Download PLATE TECTONICS

The Essential Earth
Chapter 3:
PLATE TECTONICS:
The Unifying Theory
Copyright © 2007 by W. H. Freeman and Company
The Essential Earth
Chapter 3:
PLATE TECTONICS:
The Unifying Theory
Copyright © 2007 by W. H. Freeman and Company
Map of the Earth's Tectonic Plates
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• Oceanic vessels at sea still obtain the highest
quality bathymetry data possible.
• Bruce Heezen and Marie Tharpe were marine
geologists instrumental in mapping and making
some of the first comprehensive world maps.
Plate Tectonics
• What are the 3 types of plate boundaries ?
1. Divergent
2. Convergent
3. Transform (strike-slip)
movie
Divergent Boundaries
Oceanic Plate Separation
Volcanoes and earthquakes concentrate.
ica
r
e
Am
h
t
Nor Plate
n
MidAtlantic
Ridge
Eur
asia
n
Pla
te
• Plates form at mid-ocean spreading centers where rock melts,
cools, and crystallizes
• Each new plate moves symmetrically away from the spreading
center increasing in thickness as it cools
Divergent Boundaries
Continental Plate Separation
Parallel valleys; volcanoes and earthquakes.
East African
Rift Valley
ate
l
P
an
c
i
r
f
A
Soma
li Sub
plate
Divergent Boundaries
• Mid-Atlantic Spreading Center
• Formation of Iceland (new continent)
movie
Convergent Boundaries
Ocean-Ocean Convergence
Mariana Islands
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n
i
p
p
i
Phil
e
Plat
Marianas Trench
Pacif
ic Pla
te
Convergent Boundaries
Ocean-Ocean Convergence
Deep-sea trench; volcanic island arc.
Mariana Islands
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n
i
p
p
i
Phil
e
Plat
Marianas Trench
Pacif
ic Pla
te
Convergent Boundaries
Ocean-Continent Convergence
A volcanic belt of
mountains forms.
Andes
Mountains
Peru-Chile Trench
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P
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c
N az
South
American
Plate
Convergent Boundaries
movie
Continent-Continent Convergence
Crust crumbles, creating high
mountains and a wide plateau.
Himalaya
Ind
ate
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P
n
a
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l
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r
ian-Aust
Tibetan
Plateau
Main
thrust
fault
Eurasian
Plate
Mountain Belts Formed During Convergence
• Mountain belts are chains of mountain ranges
1000s of km long
– Located along the edges of continents
• As mountains grow higher and steeper, erosion rates
increase (from running water and ice )
Continental Cratons
• Ancient mountain belts have
eroded nearly flat to form the
stable core of a continent (craton
or shield)
• Every continental plate has a
central, old, craton.
Volcanic Chains
movie
Transform-Fault Boundaries
Mid-Ocean Ridge Transform Fault
Spreading centers offset.
Euras
i an P l
ate
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t
a
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P
n
ica
r
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m
A
North
Transform-Fault Boundaries
Continental Transform Fault
Offset continental crust.
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l
P
c
i
Pacif
North
Ameri
can Pl
ate
Transform-Fault Boundaries
Continental Transform Fault – San Andreas
As plates
move past
each other...
…creek beds are
offset
Transform-Fault Boundaries
Continental Transform Fault – San Andreas
As plates
move past
each other...
San
Francisco
n
Sa
nd
A
…creek beds are
offset
re
as
Los Angeles
fau
lt
Magnetic mapping can measure the rate of seafloor spreading
An oceanic survey over the Reykjanes Ridge, part of the Mid-Atlantic
Ridge southwest of Iceland, showed an oscillating pattern of
magnetic field strength. This figure illustrates how scientists worked
out the explanation of this pattern.
A sensitive magnetometer
records magnetic anomalies,…
Mid-Atlant
ic
High inten
sity
Low intens
ity
Ridge
Magnetic mapping can measure the rate of seafloor spreading
An oceanic survey over the Reykjanes Ridge, part of the Mid-Atlantic
Ridge southwest of Iceland, showed an oscillating pattern of
magnetic field strength. This figure illustrates how scientists worked
out the explanation of this pattern.
A sensitive magnetometer
records magnetic anomalies,…
…alternating bands of high
and low magnetism.
Iceland
Mid-Atlant
ic
High inten
sity
Low intens
ity
Ridge
MidAtlantic
Ridge
Symmetrical bands on both sides.
Why?
Mid-ocean ridge
(original)
Seafloor Magnetic Anomalies
• Heat at spreading
centers melt mantle rocks
• Melting raises
temperatures above the
“Curie Temperature”
4.0
Million 3.0
years ago
• The magnetic alignment
of minerals is released
2.0
Ocean
crust
today
2.5
3
.
n 3
o
i
l
l
mi
5.0 rs old
yea
0.7
0
0.7
3.3
5
.
2
5.0
• When the melt cools to
form tectonic plates, the
new rocks adopt the
magnetic alignment of the
Earth's current magnetic
field.
• The plate spreads away
and starts the process
again.
Magnetic Reversal Record
Subchrons
5.0 Ma
4.0
Gilbert
reversed chron
3.0
Gauss
normal chron
2.0
1.0
Matuyama
reversed chron
Present
Brunhes
normal chron
Review: Three Types of Plate Boundaries
But how do we
know that plates
move at all ?
Transform
(strike-slip)
Convergent
(subduction)
Divergent
(spreading)
Early Case for Continental Drift:
continental “shape” and “fit”
225 My ago
135 My ago
• Puzzle-piece fit of
coastlines of Africa and
South America has long
been known
Today
65 My ago
Early Case for Continental Drift: fossils
Early Case for Continental Drift: fossils
•In early 1900s, Alfred Wegner noted South America, Africa, India,
Antarctica, and Australia have almost identical rocks and fossils
– Glossopteris (plant),
Lystrosaurus and
Cynognathus (animals)
fossils on five
continents
– Mesosaurus (reptile) in
Brazil and South Africa
only
Early Case for
Continental Drift:
glaciers
• Wegner reassembled continents
into the supercontinent Pangaea
• Late Paleozoic glaciation patterns
on southern continents explained
reconstruction into (Pangaea)
Gondwanaland
Early Case for Continental Drift
• Continental Drift hypothesis initially rejected
– Wegener could not come up with viable driving force
– continents should not be able to “plow through” sea floor rocks
The Earth's
Magnetic Field
Can Give Us Clues
Continental Drift: Paleomagnetism
• Paleomagnetism magnetic minerals
align and dip with Earth's internal
magnetic field lines
– Steeper dip angles indicate rocks formed
closer to the magnetic poles
• Rocks with increasing age point to
pole locations increasingly far from
present magnetic pole positions
Paleomagnetism and
Continental Drift Revived
• Apparent polar wander curves for
different continents indicate plate
movement !
• Wegner was right!
• Plate Tectonics widely
accepted by 1960's
Plate Tectonics and the Scientific Method
How does an idea become a theory ?
* When Wegner first suggest the idea of Plate Tectonics what was it ?
a) theory b) hypothesis c) data d) proof
* What data was later acquired to test this (name 5 lines of evidence) ?
* When did plate tectonics become an accepted theory ?
(What discovery proved the original predictions ?)
Check what you've learned!
1. What are the 3 types of plate boundaries ?
-Describe the motion of each.
-Give examples of each on the Earth.
2. What were the 3 original lines of evidence
to argue for plate tectonic motion on the Earth ?
- What was the 4th and final piece of evidence
that proved this to be true ?
- What are some new observations we use today
that proves plate motion
and gives precise plate velocities ?
FIGURE 3.7 Earth's surface is a mosaic of 13 major plates of rigid lithosphere, as well as a number of
smaller plates, that move slowly over the ductile asthenosphere. Only one of the smaller plates, the Juan
de Fuca Plate, off the west coast of North America, is labeled on this map. The arrows show the relative
motion of two plates at a point on their boundary.The numbers next to the arrows give the relative plate
speeds in millimeters per year. [Plate boundaries by Peter Bird, UCLA.]
FIGURE 3.5 Marie Tharp and Bruce Heezen inspect a map of the seafloor. Their discovery of active rifts
on the mid-ocean ridges provided important evidence for seafloor spreading. [The Earth Institute at
Columbia University.]
FIGURE 3.6 The Pacific Ring of Fire, with its active volcanoes (large red circles) and frequent
earthquakes (small black dots), marks plate boundaries where oceanic lithosphere is being recycled.
FIGURE 3.9 The Mid-Atlantic Ridge, a divergent plate boundary, rises above sea level in Iceland. This
cracklike rift valley, filled with newly formed volcanic rocks, indicates that plates are being pulled apart.
[Gudmundur E. Sigvaldason, Nordic Volcanological Institute.]
FIGURE 3.2 Fossils of the freshwater reptile Mesosaurus, 300 million years old, are found in South
America and Africa and nowhere else in the world. If Mesosaurus could swim across the South Atlantic
Ocean, it should have been able to cross other oceans and should have spread more widely. The
observation that it did not suggests that South America and Africa must have been joined 300 million
years ago. [After A. Hallam, “Continental Drift and the Fossil Record,” Scientific American (November
1972): 57–66.]