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Seafloor Spreading
•Sea-floor spreading is the process in which the
ocean floor is extended when two plates move
apart. As the plates move apart, the rocks break
and form a crack between the plates. Earthquakes
occur along the plate boundary. Magma rises
through the cracks and seeps out onto the ocean
floor like a long, thin, undersea volcano.
•As magma meets the water, it cools and solidifies,
adding to the edges of the sideways-moving
plates. As magma piles up along the crack, a long
chain of mountains forms gradually on the ocean
floor. This chain is called an oceanic ridge.
http://library.thinkquest.org/17457/platetectonics/4.php
• 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.
• As these plates get bigger, others become
smaller as they melt back into the Earth in
the process called subduction.
http://library.thinkquest.org/17457/platetectonics/4.php
Plate Boundaries
• Convergent
• Divergent
• Transform-Fault
Convergent Boundaries
• Boundaries between two
plates that are colliding
 
• There are 3 types…
Type 1
• Ocean plate colliding with a
less dense continental plate
• Subduction Zone: where the
more dense plate slides under
the less dense plate
• VOLCANOES occur at
subduction zones
• Oceanic-Continental Convergence When an
oceanic plate pushes into and subducts under a
continental plate, the overriding continental
plate is lifted up and a mountain range is
created. Even though the oceanic plate as a
whole sinks smoothly and continuously into the
subduction trench, the deepest part of the
subducting plate breaks into smaller pieces.
These smaller pieces become locked in place for
long periods of time before moving suddenly
and generating large earthquakes. Such
earthquakes are often accompanied by uplift of
the land by as much as a few meters.OceanicOceanic Convergence
Andes Mountains,
South America
Type 2
• Ocean plate colliding with
another ocean plate
• The more dense plate slides
under the less dense plate
creating a subduction zone
called a TRENCH
• Oceanic-Oceanic Convergence When two
oceanic plates converge one is usually
subducted under the other and in the process a
deep oceanic trench is formed. The Marianas
Trench, for example, is a deep trench created as
the result of the Phillipine Plate subducting
under the Pacific Plate.
Oceanic-oceanic plate convergence also results
in the formation of undersea volcanoes. Over
millions of years, however, the erupted lava and
volcanic debris pile up on the ocean floor until a
submarine volcano rises above sea level to form
an island volcano. Such volcanoes are typically
strung out in chains called island arcs.
Aleutian Islands, Alaska
Type 3
• A continental plate colliding
with another continental plate
• Have Collision Zones:
–a place where folded and thrust
faulted mountains form.
• Continental-Continental Convergence
When two continents meet head-on, neither is
subducted because the continental rocks are
relatively light and, like two colliding icebergs,
resist downward motion. Instead, the crust
tends to buckle and be pushed upward or
sideways. The collision of India into Asia 50
million years ago caused the Eurasian Plate to
crumple up and override the Indian Plate. After
the collision, the slow continuous convergence of
the two plates over millions of years pushed up
the Himalayas and the Tibetan Plateau to their
present heights. Most of this growth occurred
during the past 10 million years.
Divergent Boundaries
• Boundary between two plates
that are moving apart or rifting

• RIFTING causes SEAFLOOR
SPREADING
Features of Divergent
Boundaries
• Mid-ocean ridges
• Rift valleys
• Fissure volcanoes
Divergent Boundaries
Places where plates are coming apart
are called divergent boundaries. When
Earth's brittle surface layer (the
lithosphere) is pulled apart, it typically
breaks along parallel faults that tilt
slightly outward from each other. As the
plates separate along the boundary, the
block between the faults cracks and
drops down into the soft, plastic interior
(the asthenosphere).
The sinking of the block forms a central
valley called a rift. Magma (liquid rock)
seeps upward to fill the cracks. In this
way, new crust is formed along the
boundary. Earthquakes occur along the
faults, and volcanoes form where the
magma reaches the surface. Plate
separation is a slow process. For example,
divergence along the Mid Atlantic ridge
causes the Atlantic Ocean to widen at only
about 2 centimeters per year.
Transform Fault Boundaries
• Boundary between two plates
that are sliding past each other
• EARTHQUAKES along faults
• Transform Boundaries
Places where plates slide past each other are
called transform boundaries. Since the plates on
either side of a transform boundary are merely
sliding past each other and not tearing or
crunching each other, transform boundaries lack
the spectacular features found at convergent
and divergent boundaries. Instead, transform
boundaries are marked in some places by linear
valleys along the boundary where rock has been
ground up by the sliding. In other places,
transform boundaries are marked by features
like stream beds that have been split in half and
the two halves have moved in opposite
directions.
• Perhaps the most famous transform
boundary in the world is the San Andreas
fault, shown in the drawing above. The
slice of California to the west of the fault
is slowly moving north relative to the rest
of California. Since motion along the fault
is sideways and not vertical, Los Angeles
will not crack off and fall into the ocean as
popularly thought, but it will simply creep
towards San Francisco at about 6
centimeters per year. In about ten million
years, the two cities will be side by side!
• Although transform boundaries are not marked
by spectacular surface features, their sliding
motion causes lots of earthquakes. The
strongest and most famous earthquake along
the San Andreas fault hit San Francisco in 1906.
Many buildings were shaken to pieces by the
quake, and much of the rest of the city was
destroyed by the fires that followed. More than
600 people died as a result of the quake and
fires. Recent large quakes along the San
Andreas include the Imperial Valley quake in
1940 and the Loma Prieta quake in 1989.
San Andreas Fault, CA
Animations
• How Ocean Crust Forms
• Plate Boundaries
• Seafloor Spreading
• Seafloor Spreading
Animations
• Seafloor Spreading
• Seafloor Spreading Video