9.4 and 9.5 notes
Review/Preview (True or False?)
1.
The main criticism of Wegener’s hypothesis was
his lack of a mechanism.
2.
Scientists now know the mechanism of plate
motion.
3.
4.
The mantle consists of liquid magma.
Scientists cannot effectively study the mantle
because they cannot experiment with it directly.
Theory of Plate Tectonics
• 1968: updated version on Continental Drift based on
new evidence
• Contients and seafloor move
New Evidence for Plate Tectonics:
9.4 The Evidence for Plate Tectonics
Strongest new evidence came from ocean drilling (new technology)
in the 1960’s
Geochronology = seafloor age
New seafloor at crust gets older on both sides as you move away
shows plate movement.
No oceanic crust older than 180 million years shows it gets recycled
during subduction.
Paleomagnetism Preserved
in Lava Flows
Paleomagnetism
• Iron-rich minerals act as tiny magnets
• As they cool, they line up parallel to earth’s magnetic
field
• Earth’s magnetic field periodically reverses (N
becomes S)
Evidence for PT: strips of alternating polarity in a
mirror image across the ocean ridge
Hot Spot – mantle plume rises to surface
-- not at a plate boundary
-- shows that plates move
Ex. Hawaiian Islands
9.5 Mechanisms of Plate Motion
Driving force for plate movement is the unequal
distribution of heat in earth.
Convection – method of heat transfer in fluids
where hot material rises and cold material sinks
Movement is due to differences in density.
But isn’t the mantle made of solid
rock?
“On a human time scale, rocks are solid, but if you wait
for a long time – millions of years – rocks slowly
change their shape. They flow.”
-- Peter Bunge
Asst. Professor of
Geophysics
Princeton University
Mantle Convection Models
Heat sources:
1. radioactive decay of elements in mantle
2. residual heat from formation
How do scientists study mantle
convection?
Geophysicists use computer
modeling to simulate mantle
movement.
Burkavici
http://www.columbia.edu/itc/ldeo/mutter/jcm/Topic3/Topic3.html
Mantle convection model animations
http://homepage.usask.ca/~sab248/Mtemp28.WMV
http://www.gps.caltech.edu/~gurnis/Movies/movies-more.html
Proposed Mechanisms :
1. Ridge push
2.
Slab pull
At present, none is able to explain all facets of plate motion.
There are still many unanswered questions about the details
of Plate Tectonics.
Pangea Ultima
• http://www.nytimes.com/packages/html/science/20
070109_PALEO_GRAPHIC/