Transcript Seafloor Spreading and Paleomagnetism

Seafloor Spreading and
Paleomagnetism
Science Haters
• Other scientists
rejected Wegener’s
hypothesis because
he couldn’t explain
what was causing
the continents to
move
• Wegener suggested rotation of the Earth, but physicists
said no
• Wegener also thought the continents plowed through
ocean floor but there was no evidence for that
•Read pgs. 448-454 in blue
book
•Jot down the big ideas
about “Seafloor Spreading”
and “Paleomagnetism” on
worksheet
Holmes and Hess
-proof for Wegener’s theory.
Arthur Holmes in the
1940s-1950s:
Seafloor spreading:
new ocean crust is
formed at ocean ridges
and destroyed at deep sea
trenches.
Holmes proposed plate
motion is thought to be
caused by thermal
convection currents in
the asthenosphere.
Convection Currents & Plate Motion
Since the hotter material deep in the asthenosphere is less
dense it slowly rises. As it reaches the base of the
lithosphere it begins to cool, become more dense, and
begins to sink. Rising convection currents will move plates
apart (divergent). Sinking convection currents will move
plates together (convergent).
Plate Boundaries & Convection Currents
Convergent Boundary
Divergent Boundary
Convection Currents
in Mantle
Subduction Zone: When one plate sinks beneath another.
Seafloor
Spreading –
new ocean
crust is formed
at ocean ridges
and destroyed
at deep sea
trenches.
http://education.sdsc.edu/optiputer/flash/seafloorspread.htm
Harry Hess – 1940s1960s:
Used paleomagnetism
and seafloor mapping to
prove convection is a
source for ‘continental
drift’
Then came the Magnetometer. This could detect small
changes in the magnetic field in rocks and map them.
As new crust is
created the iron
bearing minerals
point to where
the magnetic
pole is at that
time.
This is called
Paleomagnetism
Mid-Ocean Ridge
This proved that new sea floor was being created at midocean ridges. Thus, the plates were moving.
http://earthguide.ucsd.edu/eoc/teachers/t_tectonics/p_paleomag.html