Transcript Plate Tectonics

Plate Tectonics
Text ref. Ch.17 (pg. 442)
Contents
Continental Drift
Seafloor Spreading
Plate Tectonics
Continental Drift
Text ref. Ch.17 (442-447)
Objectives
Describe one piece of early evidence that
led people to suggest the Earth’s
continents may have once been joined.
Discuss evidence of continental drift.
Explain why continental drift was not
accepted when it was first proposed.
Drifting Continents
Early observations
Map-makers noticed how continents fit on
either side of the Atlantic.
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Thought continents had been separated by
floods and earthquakes
Edward Suess proposed continents had once
been joined together as supercontinent known
as Gondwanaland.
Continental Drift
Wegener proposes continental drift, calling
his supercontinent Pangaea, which means
“all Earth” in Greek
Proposes supercontinent began breaking
apart 200 million year ago.
Begins collection of scientific evidence to
support his theory
Evidence from Rock Formations
Wegener hypothesized that the same
types of rock formations should exist on
both sides of the Atlantic.
Noticed rocks in the Appalachian
mountains shared features with rocks in
Greenland and Europe.
All rocks were dated older than 200 million
years; therefore found together before
continental drift began.
Evidence from Fossil Records
Fossils of the same animals and plants were can
be found on separate continents.
Kannemeyerids and Labyrinthodonts fossils can
be found in Africa, North America, South
America, Antarctica, Greenland, and Northern
Europe.
Glossopteris plants can also be found in many
continents, strengthening the hypothesis that
these continents shared the same climate.
Glossopteris
Kannemeyerid
Ancient Climatic Evidence
Wegener studied sedimentary rocks and
found evidence of widespread climate
change.
Coal deposits in Antarctica suggest that
plants once lived in Antarctica; therefore
Antarctica was once closer to the equator.
Glacial Evidence
Glacier deposits can be found in Africa,
India, Australia, and South America. This
suggests that these areas were once cold
enough for glaciers to form.
Opposition to the Hypothesis
Wegener could not explain why the
continents had moved throughout geologic
time.
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Scientists doubted that such forces could exist
on Earth to move entire continents.
Wegener could explain how continents could
move through the ocean floor to new locations
on Earth.
Wegener continued to collect evidence until
his death in 1930.
Seafloor Spreading
Text ref. Ch.17.2 (pg.448-455)
Objectives
Summarize the evidence that led to the
discovery of seafloor spreading.
Explain the significance of magnetic
patterns on the seafloor.
Explain the process of seafloor spreading.
Technological Advance
People before the 1900’s assumed the
seafloor was essentially flat.
Advances in technology in the 1940’s and
’50s led to new ideas.
Sonar can map surfaces by bouncing high
frequency sound waves in order to
calculate distances (elevation)
Ocean Floor Topography
Use of magnetometer to detect variations
in magnetic fields allow scientists to map
the seafloor.
Images revealed underwater mountain
ranges, called ocean ridges.
Ridges form the largest continuous
mountain range on Earth.
Sonar revealed ridges had corresponding
trenches
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Mariana Trench is over 11 km deep.
Questions
What could have formed the ridges and
trenches found on the seafloor?
What is the source of volcanism
associated with these locations?
What forces could be at work to move the
entire seafloor and create trenches 6 times
as deep as the Grand Canyon?
Ocean Rocks and Sediments
Scientists found predictable patterns in the ages
of rock on the seafloor.
Rock ages with distance from ocean ridges
Ocean floor is dated at its oldest around 180
million years.
Continental rocks are dated at 3.8 billion years.
Why is the seafloor so young in comparison?
Ocean Rocks and Sediments
cont…
Thickness of ocean sediments is much thinner
than expected.
Typically a few hundred meters thick.
Continents may have sediments up to 20 km
thick.
Why are the sediments so thin?
Why isn’t seafloor as thick as continental crust?
Sediment is found to be thicker the further it is
from ridges.
Magnetism
Study of the Earth’s magnetic field in ironbearing minerals is known as
paleomagnetism.
Seafloor is composed of iron-rich basalt, in
which the iron becomes aligned with
Earth’s magnetic field as molten rock
cools.
When rock hardens, the iron orients itself
to Earth’s magnetic field at that time
The Geomagnetic Time Scale
Studies in the 1960’s revealed pattern of
magnetic reversals over time.
Magnetic reversals are changes in the
Earth’s magnetic field.
Data from these reversals allowed
scientists to construct a geomagnetic time
scale.
The Investigation
Scientists towed magnetometers behind
ships to measure the magnetic field of the
ocean floor.
Scientists noticed alternating + and –
values for the magnetic field readings.
Magnetic Symmetry
Scientists noticed that the + and –
magnetic bands were symmetrical,
originating from the ocean ridges.
Scientists also discovered basaltic lava
flows on land matching these magnetic
reversals.
Age of the ocean floor was ascertained
from this data.
Isochronic Mapping
Isochron is a line on a map that connects
points that have the same age.
Young rock is found at mid-ocean ridges,
while older rock is found further away near
deep sea trenches.
Seafloor Spreading
After all this data, Harry Hess proposes
the theory of seafloor spreading.
Theory proposes that new ocean floor is
created at the ridges and destroyed at the
trenches.
Magma rises to the surface and hardens
at the ocean ridges.
Consecutive bands of seafloor are created
in this way.
Wegener’s Theory Revisited
Wegener’s theory of continental drift did
not account for the forces in action.
Seafloor spreading completes the picture
and describes the forces that shape the
drifting of the continents.
Continents are like groceries on the
conveyer belt at the checkout line; they
don’t push through the ocean floor, they
ride on top of it as the ocean floor moves.
Theory of Plate
Tectonics
Text ref. Ch.17.3 (pg. 455 – 459)
Objectives
Explain the theory of plate tectonics.
Compare and contrast the three types of
plate boundaries and the features
associated with each.
The Theory
Why do earthquakes happen?
Why do volcanoes erupt?
Why do mountains exist?
Plate Tectonics is a theory that states the
Earth’s crust and rigid upper mantle are
broken into enormous slabs called plates.
Each plate moves at a particular rate, and
measured using a system of satellites and
receivers
Plate Boundaries
Tectonic plates interact at places called
plate boundaries.
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Divergent
Convergent
Transform
Each boundary has certain geologic
characteristics.
Divergent Boundaries
Places where two tectonic plates are moving
apart.
Most are found on the seafloor as ocean ridges.
Formation of new ocean crust accounts for
volcanism along these locations.
A rift valley is formed when a divergent plate
boundary is located on continental crust.
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Ex. East African Rift Valley
Convergent Boundaries
Places where two tectonic plates are
moving toward each other.
Crust is destroyed at these boundaries.
Three types:
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Oceanic converging with oceanic crust
Oceanic converging with continental crust
Continental converging with continental crust
Subduction and Convergence
Occurs when a cooler, denser plate
descends below a less dense plate.
In the event of oceanic and oceanic plate
convergence, a trench is formed.
Subducted plate melts and becomes
magma, rises to the surface, and erupts as
a volcano.
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Ex. Mariana Trench and islands
Ex. Aleutian Trench and islands
Oceanic-Continental Convergence
Denser oceanic plate subducts under less
dense continental plate.
Forms mountain ranges of volcanoes.
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Ex. Peru-Chile Trench and Andes Mountains
Continental-Continental Plate
Convergence
Occurs when two continental plates
collide.
Produces large mountain ranges with little
volcanism.
Since continental crust is buoyant, crust
crumples rather than subducts.
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Ex. Himalayas
Transform Boundaries
Place where two plates slide horizontally past
each other.
Crust is deformed or fractured at these
boundaries, but not created or destroyed.
Characterized by long faults, hundreds of km in
length.
Earthquakes.
Most transform boundaries are not found on
continents.
Exception: San Andreas Fault