Transcript Plate Tectonic Theory

The World as it Turns
Changes in the Earth’s Crust…Plate Tectonics
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
In your journal
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?
Lithosphere. This layer varies in thickness
from 1 to 250 miles. The lithosphere is a
cool, strong and rigid layer. Its uppermost
part is called the crust and is divided into
oceanic and continental-type crusts
discussed in the next section.
Asthenosphere. The upper reaches of the
mantle are not solid; they are considered
plastic and flow very slowly. This is due to
the reduction in pressure as we approach
the top of the mantle- the rocks are more
likely to begin to melt.
Mantle. About 1860 miles thick, the mantle
is made of iron and magnesium-rich silicate
rocks and has a density of about 4.5
g/cm3. The mantle is hot, but mostly solid
because the minerals are under pressure.
Outer Core. About 1240 miles thick, the
outer core is also made mostly of iron with
a density of about 10 g/cm3. However, a
reduction in pressure makes this hot layer
a liquid.
Inner Core. About 800 miles in diameter
and composed of more than 90% iron with
a maximum density of 13 g/cm3. This iron
in this layer, although very very hot, is solid
due to the immense pressure at the center
of the Earth.
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 
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
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 and is recorded
in the rocks – this provides further evidence of sea floor
spreading
• On average, Atlantic Ocean spreads ~ 1.25 cm/yr, similar to
fingernail growth
Polarity
reversals same
N SN SNS N
SN S
S NS
distance from
NS
rift on either
side
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
• Sea Floor Spreading from BBC documentary film "Earth The
Power Of The Planet "
Things to Ponder about Sea Floor Spreading…
• Use Newton’s Laws of Motion to explain
the movement.
• What force is causing the motion?
• What happens when the plates hit each
other?
• What type of force does there have to be
to cause the movement?
Remember Flip
Videos for tomorrow!
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
• There are three types of plate boundaries:
– Divergent Boundaries -Boundary between two plates
that are moving apart.
- Convergent Boundaries -Boundary between two plates
that are moving apart.
- Transform Boundaries -Boundary between two plates
that are sliding past one another.
– animations
Convergent – oceanic to continental
• Continental vs. oceanic
– A subduction zone is the
area where a more dense
oceanic plate dives under
less dense continental plate.
– The place where the
continental crust is pulled
down by the oceanic crust is
a deep, v shaped called
called a trench.
– Where the oceanic plate
subducts and melts under
continental plate, the magma
and hot gasses are forced
upwards creating a volcanic
mountain range on the
continent.
Mt. St. Helens
Lassen peak
Convergent: oceanic to oceanic
• Oceanic vs. Oceanic
• If it is 2 oceanic crusts that
meet, then a underwater
volcanic range is formed in the
ocean.
• As they erupt and the rock
cools they will form islands.
Convergent: continental and continental
• Continental vs.
Continental
• 2 continental plates
converging will form a
mountain range like the
Himalayan 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
India Is still moving
into Asia Today
Divergent boundaries….
• When oceanic moves away from oceanic it forms a mid ocean
ridge.
• When continental moves away from continental if forms a rift
valley.
Mid Atlantic ocean ridge
Examples of Divergent Boundaries: MidAtlantic Ocean Ridge, East African Rift,
and Red Sea
Spreading Centers
Transform…
• At transform plate boundaries, plates slide/grind past each other.
• A famous example of a transform boundary is found in California. It
is called the San Andreas Fault.
Identify the Plate Boundaries below
A.
B.
C.
D.
E.