Transcript Plate Tectonic Theory

The World as it Turns
Changes in the Earth’s Crust…Plate Tectonics
Parts of the Earth…
• There are 5 main layers, from surface
to center:
– Lithosphere (crust): solid outermost
layer, about 5-50km deep;
– Asthenosphere: the lower layer of the
earth's crust; less rigid than the
lithosphere—more slushy/flexible
– Mantle: region below crust extending
down to Earth's core; some molten
rock; less flexible
– Liquid Outer Core: made of molten
iron/nickel; flows & creates Earth’s
magnetic field
– Solid Inner Core: solid iron/nickel
center of the Earth; high temperatures
& pressure; like a solid
How do we know?  Earthquakes 
The Theory of Continental Drift
Alfred Wegener, 1912
Continental Drift…
• proposed – continents slowly drift in
relation to one another; continents were
once joined, single super-continent
(Pangaea)
Alfred
Wegener
Interactive Map of Pangaea
Continental Drift: The Evidence…
• shape of the continents - puzzle-like fit
• same animal fossils – fresh water fossils-opposite
shores of Atlantic Ocean
• same sequence of rock layers - found on opposite
shores of Atlantic Ocean along coastlines
• fossils of tropical plants - found in polar areas
• glacial deposits - found in tropical areas
More on the evidence
Continental Drift: Lack of Support…
• Wegener’s theory did not go over well in the scientific
community…
– no explanation how the continents “drift”
– continents plow through sea floor , really???
– What strong force moves huge mass-large distance???
• To read an excellent historical account of
Continental Drift with informative graphics,
visit the U.S. Geological Survey.
Crustal History Confirmation
A.Describe the evidence Wegener used
to support his theory of Continental
Drift.
B.Why did the scientific community not
believe in Wegener’s theory?
Sea Floor Spreading
More Evidence is Found
Sea Floor Spreading
• 1940’s, use sonar to map
ocean floor
• Discovery of ridges
(underwater mountains),
middle Atlantic Ocean
– extended entire length (N S) of ocean
– More ridges found in Indian
and Pacific Oceans
Sea Floor Spreading: The Evidence…
• 1968, scientists drill cores of rock from ocean floor
– youngest rock at the mid-ocean rift; oldest at continental
coasts
– Earth’s magnetic field constantly shifting,
• On average, Atlantic Ocean spreads ~ 1.25 cm/yr, similar to
fingernail growth
Polarity
reversals same
distance from
rift on either
side
SN S
N SN SNS N
S NS
NS
Sea Floor Spreading: the How…
• Occurs at MID-OCEAN RIDGES
• CONVECTION – Hot, less dense magma rises, flows sideways ,
cools & sinks
• Magma/crust friction pulls crust; rift fills with magma/lava, cools creating NEW crust
• Ocean lithosphere (crust) is youngest NEAR mid-ocean ridge
Mid Ocean
youngest
Ridge
Convection
oldest
Currents
Rising
Magma
Oceanic Crust
(lithosphere)
Sea Floor Spreading: The How…
• Sea Floor Spreading animation
• Remember: convection currents CAUSE sea floor
spreading—gravity causes convection currents
• Convection Currents and Plate Movement animation
Crustal History Confirmation
A.Explain sea-floor spreading.
B.When examining rock samples from
the ocean floor, identify 2 things that
can be found that support the
hypothesis of sea floor spreading.
Where are we today?
The Plate Tectonic Theory
The Plate Tectonics Theory
• Plate Tectonics Theory combines continental drift & seafloor
spreading
• Lithosphere floats on denser, liquid rock in mantle called
asthenosphere.
• Lithosphere broken into tectonic plates
• Tectonic plates move about 1-8 cm/yr
• This movement is the “continental drift” referred to by Wegener
The Plate Tectonics Theory
• New plate added by rising magma at divergent boundaries (think
mid-ocean ridges)
• Plates are destroyed in subduction zones at convergent
boundaries
• Transform
boundaries
slide past
each other
Plate Boundary Movements…
• 3 types of plate boundary movements
– Convergent: Boundary between two plates that are pushing
together.
– Divergent: Boundary between two plates that are moving
apart.
– Transform: Boundary between two plates that are sliding past
one another.
• ANIMATIONS
When Boundaries Collide: Convergence…
• Continental vs. oceanic
– More dense oceanic plate
dives under less dense;
called subduction
– dense, leading edge of the
oceanic plate actually pulls
the continental plate down
forming a trench
– Crust heats and melts as it is
forced downward below the
continental crust
– Hot gases and magma
forced upward creating a
volcanic mountain range on
the continent
When Boundaries Collide: Convergence…
• Oceanic vs. Oceanic
– the farther a plate gets from the
mid-ocean ridge that created it,
the colder and denser it gets
– when two oceanic plates collide,
the plate that is older, therefore
colder and denser, is the one
that will sink (subduct)
– this subduction zone forms a
curved volcanic mountain chain
– Aleutian Peninsula of Alaska is
an example of a very
volcanically-active island arc
When Boundaries Collide: Convergence…
• Continental vs.
Continental
– two continental plates
meet head-on, neither
can sink; both plates
are too buoyant
– solid rock is folded and
faulted
– huge chunks of rock
many kilometers wide
are thrust on top of one
another, forming a
towering mountain
range
Himalayan Mtns. Taken by Satellite, 2004…
Tibet
(\Eurasian Continental Plate)
Nepal
India
(Indian Continental Plate)
* Himalayan mountain range
is the perfect example of how
the highest mountains in the
world continues to grow
Plate Tectonics: Boundaries
• They Move! – Convergence
• Turn to your neighbor and explain converging
tectonic boundaries
• Use handy supplies to help your explanations
When Boundaries Collide: Divergence…
• hot magma moves slowly upward, cooler magma near surface
moves slowly downward forming convection currents within the
asthenosphere
• Rising convection currents diverge where they approach the
surface pulling on the plate above it creating a divergent plate
boundary
• two sides move away in opposite directions, cracks between the
diverging plates fill with molten rock which cools and quickly
solidifies, forming new oceanic crust
When Boundaries Collide: Transform…
• At transform plate boundaries, plates slide/grind past each other.
• separates the North American plate from the Pacific plate along the
San Andreas fault, a transform plate boundary responsible for many
California’s earthquakes
The World as it Turns
In Closing…
• Turn to your neighbor and explain converging,
diverging and transform boundaries tectonic
boundaries
• Use handy supplies to help your explanations
• Be prepared to share your “demonstrations”
with the class
Review…
• As plates move, the structure of the earth changes, creating
volcanoes and building mountains