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Chapter 2
Plate Tectonics: A Unifying Theory
Introduction

Why should you know about plate tectonics?

Plate tectonics affects all of us, whether from
destruction caused by volcanic eruptions and
earthquakes, or political and economical issues
due to the formation and distribution of valuable
natural resources.

Plate tectonics is the unifying theory of geology,
tying together many seemingly unrelated geologic
phenomena, and illustrating why Earth is a
dynamic planet of interacting subsystems and
cycles.
Early Ideas About Continental
Drift

Alfred Wegener and the Continental Drift
Hypothesis
The idea that continents have
moved in the past is not new.
 The concept of continental
movement was first suggested
when it was noticed that Africa
and South America had
coastlines which appeared to be
counterparts of one another.
 This suggested they may once
have been joined and drifted
apart.

Fig. 2.3, p. 30
Early Ideas About Continental Drift

Alfred Wegener and the Continental Drift Hypothesis

Alfred Wegener originally
proposed the continental
drift hypothesis in 1912.

He postulated that all
landmasses were originally
united into a
supercontinent named
Pangaea.
Fig. 2.2, p. 29
Early Ideas About Continental Drift

Alfred Wegener and the Continental Drift
Hypothesis.

Pangaea consisted of a northern landmass called Laurasia,
and a southern landmass called Gondwana.

As Pangaea broke up, the various continents moved to
their present-day locations.
The Glossopteris fern, also
known as the “Pangaea
plant”
Fig. 2.1, p. 28
What is the Evidence for Continental
Drift?
Continental Fit
 Wegener
and others
amassed a large amount
of evidence in support of
continental drift.
 There
are close fits
between the continents
at their margins off the
coasts at depths of
about 2000 m.
Fig. 2.3, p. 30
What is the Evidence for Continental
Drift?

Similarity of Rock Sequences and
Mountain Ranges

Marine, nonmarine,
and glacial rock
sequences of
Pennsylvanian to
Jurassic age are
nearly identical on all
the Gondwana
continents
Fig. 2.4, p. 30
What is the Evidence for Continental
Drift?

Similarity of Rock Sequences and
Mountain Ranges
 The
trend of several major mountain ranges
produces a continuous mountain range
when the continents are positioned next to
each other, as they were during the
formation of Pangea.
What is the Evidence for
Continental Drift?

Glacial Evidence
 Glacial
tills and striations on the bedrock beneath
the till provide evidence of glaciation at the same
time on all the Gondwana continents, with South
Africa located at the South Pole.
Fig. 2.5, p. 31
What is the Evidence for
Continental Drift?

Fossil Evidence


Some of the most compelling evidence comes from fossils like
the Glossopteris fern.
One of the strongest examples is the Mesosaurus, a fresh
water reptile.
Fig. 2.6, p. 32
What is the Evidence for
Continental Drift?
 Wegener
could not
provide a convincing
mechanism to
demonstrate ‘how’ the
continents could have
moved.
 Continental
drift was
largely ignored
Fig. 2.2, p. 29
Features of the Seafloor
 Topography
of the seafloor often affected by
plate tectonics
 Major
features of the seafloor:
 Continental
shelves
 Continental slopes
 Continental rises
 Abyssal plains
 Oceanic ridges
 Submarine vents
 Oceanic trenches
Continental Margins
Features of the Seafloor
 Continental
margins lying below sea level
separate the continents from ocean basins.
 The
continental margins include the
continental shelf, the continental slope, and in
some places a continental rise.
Fig. 2.7, p. 33
Features of the Seafloor

Continental Shelf
 Continental
shelves extend from the shoreline
to the more steeply dipping continental slope.
They vary in width from ten's of meters to
more than 1,000 kilometers.
Fig. 2.7, p. 33
Features of the Seafloor

Continental Slope

The continental slope starts at an average depth of
135 m and is located where the slope of the sea floor
increases from less than 1° to several degrees.
Fig. 2.7, p. 33
Features of the Seafloor

Continental Rise

A continental rise with less dip than the
continental slope may be located at the base of
the continental slope.
Fig. 2.7, p. 33
Features of the Seafloor

Abyssal Plains

Abyssal plains are the flattest locations on Earth!

Cover large areas of the seafloor

Deep sea sediments accumulate on their surfaces
Fig. 2.7, p. 33
Features of the Seafloor

Oceanic ridges

Oceanic ridges are long, continuous submarine
mountain ranges composed of volcanic rock.

Oceanic ridges nearly encircle the world but are
offset at intervals by large fracture zones.
Fig. 2.7, p. 33
Features of the Seafloor

Submarine Hydrothermal Vents
 Seafloor
vents at or very near
spreading ridges where
circulating water is heated to
over 400°C and discharged
into seawater.
 As
the hot water circulates
through the oceanic crust it
dissolves metals that
discharge as plumes, called
black smokers.
Fig. 2.8, p. 34
Fig. 2.8, p. 34
Features of the Seafloor

Oceanic trenches are the seafloor
expressions of subduction zones.


They are long, narrow, features which reach the
greatest oceanic depths, sometimes >1.1 km
Characterized by low heat flow and frequent
seismic activity
Fig. 2.7, p. 33
Features of the Seafloor

Seamounts and Guyots
Fig. 2.9, p. 34
Seamounts are extinct
oceanic volcanoes. Many
rise more than a kilometer
above the ocean floor. They
formed as the oceanic crust
moved over a hot mantle
plume.
Guyots have the same origin,
but have flat tops created
when the volcanoes sank to
sea level. Once near sea
level, erosion by waves has
flattened the tops.
Features of the Seafloor

Aseismic Ridges


Aseismic ridges are long, narrow ridges and broad
plateaus that rise off the seafloor.
Commonly form near hot-spot volcanoes, consisting of
seamounts and guyots extending away from spreading
ridges
Fig. 2.9, p. 34
Earth’s Magnetic Field
 Paleomagnetic
studies during the 1950s
revived interest in continental drift


Fig. 2.12, p. 36
They indicated that either the
magnetic poles had wandered
and each continent had its own
pole (an impossibility), or the
continents had moved over time.
If the continents are moved into
different positions relative to
each other, the separate poles
could be resolved into one.
Earth’s Magnetic Field


Paleomagnetism is the remnant magnetism in ancient
rocks recording the direction and intensity of Earth’s
magnetic field at the time of the rock’s formation.
The Curie point is the temperature at which hot ironbearing minerals cool enough to gain magnetism. As
long as the rock is not subsequently heated above the
Curie point, it will preserve its remnant magnetism.
Fig. 2.11, p. 35
Paleomagnetism and
Polar Wandering

How can the apparent wandering of the
magnetic poles be best explained?
The magnetic poles have
remained near their present
locations at the geologic
north and south pole and the
continents have moved.
 Evidence: When the
continents are fitted
together, the paleomagnetic
data for all of them have the
same magnetic poles

Fig. 2.12, p. 36
Magnetic Reversals
and Seafloor Spreading

Earth’s present magnetic field is
considered "normal."

Normal - with the north and south magnetic
poles located approximately at the north and
south geographic poles, respectively

At various times in the geologic past, Earth’s
magnetic field has completely reversed

Reversed - the magnetic south pole is near the
geographic north pole and the magnetic north
pole is near the geographic south pole
Magnetic Reversals and Seafloor
Spreading


Earth’s present magnetic
field is normal
The existence of magnetic
reversals was discovered in
continental lava flows by:
 A)
age dating
 B) determining the
orientation of the remnant
magnetism
Fig. 2.13, p. 37
South magnetic
pole (normal
position)
North magnetic
pole (normal
position)
North magnetic
pole (reversed)
South magnetic
pole (reversed)
South magnetic
pole (normal)
North magnetic
pole (normal)
North magnetic
pole (reversed)
South magnetic
pole (reversed)
Stepped Art
Fig. 2-13, p. 37
Magnetic Reversals and Seafloor
Spreading

Harry Hess proposed the theory of seafloor
spreading in 1962.
He suggested that the
seafloor separates at
oceanic ridges, where
new crust is formed by
upwelling magma.
 As the magma cools,
the newly formed
oceanic crust moves
laterally away from the
ridge.

Fig. 2.15, p. 38
Magnetic Reversals and
Seafloor Spreading

Deep-Sea Drilling and the Confirmation of
Seafloor Spreading
 Seafloor
spreading was
confirmed by magnetic
anomalies in the ocean
crust that were both parallel
to and symmetric around the
ocean ridges.
 This
indicates that new
oceanic crust forms along
the spreading ridges.
Fig. 2.14, p. 38
Magnetic Reversals and
Seafloor Spreading

Deep-Sea Drilling and the Confirmation of Seafloor
Spreading
 Sea
floor spreading
confirmed by:
the ages of fossils in ocean
sediments
 radiometric dating

 Oceanic
crust is youngest
at the spreading ridges
and oldest at the farthest
points from the ridges.
Fig. 2.15, p. 38
Plate Tectonics: A Unifying
Theory

Overwhelming evidence in support of plate tectonics led to its
rapid acceptance and elaboration since the early 1970's.


The theory is widely accepted because it explains so
many geologic phenomena, including: volcanism,
seismicity, mountain building, climatic changes, animal
and plant distributions in the past and present, and the
distributions of natural resources.
For these reasons, it is known as a unifying theory.
Plate Tectonics: A Unifying
Theory

According to plate tectonic theory, the rigid
lithosphere is divided into different-sized plates.
Fig. 2.16, p. 39
Plate Tectonics: A Unifying Theory
 The
lithosphere overlies the asthenosphere,
and some type of heat-transfer system within
the asthenosphere moves the plates.
 As
the plates move over the asthenosphere,
they separate mostly at oceanic ridges and
collide and subduct into Earth’s interior at
oceanic trenches.
The Three Types of Plate Boundaries

Plate tectonics has operated since at least the Proterozoic
era. It is important to understand how the plates move and
interact with one another.

Types of Plate Boundaries
 Divergent
 Convergent
 Transform
The Three Types of Plate
Boundaries

Divergent Boundaries - Divergent boundaries form
when two plates move away from each other



New oceanic lithosphere forms at
the opening rift
Most divergent boundaries occur
along the crests of oceanic ridges
They are also present under
continents during the early stages
of continental breakup
Fig. 2.17, p. 41
Volcanic activity
Magma
Continental crust
Rift valley
Coastal mountain
range
Narrow fault-bounded sea
Continental “seaboard”
(coastal mountains gone)
Wide ocean
Stepped Art
Fig. 2-17, p. 41
The Three Types of Plate Boundaries

Divergent Boundaries
 Characteristic
features of ancient continental
rifting include: faulting, dikes, sills, lava flows,
and thick sedimentary sequences within rift
valleys
 Pillow
lavas and associated deep-sea
sediments are evidence of ancient spreading
ridges
The Three Types of Plate
Boundaries

Divergent Boundaries

Modern example: East Africa

Initial stages of continental breakup

Rift valleys and volcanism

Rift valleys expand and create
seas, such as Red Sea

As divergence continues, seas may
expand into oceans and continents
have passive margins
Fig. 2.18, p.44
The Three Types of Plate Boundaries

Convergent Boundaries

Convergent boundaries: where two plates collide

There are three types of convergent boundaries
 An
oceanic-oceanic boundary: two oceanic plates
collide, one ocean plate will subduct beneath the
margin of the other plate
 An
oceanic-continental boundary: an oceanic plate
and a continental plate collide, the oceanic plate
will subduct
 A continental-continental
boundary: two continents
collide, no subduction of continents
The Three Types of Plate Boundaries
Convergent Boundaries
 Oceanic-Oceanic Boundaries
 One oceanic plate subducts beneath the other and a
volcanic island arc forms on the non-subducted plate
 An oceanic trench forms parallel to the volcanic island
arc where the subduction occurs
 Volcanoes result from rising magma produced by the
partial melting of the subducting plate

Fig. 2.19a, p. 45
The Three Types of Plate Boundaries

Convergent Boundaries



Oceanic-Continental Boundaries
An oceanic plate and a continental plate converge, with the
denser oceanic plate subducting under the continental plate
Like an oceanic-oceanic boundary, a chain of volcanoes forms
on the nonsubducted plate
Fig. 2.19b, p. 45
The Three Types of Plate Boundaries

Convergent Boundaries
 Continental
- Continental Boundaries
 Two
continents converge,
the ocean floor separating
them subducts, the two
continents collide. Neither
plate will subduct.
 When
the two continents
collide, they are welded
together to form an interior
mountain chain.
Fig. 2.19c, p. 45
The Three Types of Plate Boundaries

Convergent Boundaries
 Recognizing Ancient Convergent Plate Boundaries

Intensely deformed rocks,
andesite lavas, and
ophiolites are all evidence
of ancient subduction
zones, marking former
convergent plate
boundaries
Fig. 2.20, p. 46
Fig. 2.20, p. 46
The Three Types of Plate Boundaries

Transform Boundaries

Plates slide laterally past each other along transform
faults

These boundaries change one type of motion between
plates into another type of motion
Fig. 2.21a, p. 49
The Three Types of
Plate Boundaries

Transform Boundaries
 These
are boundaries along
which plates slide laterally past
each other along transform faults
 Generally,
no diagnostic features
are left by transform faults
 Example:
San Andreas fault
Fig. 2.22, p. 48
Hot Spots and Mantle Plumes

What are hot spots?

A hot spot is the location on Earth’s surface
where a stationary column of magma, originating
deep within the earth (possible near the mantleouter core boundary) has slowly risen to the
surface and caused volcanism
Hot Spots and Mantle Plumes
 Hot
spot plumes apparently remain stationary within
the mantle while plates move over them
 The
resulting hot spot leaves a trail of extinct and
progressively older volcanoes that record the
movement of the plate
Fig. 2.23, p. 49
Hot Spots and Mantle Plumes

Determining rate and direction of plate movement

Hot spots may be used to determine the absolute motion
of plates. They provide an apparently fixed reference
point from which the rate and direction of plate movement
can be measured.
Plate Movement and Motion

Determining rate and direction of plate movement
The average rate of plate
movement is most commonly
determined by dividing the
distance from an oceanic ridge
axis to any magnetic anomaly in
the crust of the seafloor by the
age of that anomaly.
 Satellite-laser ranging
techniques are also used to
determine the current rate of
movement and relative motion of
one plate with respect to
another.

Fig. 2.24a, p. 49
The Driving Mechanism of Plate
Tectonics

What moves the plates?

Most geologists agree that
some type of convective
heat system is the basic
process responsible for
plate motion.

Radioactive decay in the
Earth's interior provides
heat for convection.

Additional heat comes from
the core
Fig. 2.25, p. 50
The Driving Mechanism of Plate
Tectonics

Mantle Convection Cell Model


Spreading ridges: Hot ascending limbs of cells
Trenches: Cooled part of convention cells descend
Fig. 2.25, p. 50
The Driving Mechanism
of Plate Tectonics

Gravity driven
plate motion
 Besides
convection,
gravity-driven
mechanisms may
have a major role
Fig. 2.27, p. 51
involves pulling the plate behind a
subducting cold slab of lithosphere
 “Ridge-push” involves gravity pushing the oceanic
lithosphere away from the higher spreading ridges
and toward the subduction trenches
 “Slab-pull”
Plate Tectonics and the
Distribution of Natural Resources

Petroleum

Plate movements are partly responsible for abundant
petroleum in the Middle East.

During the Mesozoic, much of the Middle East was a broad
marine shelf near the equator.

Countless microorganisms lived in the warm surface waters,
died, and accumulated in sediments.

Burial of the organic-rich sediments from subduction created
the right amount of heat to produce petroleum.

Plate collisions between Iran and the Arabian Plate folded
rocks and created traps for the petroleum to accumulate.
Plate Tectonics and the
Distribution of Natural Resources

Mineral Deposits

Many metallic mineral
deposits are related to
igneous and associated
hydrothermal activity in
convergent and
divergent plate
boundaries.

Copper, iron, lead , zinc,
gold and silver ore
deposits are associated
with plate boundaries.
Fig. 2.28, p. 53
Plate Tectonics and
the Distribution of Life



Plate tectonics affects
organic evolution.
Organisms occupy biotic
provinces controlled mostly
by:
 Climate
 Geographic barriers
Plate movements create
mountains and continental
barriers that influence
climate and organic
evolution.
Fig. 2.29, p. 54
Plate Tectonics and
the Distribution of Life
Plate tectonics and organic evolution in the
Americas


Formation of Panama by plate
movements about 5 million
years ago isolated Caribbean
and Pacific marine organisms
promoting separate evolution.
North American land animals
migrated to South America,
causing extinctions of South
American species.
Fig. 2.29, p. 54
End of
Chapter 2