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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
Young
Hot
Old
Cold
Old
Cold
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
http://www.moorlandschool.co.uk/earth/magnetism.htm
http://www.usgs.gov/125/articles/plate_tectonics.html
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
http://www.earth.northwestern.edu/people/seth/107/Ptdev/magstripes.htm
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
Rift Valley
Off set
Fracture Zone
Fracture zone marked
by change in elevation
of sea floor.
Rift Valley
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.
Evidence
Compressional earthquakes
in offset zone only.
Rift Valley
Younger rocks
Off set
Older rocks
Tensional earthquakes
in rift zone.
Older rocks
Younger rocks
Magnetic anomalies are offset
Offset is original break in crust;
never gets wider.
Rift Valley