Download Seafloor Spreading

Seafloor Spreading
By:
Continental Drift Hypothesis
•! Alfred Wegener proposed the
hypothesis that the continents
drifted to their present-day
locations. He provided
evidence of certain fossils,
rocks, mountain ranges, and
ancient climatic conditions
(stored in sedimentary rocks)
on continents he once thought
were connected (because of
their shapes) millions of years
ago on a super-continent
called Pangaea. His
hypothesis was rejected by
scientists because he couldn’t
explain how, when, or why the
continents moved.
Unanswered Questions:
!!How did the continents drift?
!!When did they drift?
!!What was a force strong
enough to make the continents
move?
Mapping the Sea Floor
•! During World War 2, enemy Uboats (submarines) would
destroy valuable American boats
with torpedoes from underwater.
These boats needed a way to
find these U-boats before they
struck. So, sonar was invented.
Sonar is the casting off of sound
waves. These waves bounce off
any surface they touch. Boats
would use sonar to find U-boats.
Later, sonar was used to map
the ocean floor. Harry Hess used
sonar to map it and discovered a
mile-high underwater mountain
chain, known as the Mid-Atlantic
Ridge.
Harry Hess’s Hypothesis
•! Harry Hess’s discovery led to even
greater ones. It was found out that
the seafloor was warmer near the
Mid-Atlantic Ridge. This was
actually heat from the magma
underneath it. This led to Harry
Hess’s even greater theory;
seafloor spreading. He
hypothesized that magma slowly
leaked out of the center of the
ridge because of convection in the
mantle. It quickly cooled and
formed new seafloor. Then, more
magma coming out of the ridge
pushed the new seafloor away
from the ridge. This would make
the continental plates move,
causing the continents on top of
them to move, too. This process
explains the unanswered question
of Wegener’s continental drift
hypothesis; how do the continents
move?
Evidence for Seafloor Spreading
•!
For the seafloor spreading hypothesis
to be accepted by people, Hess would
need evidence to support it . First of
all, the temperature underneath the
Mid-Atlantic Ridge is warmer than
lower, surrounding areas. This is
because magma is below it, which can
come out of the center of the ridge
because of mantle convection which
would make new seafloor when it
cooled. Seafloor drilling led to the
discovery that older rocks are farther
away from the Mid-Atlantic Ridge and
younger ones are closer. This means
that the magma coming out of the ridge
is pushing old seafloor away. Third,
subduction gets rid of seafloor, but it is
not slowly disappearing. The only
explanation is seafloor spreading.
Finally, magnetic polarity alternates
between normal and reversed. Igneous
rock make tiny crystals that show which
one it was at the time. So, since there
are stripes of igneous rock with
alternating crystals, it means that the
seafloor is slowly spreading.
The need for evidence in science has
not changed. Without evidence, we
just have theories and hypothesis,
which cannot be supported without
evidence to back it up.
Link Page (all links in order)
http://www.bishoplynch.org/faculty/michaeldehnisch/seafloor.jpg
http://www.wikkistix.com/images/continentaldrift.jpg
http://www.divediscover.whoi.edu/images/sonar_swath_x.gif
http://www.marinebio.net/marinescience/02ocean/mgimg/atlantic.jpg
http://www.geology.wisc.edu/courses/g115/volcano/images/atlantic.jpg
http://maps.unomaha.edu/Maher/plate/week2/miniridge2.jpeg
http://maps.unomaha.edu/Maher/plate/week2/seafloorspreadingUSGS.gif
http://tuxcafe.org/%7Eherob/images/subduction_edited.jpg
http://discovermagazine.com/1999/jun/featoil/2.jpg
The Theory of Plate Tectonics
•! Alfred Wegener had a hypothesis that the continents drifted to their presentday locations. But, it was rejected by scientists because they couldn’t think
of a force strong enough to move the continents. Seafloor spreading was a
hypothesis created by Harry Hess; this was the theory that the seafloor was
slowly spreading. Both of these hypothesis and the information discovered
from them led to another theory, called “Plate Tectonics.” This is the
hypothesis that the Earth’s crust is broken up into several pieces, called
lithospheric plates, and that the plates are slowly moving which changes the
geographic appearance of Earth’s surface. Here is how plate tectonics
works. As discovered during the investigation of seafloor spreading, new
seafloor is formed when lava cools after it comes out of a mid-ocean ridge
because of convection in the mantle. Once the lava cools, gravity pushes
the new seafloor downhill; this process is known as ridge push. Then
mantle convection currents carry the seafloor away. When the seafloor
becomes cooler and denser than the warm rock in the mantle, the seafloor
sinks into the mantle in a subduction zone, also called an ocean trench. The
process of the seafloor sinking into the mantle is called slab pull. Once
deep in the mantle, it melts back into magma.