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Sea Floor Spreading

Sea Floor Spreading. By Diana L. Duckworth Rustburg High School Campbell County, VA. The action at divergent plate boundaries. Great Puzzles in 1960. Rock in oceans always basalt, younger than 225 million years old. Ocean crust thin; continental crust thick & granitic

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Sea Floor Spreading

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  1. Sea Floor Spreading By Diana L. Duckworth Rustburg High School Campbell County, VA The action at divergent plate boundaries.

  2. Great Puzzles in 1960 • Rock in oceans always basalt, younger than 225 million years old. • Ocean crust thin; continental crust thick & granitic • Rocks on continent varied, oldest 3.8 billion years old • Volcanoes & Earthquakes limited in locations • Atlantic & Pacific different topography from land and from each other

  3. 1963 – Harry Hess • Observed rocks near ridge younger than rocks at edge of ocean basin • Observed higher heat escaping from Earth’s interior at ridge crest (rift valley) than on sides • Ridge appeared to be under tension – like rift valley was two normal faults

  4. Proposed answer: Sea Floor Spreading • New sea floor created by insertion of lava in rift valley • Pushes ridge apart in center • Center is hot & expands; edges cold & contract, sinks Young Hot Old Cold Old Cold Magma source Mantle, mafic

  5. Vine & Matthews Test & Confirm • Hypothesis: • If rocks are being put into place in rift valley and cooling through Curie Point, then should see symmetrical magnetic polarity anomalies on either side of the ridge • Conclusion • Found symmetrical magnetic polarity anomalies on either side of the ridge

  6. http://www.moorlandschool.co.uk/earth/magnetism.htm

  7. http://www.usgs.gov/125/articles/plate_tectonics.html

  8. Calculate Spreading Rates • Age of point on sea floor • Distance from ridge • Simply divide distance by date & get spreading rate • Half rate – how fast crust is being formed • Double half rate – how fast continents are moving apart • Typical half rates – 2 to 4 cm/yr

  9. Colors denote age of rocks http://www.earth.northwestern.edu/people/seth/107/Ptdev/magstripes.htm

  10. Example • Suppose distance were 250 km to the 10 my reversal line • = 25,000,000 cm in 10,000,000 years • = 25/10 • = 2.5 cm/yr (half spreading rate)

  11. Summary of Evidence • Tensional earthquakes in rift valley – shallow, low magnitude, caused by upward movement of magma • High heat flow in rift valley; low on ridge flanks • Youngest rocks in ocean basin in rift valley; oldest on edges of ocean basin • Symmetrical magnetic reversal anomalies on either side of rift valley

  12. Transform Faults Not your typical strike slip fault!

  13. Map View – transform fault Rift Valley Off set Fracture Zone Fracture zone marked by change in elevation of sea floor. Rift Valley

  14. Process – Plates slide past each other in the offset zone only; necessary to spread on a sphere. Rift Valley Plate A Off set Fracture Zone Plate B Rift Valley Caused by sea floor spreading from offset rift zones.

  15. Evidence Compressional earthquakes in offset zone only. Rift Valley Younger rocks Older rocks Off set Younger rocks Older rocks Magnetic anomalies are offset Tensional earthquakes in rift zone. Offset is original break in crust; never gets wider. Rift Valley

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