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

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Presentation transcript:

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

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

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

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 Old Cold Old Cold Young Hot Magma source Mantle, mafic

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

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

Colors denote age of rocks

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)

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

Transform Faults Not your typical strike slip fault!

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

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

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