1. The document discusses sea-floor spreading, which is the process where new oceanic crust forms at mid-ocean ridges as tectonic plates move away from each other. 2. Evidence that supports sea-floor spreading includes magnetic stripe patterns in the ocean floor and samples from ocean crust that show it is younger near ridges and older further away. 3. Oceanic crust is basaltic rock that forms at ridges and is then recycled in subduction zones, making it generally younger than continental crust which does not undergo this recycling process.

2. TOPIC OBJECTIVES FOR GROUP
REPORTERS:
C. Sea-floor Spreading
 Enumerate and explain the initial and
unusual observations that led to the
formulation of the sea-floor spreading
hypothesis.
 Enumerate and explain the evidences that
support the Sea-Floor Spreading Process.
 Differentiate continental crust from oceanic
crust and explain why the continental crust
is older than the oceanic crust.

5. • Seafloor spreading is the
movement of two
oceanic plates away
from each other (at a
divergent plate
boundary), which results
in the formation of new
oceanic crust (from
magma that comes from
within the Earth's
mantle) along a mid-
ocean ridge.
Where the oceanic plates are moving away
from each other is called a zone of divergence.

6. • Proposed by Harry H. Hess in 1960,
he hypothesized that the formation
of underwater mountains and new
ocean floors were caused by the
Earth’s mantle rising up between
two plates.

9. The boundaries where the
plates move apart are
'constructive' because new
crust is being formed and
added to the ocean floor.
The ocean floor
gradually
extends and thus
the size of these
plates increases.

12. 1. Molten material
2. Drilling samples
3. Magnetic stripes

16. Evidence from Magnetic Stripes
By the mid-1960s studies of the earth's magnetic field
showed a history of periodic reversals in polarity.
The magnetic history of the earth is thus recorded
in the spreading ocean floors as in a very slow
magnetic tape recording, forming a continuous
record of the movement of the ocean floors.

17. • Magnetic surveys
conducted near
the mid-ocean
ridge showed
elongated
patterns of
normal and
reversed polarity
of the ocean floor
in bands
paralleling the
rift and
symmetrically
distributed as
mirror images on
either side of it.

19. • "A dramatic proof of sea-floor spreading was discovered in the
mid 1960s when data revealed alternating stripes of magnetic
orientation on the sea floor, parallel to the mid-ocean ridges and
symmetric across them -- that is, a thick or thin stripe on one side
of the ridge is always matched by a similar stripe at a similar
distance on the other side. This mirror-image magnetic orientation
pattern is created by steady sea-floor spreading combined with
recurrent reversals of Earth's magnetic field. Iron atoms in liquid
rock welling up along a mid-ocean ridge align with Earth's
magnetic field. "

20. Oceanic Crust Continental Crust
• The oceanic crust is • The continental crust
that part of the Earth’s accounts for 40% of the
crust that covers the surface of the Earth. It
ocean basins. It consists is made up of granite
rock. This rock is rich in
of dark-colored rocks constituents like silicon,
made up of basalt. aluminum, and oxygen.
• The oceanic crust • The continental crust is
scarcely floats on the much thicker when
mantle. compared to the oceanic
crust.
• The continental crust
floats much more freely
on the magma.

21. • The density of the continental crust is much less as
compared to the oceanic crust. It has an
approximate value of 2.6 g/cm3. Due to this
difference in densities in magma between the
oceanic crust and the continental crust, the
continents stay in their places, and both crusts are
able to float on the magma.
• With age, the oceanic crust gathers a layer of
cooled mantle on the underside. This causes the
two-layered structure to sink into the hot, molten
mantle. Once in the mantle, the oceanic crust melts
and is thus recycled. Due to this process there is an
absence of aged oceanic crust.

22. • The best theory fitting all this evidence together is a
mechanism known as "sea-floor spreading," an idea formalized
in the 1960s by North Americans Harry Hess, Robert Dietz,
and Tuzo Wilson.
• This developing theory received further confirmation by the
mapping of 20th-century seismic activity and recognition that
it is concentrated at shallow depths of the lithosphere along
the "axial rift" of mid-ocean ridges and at deeper depths
around subduction zones, just as the sea-floor spreading
theory would predict.
• Once again, the military played a central role in these scientific
discoveries, for it was the 1963 treaty banning above-ground
nuclear-weapons testing that created the need for a
Worldwide Standardized Seismograph Network (WWSSN), the
very instruments used to detect this faint earthquake activity
all around the planet.

23. References
• http://library.thinkquest.org/17457/platetectonics/4.php
• http://www.britannica.com/EBchecked/topic/530828/seafloor-spreading-
hypothesis
• http://mail.colonial.net/~hkaiter/MCASreview.html
• http://library.thinkquest.org/17175/current.html
• http://www.ucmp.berkeley.edu/fosrec/Metzger3.html
• http://www.sciencephoto.com/image/167059/530wm/E3500088-
Sea_floor_spreading-SPL.jpg
• http://www.aoi.com.au/bcw/Seafloor/index.htm
• http://www.ucmp.berkeley.edu/fosrec/Metzger3.html
• http://www.aoi.com.au/bcw/Seafloor/index.htm
• http://www.geosociety.org/educate/LessonPlans/SeaFloorSpreading.pdf
• http://home.comcast.net/~steveriddle2/6th/documents/!workbooks/ch4sec4
KEY.pdf
• http://www.encyclopedia.com/topic/seafloor_spreading.aspx
• http://library.thinkquest.org/17457/platetectonics/4.php