plate tectonics - weeblyghsacceleratedchemistry.weebly.com/.../plate_tectonics_2012.pdf · once...
TRANSCRIPT
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2/15/2012 1
Plate Tectonics
Compiled by
Susan Weidkamp
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Presentation Objectives:
Describe how continents have moved over the past
250 million years.
Explain evidence supporting the Theory of Plate
Tectonics.
Explain the Physics of Plate Movement.
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The world is like a jigsaw puzzle
Once people looked at a map of the world, they noticed the land fit together like jigsaw puzzles pieces.
Australia
Africa
Antarctic
a
Greenlan
d
South
America
Europ
e
AsiaNorth
America
Central
America
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2/15/2012 4
Moving Continents? No Way!
The land is solid, what forces moved the land
into the current pattern? Ask Alfred Wegener.
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2/15/2012 5
Alfred Wegener
He was the meteorologist
who collected puzzle
evidence in the early
1900’s. In 1912 proposed
the first scientific theory of
continental drift known as
the Theory of Plate
Tectonics.
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2/15/2012 6
Pangaea the Supercontinent
Alfred Wegener proposed that the continents were once gathered together in one large massive form he named Pangaea
Permian 225 million years ago
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Vocabulary
Plate tectonics- Tectonics comes from the Greek root meaning “to build.”
Pangaea (pan JEE uh) Alfred Wegener’s proposed supercontinent composed of all the world’s land masses.
Glossopteris- ancient fern-like plant found in fossil remains in South America, Africa India, Australia, and Antarctica.
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2/15/2012 8
Moving Continents- Ancient Time
Computer modeling
was used to predict
placement of plates
over time
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2/15/2012 9
Dinosaur Days
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Present Day
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2/15/2012 11
Hola Tierra! ¿Como estás? Su cara ha cambiado con el
movimiento de la Tierra y yo no lo reconozco a usted.
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Fossils of the same species were found on
several different continents.
Wegener’s Fossil Evidence
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2/15/2012 13
Evidence from the Rocks
Similar layers of rock
were formed in
Antarctica, Australia,
South America, Africa
and India before
Pangaea broke apart.
Glossopteris fossils were
found in the rocks on
each continent.
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Solving the Fossil Puzzle
Wegener proposed that the species dispersed when the continents were connected.
Later as the continents drifted they were carried to their present positions.
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Glacier Evidence
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2/15/2012 16
Running the Movie Backwards!
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2/15/2012 17
Glacier Analysis and Theory
Wegener proposed that the
continents were adjacent to
each other during the
glacial event.
Therefore, glaciers spread
over a much smaller area
in the southern hemisphere
and probably did not
influence the climate of the
northern hemisphere.
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Did continents move? Do you see a pattern?
Using the distribution of rock
types, Wegener reconstructed
the distribution of climates
zones at specific times in the
geologic past.
Climates are parallel to
equator, and relative to poles.
What about ancient times?
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2/15/2012 19
Did the poles move? He found that, unlike the present
distribution, in which zones parallel the equator, the past zones occupied very different positions.
This implies that the rotational pole was in very different locations relative to today.
Wegener proposed an alternative interpretation. He believed that the climate zones remained stationary and the continents drifted to different locations.
The drift of the continents caused the apparent movement of the climate zones.
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2/15/2012 21
Still Moving?
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2/15/2012 22
Evidence from space- Still Moving!
A satellite view of the Sinai shows two arms of the Red Seaspreading ridge, exposed on land.
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2/15/2012 23
For Review
What was the first clue that gave people the idea of
plate tectonics?
Who first proposed the theory?
Describe the evidence used to support the
theory of plate tectonics.
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2/15/2012 24
Problems with Wegener's Model of
Continental Drift
Wegener's model was not accepted by all
geologists. Some thought that dispersion by
winds or ocean currents could explain the
distribution of fossil species.
Other geologists thought the poles might wander
and continents remain stationary.
Many geologists thought Wegener's evidence
was insufficient.
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Problems
The greatest shortcoming, at least in the eyes of
American geologists, was the lack of an adequate
mechanism for moving the continents.
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• Wegener proposed that the Earth's spin caused the continents to move, plowing through the oceanic plate and producing mountains on their leading edges.
• Geologists at that time understood enough about the strength of rocks to know that this was highly unlikely.
• Wegener's work was largely unaccepted in the northern hemisphere.
• In the southern hemisphere, where geologists were familiar with the rocks that Wegener used to support his hypothesis, continental drift was generally accepted.
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A mechanism to move continents was proposed by Arthur
Holmes, Scottish geologist in 1928.
He believed heat trapped in the Earth caused convection
currents, areas where fluids beneath the Earth's crust rise, flow
laterally, and then fall.
The currents would rise beneath continents, spread laterally,
then plunge beneath the oceans.
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The Physics of Plate Movement
Sea-Floor Spreading
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Harry Hammond Hess, professor of
geology at Princeton University
Was very influential in setting the stage for the emerging plate-tectonics theory in the early 1960s.
Believed in the evidence Wegener used to defend his theory of continental drift.
Had very different views about large-scale movements of the Earth.
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Sea-Floor Spreading
In 1960 Henry Hess proposed the theory of Sea-Floor Spreading.
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History of Ocean Basins In 1962 Hess published a
paper titled "History of Ocean Basins," which was one of the most important contributions in the development of plate tectonics.
He explained how molten rock (magma) oozes up from the Earth's interior along the mid-oceanic ridges, creating new seafloor that spreads away from the active ridge crest and, eventually, sinks into the deep oceanic trenches.
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If the oceans have existed for at least 4
billion years, as most geologists
believed, why is there so little sediment
deposited on the ocean floor?
Answer: The sediment has been
accumulating for about 300 million years
at most. This interval is approximately
the time needed for the ocean floor to
move from the ridge crest to the trenches,
where oceanic crust descends into the
trench and is destroyed.
Hess’ concept explains puzzles
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A ridge is
created
where the
molten rock
rises.
A trench
forms where
the molten
rock sinks
back into
the mantle.
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How old are the oldest sea floor
fossils?The sea floor is constantly being
recycled. This explains why the
oldest fossils found on the seafloor
are no more than about 180 million
years old.
In contrast, marine fossils in rock
strata on land -- some of which are
found high in the Himalayas, over
8,500 m above sea level -- can be
considerably older.
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Magnetic Pole Reversal
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Magnetic
clues to solve
the mystery of
ridge
formation
were left in
the newly
formed rock.
Pole reversal
occurs
periodically.
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Core sampling has determined the pattern is symmetrical.
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The Mid-Atlantic Ridge is 6,500 feet (2,000 m) above the
adjacent sea floor, which is at a depth of about 20,000 feet
(6,000 m) below sea level.
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Mid-Atlantic Ridge
emerges as the South
American and African
plates pull apart at the
"slow" rate of
approximately 30
millimeters per year.
The axis of the ridge is
marked by a 2 km deep
rift valley, which is
typical of most slow-
spreading ridges.
The map reveals a 12 km jog of the rift valley, a second-order
discontinuity, and also shows a first-order discontinuity called
the Cox transform fault. Colors indicated depths of from 1900
(pink) to 4200 meters (dark blue).
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Theory of Plate Tectonics
The entire lithosphere
of the Earth is divided
into sections called
“plates.”
The plates are moving
slowly.
Plates move at
different rates and in
different directions.
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Plate Boundaries
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Plate Structure
Continental crust is present either in middle or
end of continental plate.
Continental plate has both continental and
oceanic crust. (Land is mixed)
Oceanic plate is entirely oceanic crust. (Oceans
are pure)
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Earthquakes occur along plate boundaries.
Plates can include land and ocean so they are
not the same size as a continent.
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Ring of Fire
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The Physics of Plate Movement
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Vocabulary II
Convergent Boundary- Two plates slide toward
each other.
Divergent Boundary- Two plates slide apart
creating a gap where new plate is made.
Transform Boundary- Two plates slip past each
other.
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The Juan de Fuca
ridge off the
Oregon Coast .
What kind of fault
is the San Andreas
Fault?
Convergent
Divergent
Transverse
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Plate Boundaries
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When a less dense, lighter
(continental) plate overrides
a more dense (oceanic)
plate a subduction zone
forms.
Because the oceanic plate is
bent and driven down, a
deep trench forms at this
collision point.
Trench Formation
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Volcanic Activity
Why do volcanoes form near trenches?
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As the mantle rocks melt they form magma which is
less dense than the surrounding mantle material so it
will rise. Pressure in the magma cracks the overlying
rocks. Then the magma injects into the crack.
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A volcano will form if the magma reaches the
surface. When magma does reach the surface it is
then called lava.
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References
John N. Louie, Phd. Plate Tectonics, the Cause of Earthquakes.
http://www.seismo.unr.edu/ftp/pub/louie/class/100/plate-
tectonics.html May 2, 2001
http://volcano.und.nodak.edu/vwdocs/vwlessons/plate_tectonics/intr
oduction.html
http://volcano.und.nodak.edu/vwdocs/vwlessons/plate_tectonics/par
t5.html
Geology: Plate Tectonics.
http://www.ucmp.berkeley.edu/geology/tectonics.html
http://pubs.usgs.gov/publications/text/HHH.html