Download KICKS Plate Tectonics

The Development of the
Theory of Plate Tectonics
http://www.ridge2000.org/SEAS/for_teacher
s/curriculum/unit1.html
8.1.2.a Recognize science is an ongoing process and the scientific community
accepts and uses explanations until they encounter new experimental
evidence not matching existing explanations
8.1.2.c Recognize scientists from various cultures have made many
contributions to explain the natural world
8.4.2.a Describe the layers of Earth (core, mantle, crust, atmosphere)
8.4.2.d Describe evidence of Earth’s magnetic field
8.4.2.e Compare and contrast constructive and destructive forces (deposition,
erosion, weathering, plate motion causing uplift, volcanoes, earthquakes) that
impact Earth’s surface
Nebraska Standards
8.1.2.a Recognize science is an ongoing process and the
scientific community accepts and uses explanations until
they encounter new experimental evidence not matching
existing explanations
8.1.2.c Recognize scientists from various cultures have
made many contributions to explain the natural world
8.4.2.a Describe the layers of Earth (core, mantle, crust,
atmosphere)
8.4.2.d Describe evidence of Earth’s magnetic field
8.4.2.e Compare and contrast constructive and destructive
forces (deposition, erosion, weathering, plate motion causing
uplift, volcanoes, earthquakes) that impact Earth’s surface
SEAS curriculum: Unit 1
Plate tectonics and the
discovery of hydrothermal
vents
http://www.ridge2000.org/SEAS/for_teachers/curriculum/uni
t1.html
Activity 1: Our Changing View of
the Earth
• Students participate in the scientific
debate that followed the publication of
Alfred Wegener's book, The Origins of
Continents and Oceans, in 1912.
• Part 2 students participate in a
hypothetical scientific conference and
piece together for themselves the
subsequent evidence and hypotheses that
led to the theory of plate tectonics.
Activity 2: Finding Plate
Boundaries
• On a world map, students mark areas
where they think earthquakes and
volcanoes occur. Using lists of recent
earthquakes and volcanoes, they plot the
occurrences of earthquakes and look for
patterns in the locations. Using a map that
shows the tectonic plate boundaries,
students compare plate boundaries with
the earthquake and volcano location
patterns and speculate on the significance.
Activity 3: Modeling Plate
Tectonics
• Using simple, available materials, students
create models to simulate the plate
boundary interactions to gain further
insight into what happens at these
boundaries.
Activity 4: Mapping the Unknown
• Students simulate "soundings" to visualize and map an
unknown object inside a Mystery Box, a cardboard box
with a shoe or some other familiar object attached to the
inside bottom. In Part 1, students use "Sonar Sticks" to
scan the box bottom surface and determine depths, and
then construct a rough map of the bottom using color to
indicate relative differences in depth. The resulting map
reveals peaks and valleys but not enough detail of the
shape to determine the identity of the object. In Part 2,
students remap portions of the box bottom with higher
resolution and work together as a class (i.e., a
community of scientists) to share their data to determine
the box contents.
Alfred Wegener – proposed that in the distant
past, the Earth’s continents were all joined as a
single landmass, Pangaea.
Evidence for his theory -a. South America and Africa would fit remarkably well,
shoreline to shoreline.
b. Mountain formations appear to be folds in the Earth’s
crust.
c. Matching fossils on separate continents.
d. Mountain ranges, ages and kinds of rocks match along
edges of continents. (diamonds)
e. Glacier markings match on separate continents, some in
areas that are now tropical.
Fossil Evidence
Alfred Wegener
http://learner.org/resources/series78
.html?pop=yes&pid=315# video 5
2:40-9:40 Wegener
Discussion Questions:
1. Wegener’s hypothesis of Continental Drift was a first
step toward our current understanding of plate tectonics
but it was dismissed by the scientific community. What
were weaknesses in his theory?
Why did that matter?
2. Increased exploration of the ocean floor and the oceanic
ridges has provided the information necessary to refine
and gain acceptance for many of Wegener’s hypotheses.
What advantage is there to a scientific system that
requires so much evidence and does not accept new
hypotheses without it? What are the disadvantages?
Plate Tectonics Players
Seismic Waves and the Structure of the Earth
Richard Oldham 1906 and Andrija Mohorovicic 1909
Several scientists, working before Wegener proposed his
Continental Drift Hypothesis, used seismic waves—the
waves caused by explosions or rock breaking deep in
the earth during an earthquake—to make observations
on the structure of the Earth. Two important
ones were:
• Richard Oldham discovered that, at a certain
depth, one type of seismic wave—P-waves—
slowed down and that another type—S-waves—
were either reflected back to the surface or disappeared
altogether. He hypothesized that the solid
mantle ended at this depth: inside was the liquid
core.
• In 1909, Andrija Mohorovicic discovered discovered
that a marked change in density occurs between the crust and the mantle. We
now call this area the Mohorovicic discontinuity, or Moho for short.
http://aspire.cosmic-ray.org/labs/seismic/index.htm
Cruise 1 Activity 1 – Part 2
Composition of
the Earth: The
Four Layers
• as the Earth cooled the heavier, denser materials sank to the
center and the lighter materials rose to the top
• crust is made of the lightest materials (rock- basalts and
granites) and the core consists of heavy metals (nickel and
iron).
• The mantle is much hotter than crust and has the ability to
flow.
• The Outer and Inner Cores are hotter still with pressures so
great that you would be squeezed into a ball smaller than a
marble if you were able to go to the center of the Earth!!!!!!
Plate Tectonics Players
Convection and Rock Deformation and Flow
Arthur Holmes 1929 and David Griggs late 1930s
Wegener’s Continental Drift Hypothesis was rejected because it lacked a
convincing mechanism for moving the “continents.” The observations of these two
scientists ultimately helped solve that problem:
• In 1929, Arthur Holmes suggested that continental drift might be caused by
thermal convection in the mantle. The idea of thermal convection is that
when a substance is heated from below —as the mantle would be by
radioactivity at the Earth’s core—its density decreases and it rises. Once at
the surface, it cools, becomes denser and sinks. Holmes proposed that the
repeated heating and cooling of fluids beneath the Earth’s crust could set up
a conveyor belt-like action powerful enough to break apart a continent and
carry the pieces in opposite directions. This theory was not recognized by the
scientific community at the time
In the 1930s, David Griggs created an apparatus that showed that:
• Solid rock can flow if it is kept at very high temperatures and pressures.
Thus, the convection of solid rock in the mantle, acting as a fluid, could be
responsible for plate movement.
Cruise 1 Activity 1 - Part 2
Earth Structure
Solid
Liquid
Solid-ish
Solid
Liquid
Mechanical
Earth: What
makes the Earth
Move?
Continents ride on the denser oceanic plates. The
crust and the upper layer of the mantle together
make up a zone of rigid, brittle rock called the
Lithosphere. The layer below the rigid lithosphere
is a zone of asphalt-like consistency called the
Asthenosphere. The asthenosphere is the part of
the mantle that flows and moves the plates of the
Earth.
Plate Tectonics Players
The Age and Shape of the Ocean Floor
1940s and 1950s
During and after the Second World War, technology developed to find submarines
was used to explore the ocean floor. Several important observations were made:
• The sediment on the ocean floor is not as deep or as even as would be
expected if all of the ocean floor were the same age and created when the
Earth cooled. Some areas with much less sediment might be much younger.
• The ocean floor is not flat. There are long chains of mountains running
through both the Atlantic and Pacific Oceans, often with deep rifts running
along their ridges.
• Volcanoes and earthquakes are common along the underwater ridges.
• There are deep trenches in the ocean floor.
Plate Tectonics Players
Seafloor Spreading
Harry Hess and Robert Dietz 1959
Hess and Dietz observed that:
• There are many volcanoes along the ridges of the underwater mountain
chains.
• Based on rock samples, the crust is very young near the ridges and gets older
as one moves away from the ridges.
• Crust a given distance away from a ridge is about the same age as crust the
same distance away from the ridge on the opposite side of the ridge.
Conclusions:
• This suggests that new ocean crust is being created at the mid-ocean ridges.
• Since the Earth isn’t getting bigger, old ocean crust must be destroyed
somewhere.
Since old ocean crust is cold and dense it would probably sink
beneath continental crust or younger oceanic crust creating a subduction zone.
Plate Tectonics Players
Subduction Zones
Kiyoo Wadati and Hugo Benioff 1950-60s
Wadati and Benioff plotted the depths of earthquakes in areas that were suspected to
be subduction zones (areas where the crust collides and one side bends and slides
under the other, plunging deep into the mantle and melting). They found that:
• Earthquakes tend to lie along a sloping
plane — in 3 dimensions, it looks
like a ramp going down into the
mantle.
Conclusion:
• Earthquakes probably occur as the
subducting plate rubs and grinds
against the plate on top of it and is
evidence that oceanic crust is
destroyed at subduction zones.
Deep Earthquake zone
Plate Tectonics Players
The Crust is Moving!
J. Tuzo Wilson 1963
Flying over the
Hawaiian Islands,
Wilson noticed that while the
big island of Hawaii itself was
still a very active volcano, the more
distant island of Niihau was no longer
active, was more eroded and seemed much
older. Wilson hypothesized that there must be a stationary
“hot spot” over which the islands moved.
The crust carrying the chain of islands must be moving!
Plate Tectonics Players
Transform Faults
J. Tuzo Wilson 1965
Wilson published an observation
that, in addition to the
spreading zones proposed by Hess
and Dietz and the Subduction
Zones investigated by Wadati and
Benioff, there must be a third type
of plate boundary where plates
end abruptly and “transform” into
major faults that slip horizontally.
An example of such a transform fault
boundary is the San Andreas Fault zone. Unlike ridges and trenches, transform
faults offset the crust horizontally, causing earthquakes, but without creating
or destroying crust.
Plate Tectonics Players
Magnetic Stripes
Fred Vine and Drummond Matthews 1960s
Rocks in the ocean seafloor have a pattern of magnetic signals. Vine, Matthews
and others found that:
• The crust surrounding the mid-ocean ridges showed alternating bands going
away from each side of the ridge. The band closest to the ridge showed
“normal” magnetic polarity (aligned with the
Normal Polarity
Earth’s current magnetic field.). The next band out
showed reversed polarity, followed by a band of
Reversed Polarity
normal polarity and so on.
Conclusions:
• It is known that the magnetic polarity of the planet has
gone through repeated reversals, although the mechanisms
for this are unknown. As new seafloor crust
is formed around the rift in the mid-ocean ridge, it
magnetizes according to the polarity of the time.
• Magnetic striping might be used to correlate geologic
time with magnetic polarity.
Lithosphere Magma
Ridge
So what?
• So what? When liquid rock on/in the crust cools,
iron containing minerals will line up with the
magnetic field lines of the earth.
– Parallel to Earth magnetic field lines.
– Just like the metal pieces on the demo
• These rocks are like a recording of a
conversation. . .
– They are an account of the magnetic polarity of the
earth (which way is north/south)
– They are an account of the location of the rock during
its formation. (has it been moved since?)
• Called paleomagnetism
Reversal Model
• Reversal begins at
the core
• As the core becomes
more disorganized, so
does the magnetic
field at the surface.
• Field is weakened as
“islands” form
http://www.pbs.org/wgbh/nova/eart
h/when-our-magnetic-fieldflips.html
Plate Tectonics Players
Tectonic Plates
W. Jason Morgan and Dan Mackenzie 1967
The current theory of the structure of the Earth held that the Earth’s surface was a
single piece that can be deformed. However, two scientists Dan Mackenzie and
W. Jason Morgan, working separately, hypothesized that the crust is broken up
into separate pieces, like ice on a lake or jigsaw puzzle pieces. These plates would
be similar to what Wegener proposed.
.• After reading a paper on the massive fracture zones on the Pacific seafloor,
W. Jason Morgan recognized a pattern and created a model of plate boundaries
that showed that the plates themselves behaved like rigid bodies on a
sphere.
• Working separately Dan Mackenzie made the same observation.
Plate Tectonics Players
Convection in the Mantle
Jack Oliver, Bryan Isacks and Lynn Sykes 1968
Oliver, Isacks and Sykes, using David Griggs’ 1930 observations that solid
rocks under pressure can flow like fluid, proposed a more believable mechanism
for Wegener’s hypothesis that “continents” drift:
• The mantle of the Earth,— the layer of rock that
lies between the crust and core— might be
convecting. This means that rock in the
lower mantle, near the core, rises because
it is hotter and less dense. As it moves
toward the crust, it cools and becomes
denser. Eventually it begins to sink
again. This movement throughout the
mantle produces convection cells. Since
the crustal plates “float” on top of the
mantle, the movement of the mantle
could cause the plates to move around on
Trench
the Earth’s surface.
Lithosphere
Trench
Ridge
Convection
Mantle
Convection
Convection currents!
Density! D= M/V
How does it work?
• Mantle is a liquid.
• Recall: Temperature of Earth becomes warmer as you
go deeper into Earth.
• So, mantle rock near the core heats up and rises. As the
rock rises, it cools; cool mantle rock then sinks.
• Convection currents within the mantle drive the plate
movement. IT’S ALL ABOUT DENSITY!
– Hot fluids rise, cold fluids sink.
The History of a New View of the Structure of the Earth
1906 - Richard Oldham discovered that, at a certain depth, one type of seismic wave—Pwaves—slowed down and that another type—S-waves—was either reflected back to the
surface or disappeared altogether. He hypothesized that the solid mantle ended at this depth
and inside was a liquid core.
1909 - Andrija Mohorovicic discovered that a marked change in density occurs between the
crust and the mantle. We now call this area the Mohorovicic discontinuity, or Moho for short.
1912- Alfred Wegener proposed the Continental Drift hypothesis
1929 - Arthur Holmes observed that continental drift might be caused by thermal convection
in the mantle. The idea of thermal convection is that when a substance is heated from below
—as the mantle would be by radioactivity at the Earth’s core—its density decreases and it
rises. At the surface, it cools, becomes denser and sinks. Holmes proposed that the repeated
heating and cooling of fluids beneath the Earth’s crust could set up a conveyor belt-like
action powerful enough to break apart a continent and carry the pieces in opposite directions.
This theory was not recognized by the scientific community at the time
late 1930s - David Griggs created an apparatus that showed that solid rock can flow if it is
kept at very high temperatures and pressures. Thus, the convection of solid rock in the
mantle, acting in a fluid manner, could be responsible for plate movement.
http://learner.org/resources/series78.html?pop=yes&pid=315# video 5 2:409:40 Wegener, 9:40-26:19 WWII and Hess
1940-50 - Technology developed during WW2 to track submarines revealed that:
• The sediment on the ocean floor is not as deep or as even as would be expected if all of
the ocean floor were the same age and created
when the Earth cooled. Some areas with much less sediment might be much younger.
• The ocean floor is not flat. There are long chains of mountains running through both the
Atlantic and Pacific Oceans, often with deep rifts running along their ridges.
• Volcanoes and earthquakes are common along the underwater ridges.
• There are deep trenches in the ocean floor.
1959- Harry Hess proposes Seafloor Spreading hypothesis.
1950s and 1960s -Kiyoo Wadati and Hugo Benioff, plot earthquakes on the ocean floor in
areas suspected of being subduction zones.
1963 - J. Tuzo Wilson hypothesized that the islands of Hawaii were created as the crust
moved over a “hotspot” in the mantle.
1965 - J. Tuzo Wilson published an observation that there must be a third type of plate
boundary where plates end abruptly and “transform” into major faults that slip
horizontally.
1967 - W. Jason Morgan and Dan MacKenzie theorize that the Earth’s crust is broken up
into large “plates.”
1968 - Fred Vine and Drummond Matthews discover magnetic stripes on the ocean floor.