Download Continental drift: An idea before its time Pangaea approximately 200

Chapter 2 Plate Tectonics:
A Scientific Revolution Unfolds
Continental drift:
An idea before its time
• Alfred Wegener
Figure 2.3
• First proposed his
continental drift
hypothesis in 1915
• Published The Origin of
Continents and Oceans
• Continental drift
hypothesis
• Supercontinent called
Pangaea began breaking
apart about 200 million
years ago
Banff National Park, Alberta, Canada
Pangaea approximately 200
million years ago
Continental drift: An idea
before its time
• Continental drift hypothesis
• Continents "drifted" to present positions
• Evidence used in support of continental
drift hypothesis
• Fit of the continents
• Fossil evidence
• Rock type and structural similarities
• Paleoclimatic evidence
Figure 2.2
Figure 2.4
Matching
mountain
ranges
Figure 2.6
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The great debate
Objections to the continental drift hypothesis
Paleoclimatic
evidence
c.f. 2.7
• Data from the forms and materials of the Ocean Floor:
• extensive mapping of the ocean floor during the 1950s and 1960s
• ‘Seafloor spreading hypothesis,’ Harry Hess in the early 1960s
• Lack of a mechanism for moving continents (a
good hypothesis must be consistent with the
basic principles of physics)
• Wegener incorrectly suggested that continents
broke through the ocean crust, much like ice
breakers cut through ice
• Strong opposition to the hypothesis from all
areas of the scientific community (particularly
among American scientists)
Data from Earth’s magnetic field
• Renewed interest in
Figure 2.8
continental drift initially
came from rock
magnetism
• Magnetized minerals
in rocks
• Show the direction
to Earth’s magnetic
poles
• Provide a means of
determining their
latitude of origin
Note abyssal plains, spreading ridges, seamounts, and
trenches… the ocean floor is YOUNG (see p. 46).
Figure 2.9 and 2.10
Inclinations of
Earth’s magnetic
field with latitude
1.17
Magnetic Field Data: ‘Apparent’ Polar wandering
The apparent movement of the magnetic poles illustrated
in magnetized rocks indicates that the continents have
moved
Figure 2.11
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Magnetic Field Reversals
Geomagnetic reversals
• Earth's magnetic
field periodically
reverses polarity –
the north magnetic
pole becomes the
south magnetic
pole, and vice versa
2.8, 2.13, 2.14
Geomagnetic reversals are recorded in the ocean crust
•In 1963 Vine and Matthews tied the discovery of magnetic
stripes in the ocean crust near ridges to Hess’s concept of
seafloor spreading
Figure 2.17
2.16
Figure 2.15
Plate Tectonic Theory arose as an explanation of all the
data used to propose the (incomplete) hypotheses of
continental drift and sea floor spreading.
Figure 2.12
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Plate Tectonic Theory: Earth’s lithosphere is divided into
seven major plates (and a number of smaller plates) that
move relative to each other and overlie the
asthenosphere.
Earth’s tectonic plates, with divergent,
convergent, and transform boundaries.
Figure 2.12
Figure 2.20
Divergent plate boundaries
• Most are located
along the crests of
oceanic ridges
• Oceanic ridges and
seafloor spreading
2.21
Divergent plate boundaries
• Continental rifting
• Splits landmasses
into two or more
smaller segments
along a continental
rift
• Examples include
the East African
rift valleys and the
Rhine Valley in
northern Europe
• Produced by
extensional forces
acting on
lithospheric plates
• Along welldeveloped
divergent plate
boundaries, the
seafloor is
elevated
forming oceanic
ridges
Convergent plate boundaries
Figure 2.22
Figure 2.24
• Older portions of oceanic
plates are returned to the
mantle in these destructive
plate margins
• Surface expression of the
descending plate is an
ocean trench
• Also called subduction
zones
• Average angle of
subduction = 45°
Figure 2.23
4
Transform fault boundaries
• Plates slide past one
another and no new
lithosphere is created or
destroyed
• Transform faults
Figure 2.25
Figure 2.26
• Plates slide past one another
and no new lithosphere is
created or destroyed
• Transform faults
• Most join two segments of
a mid-ocean ridge along
breaks in the oceanic crust
known as fracture zones
• A few (the San Andreas
fault and the Alpine fault
of New Zealand) cut
through continental crust
• Most join two
segments of a midocean ridge along
breaks in the oceanic
crust known as
fracture zones
• A few (the San
Andreas fault and the
Alpine fault of New
Zealand) cut through
continental crust
Testing plate tectonics: birth of
a new theory
• Evidence from ocean drilling
• Some of the most convincing
evidence confirming seafloor
spreading has come from drilling
directly into ocean-floor sediment
– Age of deepest sediments
– Thickness of ocean-floor
sediments verifies seafloor
spreading
Transform fault boundaries
Testing the plate tectonics model
• Hot spots and mantle
plumes
Global Marine, a Hughes company,
commissioned the Glomar Challenger
and Glomar Explorer in 1973.
• Caused by rising
plumes of mantle
material
• Volcanoes can form
over them (Hawaiian
Island chain)
• Mantle plumes
– Long-lived structures
– Some originate at great
depth, perhaps at the
mantle-core boundary
‘Box 2.4’ and fig. 2.27
Figure 2.28
The JOIDES Resolution
Measuring plate
motion
What drives plate motions?
•Researchers agree that convective flow in the mantle is the basic
driving force of plate tectonics
–Very Long Baseline Interferometry (VLBI)
–Global Positioning System (GPS)
Figure 2.30
Figure
2.29, 2.30
• Forces that drive plate motion
• Slab-pull
• Ridge-push
• Slab suction
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• Models of plate-mantle
convection
• Any model must be
consistent with
observed physical
and chemical
properties of the
mantle
• Models
– Layering at 660
kilometers
– Whole-mantle
convection
– Deep-layer model
Figure 2.32
Importance of plate tectonics
• The theory provides explanations for
• Earth’s major surface processes;
• The geologic distribution of earthquakes,
volcanoes, and mountains;
• The distribution of ancient organisms, fossil fuels
and mineral deposits;
Plate tectonics is the central unifying theory
of Earth’s geology, and provides context for
interpreting Earth’s history… including
climate change.
Matter and Minerals
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