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Chapter 2
Plate Tectonics:
A Scientific
Revolution Unfolds
Continental Drift: An Idea
Before its Time

Alfred Wegener
• First proposed his continental drift
hypothesis in 1915 before the theory of
Plate Tectonics

Continental drift hypothesis
• Supercontinent called Pangaea began
breaking apart about 200 million years ago
• Continents "drifted" to present positions
Continental Drift: An Idea
Before its Time, cont’d
Evidence used in support of
continental drift hypothesis
Fit of the continents
 Fossil evidence
 Rock type and structural similarities
 Paleoclimatic evidence

The Great Debate
Objections to the continental drift
hypothesis



Lack of a mechanism for moving continents
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
Continental Drift and
Paleomagnetism
Renewed interest in continental drift
initially came from rock magnetism
 Magnetized minerals in rocks

• Declinations show the direction to Earth’s
magnetic poles when rocks formed
• Inclinations provide a means of
determining the rocks latitude of origin and
how much it moved north or south since it
was created.
Continental Drift and
Paleomagnetism

Polar wandering
• The apparent movement of the magnetic
poles indicates that the continents have
moved
• Indicates Europe was much closer to the
equator when coal-producing swamps
existed
Continental Drift and
Paleomagnetism, cont’d

Polar wandering
• Curves for North America and Europe have
similar paths but are separated by about
24 of longitude
• Differences between the paths can be reconciled
if the continents are placed next to one another
A 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

• Oceanic crust created and spread apart at
shallow mid-oceanic ridges
• Oceanic crust destroyed at deep trenches
A Scientific Revolution
Begins, cont’d

Geomagnetic reversals
Earth's magnetic field periodically
reverses polarity – the north magnetic
pole becomes the south magnetic pole,
and vice versa. This flips the remnant
magnetic inclination of the rocks 180
degrees.
 Dates of geomagnetic reversals
determined from lava flows.

A Scientific Revolution
Begins, cont’d

Geomagnetic reversals



Geomagnetic reversals are recorded in the ocean
crust as alternating linear stripes of high and low
magnetic anomalies (highs for normal polarity and
lows for reversed polarity).
These linear anomalies parallel the mid-ocean
ridges and are symmetrical with respect to them.
In 1963 Vine and Matthews tied the discovery of
these “magnetic stripes” in the ocean crust near
mid-ocean ridges to Hess’s concept of seafloor
spreading.
A Scientific Revolution
Begins, cont’d

Earthquakes and Subduction Zones



Wadati and Benioff mapped out the depths
and locations of earthquakes along trenches
and beneath arcuate trends of volcanoes that
parallel them.
They found shallower earthquakes closer to
the trench, and progressively deeper
earthquakes towards the volcanic arcs and
beyond them.
Postulated that the oceanic crust and uppermost mantle is sinking or subducting down
into the rest of the mantle along these zones
and is being destroyed.
Plate Tectonics: The
New Paradigm

Earth’s major tectonic plates
• Earth is divided into seven major
lithospheric plates and several other minor
plates that are relatively rigid.
• Plates are composed of the oceanic and/or
continental crust and the uppermost
mantle.
• Several plates include an entire continent
plus a large area of seafloor.
Plate Tectonics: The
New Paradigm, cont;d

Earth’s major tectonic plates cont.
• Plates are in constant slow motion with
respect to one another and are continually
changing in shape and size.
• Earthquakes, deformation and volcanic
eruptions occur where these plates meet.
• The interior of plates generally have fewer
earthquakes, less deformation and less
volcanic activity compared to the margins.
Plate Tectonics: The
New Paradigm, cont’d

Plate boundaries
• Interactions between individual plates occur
along their boundaries

Types of plate boundaries
• Divergent plate boundaries (constructive
margins formed by extension)
• Convergent plate boundaries (destructive
margins formed by compression)
• Transform fault boundaries (conservative
margins formed by side-to-side shear)
Divergent Plate Boundaries


Most are located along the crests of
oceanic ridges
Oceanic ridges and seafloor spreading
• The seafloor is elevated along well-developed
divergent plate boundaries, forming oceanic
ridges

Geologic features include
• crustal and lithospheric thinning
• rift zones formed by extensional faulting
• volcanic activity
• shallow earthquakes
Divergent Plate Boundaries,
cont’d
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

Convergent Plate
Boundaries

Older portions of oceanic plates are
returned to the mantle or subducted
in these destructive plate margins
• Surface expression of the descending plate
is an ocean trench
• Oceanic plate moves downward along
subduction zones
• Geologic features include
• volcanic arcs
• shallow-deep earthquakes
• mountain belts
Types of Convergent Plate
Boundaries

Oceanic-continental convergence
• Denser oceanic slab sinks into the
asthenosphere
• Along the descending plate partial melting
of mantle rock generates magma
• Resulting volcanic mountain chain is called
a continental volcanic arc (Andes and
Cascades)
Types of Convergent Plate
Boundaries, cont’d

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)
Types of Convergent Plate
Boundaries, cont’d

Continental-continental convergence
• Continued subduction can bring two
continents together
• Less dense, buoyant continental lithosphere
does not subduct
• Resulting collision between two continental
blocks produces mountains (Himalayas,
Alps, Appalachians)
Transform Fault Boundaries


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Plates slide past one another and no new
lithosphere is created or destroyed
Most join two segments of a mid-ocean ridge
along breaks in the oceanic crust known as
fracture zones
A few (e.g. the San Andreas fault) cut through
continental crust
Geologic Features include

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strike slip faults
lack of volcanic activity
shallow earthquakes
inactive fracture zones which extend outward from oceanic
transform faults (active fracture zones) and separate
lithosphere of different age and density
Testing the Plate
Tectonics Model
Hot spots and mantle plumes
• Caused by rising plumes of heat or hot
mantle material
• Volcanoes and calderas can form over them
(Hawaiian Island chain, Yellowstone
caldera)
• Mantle plumes
• Long-lived structures that are fixed in one
subsurface location while the plates drift over
them
• Some originate at great depth, perhaps at the
core-mantle boundary
Measuring Plate Motion

Paleomagnetism and plate motions
• Paleomagnetism stored in rocks on the
ocean floor provides a method for
determining plate motions
• Both the direction and rate of seafloor
spreading can be established
Measuring Plate Motion,
cont’d

Measuring plate velocities from a
fixed reference frame in space
• Accomplished by establishing exact
locations on opposite sides of a plate
boundary and measuring relative motions
• Two methods are used
• Very Long Baseline Interferometry (VLBI)
• Global Positioning System (GPS)
What Drives Plate Motions
Many, but not all researchers agree
that convective flow in the mantle is
the basic driving force of plate
tectonics
• Theory I: Causes elevation and density
differences due to temperature variations,
which create gravitational forces
• Theory II: Causes frictional drag on the
plates above the convecting mantle
End of Chapter 2