Plate Tectonics
Chapter 16
Essentials of Geology, 8e
Stan Hatfield and Ken Pinzke
Southwestern Illinois College
Birth of a Theory
 In
1911 astronomer, meteorologist, climatologist Alfred
Wegener (1880-1930) found earlier studies noting the same
fossils on opposite sides of the Atlantic.
 Earlier theorists attributed this to now sunken bridges
 Wegener noted the fit of South America and Africa and became
fixed on one continent



His search for other evidence resulted in The Origin of the Continents
and Oceans in 1915, with expanded versions published in 1920, 1922,
and 1929
He postulated a single continent Pangaea (Greek for “all the earth”)
Wegener was not the first to suggest the idea but the first to include
extensive evidence. American F.B. Taylor included little evidence with
his 1910 work and was ignored.
Wegener’s Arguments
 Fit
of continents
 Appalachian
and Caledonian mountains
Fit of Continents
Matching Mountain Ranges
Matching Mountain Ranges
 Fossil
evidence
 Mesosaurus,
Glossopteris, Cynognathus
Fossil Evidence
 Paleoclimatic
evidence
 Widespread
glaciation
Glacial Evidence
 Wegener’s
ideas were largely unaccepted
 Argued
that continents were driven through solid oceanic crust like
icebreakers
Centrifugal or tidal forces
 No acceptable mechanism
Paleomagnetism
rocks (basalt) acquire a record of earth’s magnetic field
upon cooling below the Curie Point (580° C)
 Results in two types of information
 Iron-rich
 Location
of deposition
 Time of deposition
Temporal Paleomagnetism




Earth’s magnetic field reverses polarity at irregular intervals
Polarities last 105 to 106 years
Rocks acquire current field at cooling
Polarities correlative and can be used for dating
Oceanic Paleomagnetism
Work on mid-oceanic ridges shows strong and weak magnetic anomalies
later determined to be normal and reversed magnetic events
 Events form mirror images across the ridges
 Dating determines that the rocks age away from ridge

Tying the Ends Together
 Wegener’s
Continental Drift may be combined with Sea-Floor
Spreading
 Combination provides a mechanism for movement of continents
suggested by Wegener
 Combination has been developed into the Theory of Plate
Tectonics
Plate tectonics: A modern version of an old idea
 Earth’s
major plates
Associated
 Known
with Earth's strong, rigid outer layer
as the lithosphere
 Consists of uppermost mantle and overlying crust
 Overlies
 Earth’s
a weaker region in the mantle called the asthenosphere
major plates
Seven
major lithospheric plates
Plates are in motion and continually changing in shape and size
Largest plate is the Pacific plate
Several plates include an entire continent plus a large area of
seafloor
 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
 Plate
boundaries
All
major interactions among individual plates occur along their
boundaries
Types of plate boundaries
 Divergent
plate boundaries (constructive margins)
plate boundaries (destructive margins)
 Transform fault boundaries (conservative margins)
 Convergent
 Plate
boundaries
Each
plate is bounded by a combination of these three types of
boundaries
New plate boundaries can be created in response to changes in
the forces acting on these rigid slabs
Divergent plate boundaries
 Most
are located along the crests of oceanic ridges and can
be thought of as constructive plate margins
 Oceanic ridges and seafloor spreading
Along
well-developed divergent plate boundaries, the seafloor is
elevated forming oceanic ridges
Seafloor spreading occurs along the oceanic ridge system
 Spreading
rates and ridge topography
Topographic
 At
differences are controlled by spreading rates
slow spreading rates (1-5 centimeters per year), a prominent rift
valley develops along the ridge crest that is wide (30 to 50 km) and deep
(1500-3000 meters)
 At intermediate spreading rates (5-9 centimeters per year), rift valleys
that develop are shallow with subdued topography
 At
spreading rates greater than 9 centimeters per year no median rift
valley develops and these areas are usually narrow and extensively
faulted
 Continental
rifts
Splits landmasses into two or more smaller segments
Examples
include the East African rifts valleys and the Rhine
Valley in northern Europe
Produced by extensional forces acting on the lithospheric plates
Not all rift valleys develop into full-fledged spreading centers


Convergent plate boundaries
 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
Called subduction zones
Average angle at which oceanic lithosphere descends into the
mantle is about 45
 Types
of convergent boundaries
Oceanic-continental
convergence
 Denser
oceanic slab sinks into the asthenosphere
 As the plate descends, partial melting of mantle rock generates magmas
having a basaltic or, occasionally andesitic composition
 Mountains produced in part by volcanic activity associated with
subduction of oceanic lithosphere are called continental volcanic arcs
(Andes and Cascades)
 Types
of convergent boundaries
Oceanic-oceanic
convergence
 When
two oceanic slabs converge, one descends beneath the other
forms volcanoes on the ocean floor
 If the volcanoes emerge as islands, a volcanic island arc is formed
(Japan, Aleutian islands, Tonga islands)
 Often
 Types
of convergent boundaries
Continental-continental
 Continued
convergence
subduction can bring two 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 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 prominent linear
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 the plate tectonics model
 Paleomagnetism
Polar
wandering
 The
apparent movement of the magnetic poles illustrated in magnetized
rocks indicates that the continents have moved
 Polar wandering curves for North America and Europe have similar
paths but are separated by about 24 of longitude
 Different paths can be reconciled if the continents are placed next to
one another
 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 plate tectonics theory
 Deep-focus earthquakes are closely associated with subduction zones
 The pattern of earthquakes along a trench provides a method for
tracking the plate's descent
 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
of ocean-floor sediments verifies seafloor spreading
 Thickness
 Hot
spots
Caused
by rising plumes of heat from the at least the mantle
(some say the core)
Volcanoes can form over them (Hawaiian Island chain)
Most mantle plumes are long-lived structures and at least some
originate at great depth, perhaps at the mantle-core boundary
Measuring plate motions
A
number of methods have been employed to establish the
direction and rate of plate motion
Volcanic
chains
Paleomagnetism
Very Long Baseline Interferometry (VLBI)
Global Positioning System (GPS)
 Calculations
show that
Hawaii
is moving in a northwesterly direction and approaching
Japan at 8.3 centimeters per year
A site located in Maryland is retreating from one in England at
a rate of about 1.7 centimeters per year
The driving mechanism
 No
one driving mechanism accounts for all major facets of
plate tectonics
 Researchers agree that convective flow in the rocky 2,900
km-thick mantle is the basic driving force of plate tectonics
 Several mechanisms generate forces that contribute to plate
motion
Slab-pull
Ridge-push
 Models
of plate-mantle convection
Any
model describing mantle convection must explain why
basalts that erupt along the oceanic ridge
Models
 Layering
at 660 kilometers
convection
 Deep-layer model
 Whole-mantle
End of Chapter 16