lecture 4 Plate Tectonics
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
SIO15-17: Lecture 4 Plate Tectonics
SIO15-17: Lecture 4 Plate Tectonics
SIO15-17: 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”
SIO15-17: Lecture 4 Plate Tectonics
Why do continents have roots?What keeps the lithosphere afloat?
SIO15-17: 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)
SIO15-17: 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
SIO15-17: 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)
SIO15-17: 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
SIO15-17: Lecture 4 Plate Tectonics
slab pull ridge push
slab pull more dominant
slab pull
SIO15-17: Lecture 4 Plate Tectonics
• 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
SIO15-17: Lecture 4 Plate Tectonics
http://www.nhc.noaa.gov/
SIO15-17: Lecture 4 Plate Tectonics
• 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
SIO15-17: 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
SIO15-17: 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
SIO15-17: 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
SIO15-17: Lecture 4 Plate Tectonics
Length of day does not change!
SIO15-17: 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
SIO15-17: 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