Transcript Ch19_PlateTectonics

Plate Tectonics
Continental drift: An idea
before its time
Alfred Wegener
• Proposed hypothesis in 1915
• Published The Origin of
Continents and Oceans
Continental drift hypothesis
• Supercontinent Pangaea began breaking
apart about 200 million years ago
Pangaea approximately 200
million years ago
What would the world have been like as a supercontinent?
A)
Different climate - continental interiors hotter and
drier (like Mongolia today)
B)
Lower species diversity due to connectivity of
continents and fewer shorelines
C)
Differences in marine environments - less shelf area
D)
All of the above
What would the world have been like as a supercontinent?
A)
Different climate - continental interiors hotter and
drier (like Mongolia today)
B)
Lower species diversity due to connectivity of
continents and fewer shorelines
C)
Differences in marine environments - less shelf area
D)
All of the above
Watch Pangaea Breakup
200 Million Years Ago to present
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Which continent moves the farthest during the breakup of Pangaea?
A)
B)
C)
D)
E)
North America
South America
Africa
India
Australia
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Which continent moves the farthest during the breakup of Pangaea?
A)
B)
C)
D)
E)
North America
South America
Africa
India
Australia
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Ocean opens between rigid plates
But oceans are parts of plates and not indepen
Of continents…
Continental drift hypothesis
• Continents "drifted" to present positions
Evidence used in support of continental
drift hypothesis
• Fit of continents
• Fossil evidence
• Rock type and mountain belts
• Paleoclimatic evidence
The great debate
Objections to drift hypothesis
• Inability to provide a mechanism capable of
moving continents across globe
• Wegner suggested that continents broke through
the ocean crust, much like ice breakers cut
through ice
• Geophysics suggests this was unlikely
Matching of
mtn ranges
on continents
Once connected in
one belt, now
separated…
Paleoclimatic evidence for
Continental Drift
Dismembering
of an ancient ice
sheet centered
over the south
pole
Why is there no evidence for a dismembered ice sheet
at the north pole at this time also?
A) Continents were not
located over the north pole
at this time
B) An ice sheet only
formed over the south
pole and not the north
C) Global warming from
greenhouse gas emissions from
countries at northern latitudes
melted the northern ice sheet
D) Solar energy in the
northern hemisphere was
higher then due to sunspot
activity
Why is there no evidence for a dismembered ice sheet
at the north pole at this time also?
A) Continents were not
located over the north pole
at this time
B) An ice sheet only
formed over the south
pole and not the north
C) Global warming from
greenhouse gas emissions from
countries at northern latitudes
melted the northern ice sheet
D) Solar energy in the
northern hemisphere was
higher then due to sunspot
activity
The great debate
Continental drift and the scientific
method
• Wegner’s hypothesis was correct in
principle, but contained incorrect details
• For any scientific viewpoint to gain wide
acceptance, supporting evidence required
Continental drift and paleomagnetism
Renewed interest in continental drift
came from rock magnetism
Magnetized minerals in rocks
• Show direction to Earth’s magnetic poles
• Provide a means of determining their
original latitude
What is the orientation of
the Earth’s current
magnetic field in
Boulder, CO?
A) North, inclined upward
at about 45 deg
B) North, inclined down at
about 45 deg
C) North, inclined down at
about 90 deg
D) North, inclined upward
at about 90 deg
E) None of the above
What is the orientation of
the Earth’s current
magnetic field in
Boulder, CO?
A) North, inclined upward
at about 45 deg
B) North, inclined down at
about 45 deg
C) North, inclined down at
about 90 deg
D) North, inclined upward
at about 90 deg
E) None of the above
Continental drift and paleomagnetism
Polar wandering
• Apparent movement of magnetic poles in
volcanic rocks indicates continents move
• Shows Europe was closer to equator when
coal-producing swamps existed
Continental drift and paleomagnetism
Polar wandering
• Polar wandering curves for North
America and Europe have similar paths
but are separated by about 24 of
longitude
– Differences between paths reconciled if
continents next to one another
Apparent polar-wandering paths
for Eurasia and North America
The scientific revolution begins
During the 1950s and 1960s technological
strides permitted extensive mapping of
the ocean floor
Seafloor spreading hypothesis was
proposed by Harry Hess in the early
1960s
The scientific revolution begins
Geomagnetic reversals
• Earth's magnetic field periodically
reverses polarity – north magnetic pole
becomes south magnetic pole, vice versa
• Dates when polarity of Earth’s magnetism
changed were determined from lava flows
Geomagnetic reversals
• Geomagnetic reversals are recorded in the
ocean crust
• In 1963 Fred Vine and D. Matthews tied
the discovery of magnetic stripes in the
ocean crust near ridges to Hess’s concept
of seafloor spreading
Paleomagnetic reversals recorded
by basalt at mid-ocean ridges
Maps of Magnetic Stripes in Oceanic Crust
Why are seafloor magnetic anomalies
symmetrically oriented about ridges?
A) Rates of plate motions on the two divergent
plates are the same for both plates
B) Magma is intruded into the centers of
spreading ridges and then pulled apart
in the middle
C) Convective cells in the mantle transfer
heat and move rock straight up beneath
ridges and then move outward
D) All of the above
Why are seafloor magnetic anomalies
symmetrically oriented about ridges?
A) Rates of plate motions on the two divergent
plates are the same for both plates
B) Magma is intruded into the centers of
spreading ridges and then pulled apart
in the middle
C) Convective cells in the mantle transfer
heat and move rock straight up beneath
ridges and then move outward
D) All of the above
Structure of
Juan de Fuca Plate
Age of Oceanic Crust
Why is there no oceanic crust older that 180Ma?
A) Plate tectonics didn’t occur prior to this
B) Spreading ridges ran much more slowly in the past
C) Older oceanic crust has all been subducted
D) Older oceanic crust probably exists somewhere, we just
haven’t found it under the ocean yet.
Why is there no oceanic crust older that 180Ma?
A) Plate tectonics didn’t occur prior to this
B) Spreading ridges ran much more slowly in the past
C) Older oceanic crust has all been subducted
D) Older oceanic crust probably exists somewhere, we just
haven’t found it under the ocean yet.
Geomagnetic reversal
• Paleomagnetism was the most convincing
evidence to support concepts of
continental drift and seafloor spreading
Plate tectonics: The new paradigm
More encompassing theory than
continental drift
Mix of ideas that explained motion of
Earth’s lithosphere by subduction and
seafloor spreading
Plate tectonics: The new paradigm
Earth’s major plates
• Associated with Earth's strong, rigid
outer layer
– Known as the lithosphere
– Consists of uppermost mantle and overlying
crust
– Overlies a weaker region in the mantle called
the asthenosphere
Basal tractions drive plate motions
Earth’s major plates
• Seven major lithospheric plates
• Plates are in motion and change in shape
and size
• Largest plate is the Pacific plate
• Several plates include an entire continent
plus a large area of seafloor
Why did Pangaea break up?
A) Supercontinents act as thermal blankets for heat escaping from the
mantle, which leads to extension and breakup
B) The Earth was hit by a giant asteroid at the time, pushing the
plates apart
C) Stresses caused by orbital forcing
D) Conservative politics in Russia caused it to slide towards the right,
while liberals in North America caused it to slide to the left
Why did Pangaea break up?
A) Supercontinents act as thermal blankets for heat escaping from the
mantle, which leads to extension and breakup
B) The Earth was hit by a giant asteroid at the time, pushing the
plates apart
C) Stresses caused by orbital forcing
D) Conservative politics in Russia caused it to slide towards the right,
while liberals in North America caused it to slide to the left
Earth’s Tectonic Plates
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Earth’s major plates
• Plates move relative to each other at a
very slow but continuous rate
– Average about 5 centimeters (2 inches) per
year
– Cooler, denser slabs of oceanic lithosphere
descend into the mantle
– Motion defined by rotation around a pole
Plate boundaries
• Interactions among individual plates
occur along their boundaries
• Types of plate boundaries
– Divergent plate boundaries
– Convergent plate boundaries
– Transform fault boundaries
Types of Plate Margins
Plate boundaries
• Each plate bounded by combination of all
three boundary types
• New plate boundaries created in response
to changes in forces acting on rigid slabs
Divergent plate boundaries
Most are located along the crests of
oceanic ridges
Oceanic ridges and seafloor spreading
• seafloor is elevated forming oceanic ridges
Oceanic ridges and seafloor spreading
• Seafloor spreading occurs along the oceanic ridge
system
Spreading rates and ridge topography
• Ridge systems exhibit topographic differences
• Topographic differences are controlled by
spreading rates (see map of age of oceanic crust
for width of ridges relative to their age)
Which ridge spreads the fastest?
A)Mid Atlantic ridge
B) East Pacific Rise
C) Australia-Antarctic Ridge
D) Arctic Ridge
Which ridge spreads the fastest?
A)Mid Atlantic ridge
B) East Pacific Rise
C) Australia-Antarctic Ridge
D) Arctic Ridge
Divergent boundaries are located
mainly along oceanic ridges
Divergent boundaries in Continents
Continental rifts
• Splits landmasses into two or more
smaller segments
Divergent boundaries
Continental rifts
• Example includes East African rifts
• Produced by extensional forces acting on
the lithospheric plates
• Not all rift valleys develop into spreading
centers
• Otherwise Nevada would be an ocean!
The East African Rift
Development of Continental Rift into Ocean Basin
What is the geologic evidence here (New Mexico)
for rifting?
A) volcanoes
B) normal faults
C) calderas
D) basaltic lava flows
E) all of the above
Convergent plate boundaries
Old portions of oceanic plates are
returned to the mantle
• Surface expression of descending plate is
an ocean trench
• Called subduction zones
• Average angle at which oceanic
lithosphere descends into the mantle is
about 45
All have same basic characteristics, but can
have highly variable features
Types of convergent boundaries
• Oceanic-continental convergence
– Denser oceanic slab sinks into the
asthenosphere
– Bathymetry marked by trench
– As plate descends, partial melting of mantle
rock makes basaltic or andesitic magmas
– Volcanic mountains associated with
subduction of oceanic lithosphere are called
continental volcanic arcs (Andes and
Cascades)
Types of Arcs
Types of convergent boundaries
• Oceanic-oceanic convergence
– When two oceanic slabs converge, one
descends beneath the other
– Often forms volcanoes on the ocean floor
– If the volcanoes emerge as islands, a volcanic
island arc is formed (Japan, Aleutian islands,
Tonga islands)
Fly through the Marianas Trench: Which direction is the camera flying?
A) To the West, B) To the East, C) To the North, D) To the South
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Fly through the Marianas Trench: Which direction is the camera flying?
A) To the West, B) To the East, C) To the North, D) To the South
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Continental Arc
Oceanic Arc
Continental
Collision
Types of convergent boundaries
• Continental-continental convergence
– Continued subduction brings continents together
– Less dense, buoyant continental lithosphere does
not subduct
– Result is a collision between two continental
blocks
– Process produces mountains (Himalayas, Alps,
Appalachians)
The collision of India and Asia
produced the Himalayas
Transform fault boundaries
Third type of plate boundary
Plates slide past one another and no new
lithosphere is created or destroyed
Transform faults
• Most join two segments of a mid-ocean ridge as
parts of linear breaks in the oceanic crust known
as fracture zones
• Accommodate simultaneous movement of offset
ridges
Transform faults accommodate
movement on offset ridge segments
ridge
Map view of
ridge-transform-ridge
system
transform
ridge
ridge
Map view of
ridge-transform-ridge
system
transform
What is the sense of slip on the
transform fault?
A) Left lateral strike slip
B) Right lateral strike slip
C) Thrust
D) Normal
E) Oblique
ridge
ridge
Map view of
ridge-transform-ridge
system
transform
?
left lateral fault?
ridge
ridge
Map view of
ridge-transform-ridge
system
transform
Right lateral strike slip!
ridge
ridge
Map view of
ridge-transform-ridge
system
transform
fracture zone
fracture zone
Right lateral strike slip!
ridge
Why aren’t there earthquakes on the
transform fault away from the ridge?
ridge
A)
B)
C)
D)
Volcanic rocks weld plates together
Plates are moving at same rate
Plates are hotter here
All of the above
transform
fracture zone
fracture zone
Right lateral strike slip!
ridge
Why aren’t there earthquakes on the
transform fault away from the ridge?
ridge
A)
B)
C)
D)
Volcanic rocks weld plates together
Plates are moving at same rate
Plates are hotter here
All of the above
transform
fracture zone
fracture zone
Right lateral strike slip!
ridge
Testing the plate tectonics model
Plate tectonics and earthquakes
• Plate tectonics model accounts for the
global distribution of earthquakes
– Absence of deep-focus earthquakes along the
oceanic ridge is consistent with tectonic theory
– Deep-focus earthquakes associated with
subduction zones
– The pattern of earthquakes along a trench
provides method to track plate's descent
Deep-focus earthquakes occur
along convergent boundaries
Earthquakes near Japan trench
Locations of E-quake
Hypocenters
Evidence from ocean drilling
• Most convincing evidence confirming
seafloor spreading comes from drilling
directly into ocean-floor sediment
– Age of deepest sediments
– Thickness of ocean-floor sediments verifies
seafloor spreading
Hot spots
• Caused by rising plumes of mantle
material
• Volcanoes form over them (Hawaiian
Island chain)
• Mantle plumes are long-lived structures
and originate at great depth, perhaps at
core-mantle boundary
The Hawaiian Islands form over
stationary hot spot
What caused hot spot Hawaii’s
track to change direction
at ~45Ma?
A) Location of hot spot in
mantle moved to north.
B) Direction of plate motion
changed.
C) Two different hot spots
formed the bent pattern
D) Northern section of track
is actually an old, dead
spreading ridge.
What caused hot spot Hawaii’s
track to change direction
at ~45Ma?
A) Location of hot spot in
mantle moved to north.
B) Direction of plate motion
changed.
C) Two different hot spots
formed the bent pattern
D) Northern section of track
is actually an old, dead
spreading ridge.
No one driving mechanism accounts for all
major facets of plate tectonics
Researchers agree that convective flow in 2,900
km-thick mantle is main driving force of plate
tectonics (by basal traction)
Other mechanisms generate forces that
contribute to plate motion
• Slab-pull
• Ridge-push
Importance of plate tectonics
Provides a unified explanation of Earth’s major
surface processes, especially oceans
Within framework of plate tectonics, we find
explanations for the distribution of
earthquakes, volcanoes, and mountains
Plate tectonics provides explanations for
distribution/evolution of plants and animals
and climate record