Transcript Chapter 17 - plate tectonics

Ch. 17 Plate Tectonics
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Are land masses actually moving?
– How many years would it take these land masses
to move a mile?
– S. America = 53,333 years or 5,333,300 years/100 miles
– Examples:
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1. S. America is moving away from Africa at a rate of 2
to 3 cm/yr.
2. Hawaiian islands are moving NW at 8 to 9 cm/yr.
– Hawaiian islands = 17,778 years or 1,777,800 years/100miles
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In the 1500s a scientist had already noticed
that the land masses seemed to fit together
like a puzzle, but the understanding of why
was not developed yet.
• Continental Drift – the concept of the Earth’s
continents been joined as a single landmass.
– Video-continental drift
• The single land mass (supercontinent) was called
Pangaea.
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Gondwanaland – Southern continents
Laurasia – Northern continents
Greek for “all the earth”
This separation occurred approximately 200 MYA
Fig. 17-1 (pg.444)
• Wegener was the founder of the theory of Pangaea.
He hypothesized that rock types were similar on both
sides of the Atlantic.
– Observations were made to show that rocks from the
Appalachian Mountains were similar to rocks in Greenland
and Europe.
• Wegener also found fossils that were similar in
the U.S., Europe, & Greenland.
• Evidence of coal deposits in Antarctica also
gives evidence of Pangaea. How?
– Swampy plant die off to produce coal deposits over
time. This means that Antarctica must have been
closer to the equator to support plant life.
– Fig. 17-2 (pg. 445)
• Most scientist people and scientist didn’t
believe Wegener’s theory of continental drift; he
couldn’t explain what caused the land masses
to move and how could they move without
shattering.
• Wegener die in 1930. In 1960 scientist
discovered evidence on the ocean floor that
could explain why and how continents move.
• Sonar allowed scientist to study the ocean floor,
which allowed scientists to evaluate the peaks
and valleys along the ocean floor.
– Fig. 17-6 (pg. 449)
• The longest continuous mountain ranges are
found along the ocean floor.
• Volcanoes and earthquakes commonly occur in
these areas.
• Huge trenches (11 km deep) were also found
from the sonar. 6 times greater than the Grand
Canyon.
• Other technology that supported the evidence
was a magnetometer. This device is used to
detect small changes in magnetic fields.
• This instrument was towed by ships to map the
ocean floor.
• Rock sediments were also taken from the
ocean floor. This proved that rocks near the
ocean ridges were younger (volcanoes) than
the trenches. Fig. 17-7
– Video – Sea-floor spreading
• Another discovery showed that the floor
sediments on the oceanic floor ( a few 100 m)
weren’t as thick as continental deposits (about
20 km).
Magnetism
• Before ocean studies were done scientists
knew that rocks containing iron-bearing
minerals provided a record of the Earth’s
magnetic field.
• Paleomagnetism is the study of the Earth’s
magnetic record.
– Basalt is used a lot because of its iron composition.
• As basalt cools, the iron-bearing minerals
become oriented parallel to the Earth’s
magnetic field.
• The Earth’s magnetic field can change
direction, this is called a magnetic reversal.
• In the 1960s scientists found some basalt flows
that had showed the Earth’s magnetic field was
reversed at one time.
• A magnetometers showed that the magnetic
field of the ocean floor was much stronger in
some areas and very weak in others. Why?
– Strong areas match the present magnetic field.
– Weak areas show a reversed magnetic field.
• Fig. 17-8
• A pattern is evident as the polarity is constant
as you work your way out from an ocean ridge.
– Can you explain Fig. 17-10
– The polarity matches the basalt flows on Fig. 17-8
• Scientists could easily determine the age of the
ocean floor due to the magnetic recordings.
• The age of the ocean floor is developed as an
isochron. Fig. 17-11.
• The theory that new ocean crust is formed at
ocean ridges and destroyed at deep-sea
trenches is called seafloor spreading.
• As magma comes up from the ridge it fills and
is dispersed out making new crust. This
explains the questions of how continents move.
– Fig. 17-12
• Continents move with the ocean crust as it
moves away from the ocean ridges.
Theory of Plate Tectonics
• The theory of plate tectonics refers to the Earth’s crust
and rigid upper mantle are broken into huge slabs
called plates.
– Video-plates
– Fig. 17-13 (pg. 455)
– Notice that the plates move in several directions and at
different rates over the Earth’s surface.
• These tectonic plates have plate boundaries where
they come together, move away from one another, or
move horizontally past one another.
– Video1
– Video2
Divergent Boundaries
• Plates move away from one another
– Fig. 17-14 (pg. 456)
• New crust forms. Associated with high heat
flow, volcanism, and earthquakes.
• The Atlantic Ocean, which has a large divergent
boundary, is moving at about 2 to 3 cm/yr.
– Video – Mid-oceanic ridge
Convergent Boundaries
• The plates are moving towards each other.
• 3 Types
– 1. oceanic crust converging on oceanic crust.
• Old crust is recycled by subduction.
– 2. oceanic crust converging on continental crust.
• Can produce volcanoes from Mt. ranges.
– 3. continental crust colliding with continental crust.
• Large Mt. ranges form.
• Video - convergence
• Subduction occurs when one of the 2 plates descends
beneath the other plate
– Fig. 17-15 (pg. 457)  3 Types
– Fig. 17-16 (pg. 458)  ocean to ocean
• Subduction results in the formation of a deep-sea
trench.
Transform Boundaries
• Where plates slide horizontally past each other.
• At a transform boundary the crust is only
deformed or fractured.
– Fig. 17-17 (pg. 459)
• At a divergent boundary new crust formed
• At a convergent boundary old crust is destroyed
by subduction.
Causes of Plate Motions
• The question still not totally understood is what cause
the plates to move.
– It is proposed that large-scale motion in the mantle is the
mechanism that drives the movement of tectonic plates.
– Convection current in the mantle are the driving force of this.
This is caused by the cooler exterior mantle and hotter inner
mantle.
• Fig. 17-18 (pg. 460) – convection
• The unanswered questions include:
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1. How these currents originate?
2. How big are they?
3. Are mantle convection currents permanent?
4. Do they change positions over time?
5. How do they start and stop?
6. Does this happen only in the upper mantle?