Chapter 3
Plate Tectonics:
A Unifying Theory
or,
“How the map was
made”
Life on a Plate Boundary
Dec. 26, 2003 Earthquake
Bam, Iran
Life on a Plate Boundary
Dec. 26, 2004 Indonesia
Tsunami
Life on a Plate Boundary Haiti
January 12, 2010
Earthquake
>
Life on a Plate Boundary
In California, a stream bends as plates move on either side
Long Lakes fill the area along the fault
Source: wikimedia
Hollister, CA
on the San Andreas fault
Hollister
UC Berkeley, CA
Hawyard fault line
Univ. of Calif. At Berkeley
In the Middle East,
the Red Sea
Gulf of Aden
East African Rift
form a triple
junction of spreading
and divergent boundaries
Volcanism beneath the Red Sea
and Gulf are forming new ocean
basins
On the continent, in the rift
zone,
volcanoes line the valley
Mt. Pinatubo, Phillippines
June 15, 1991
Mt. St. Helens, Washington
Mt. St. Helens today
Volcanism along a mid-ocean ridge
2004 Icelandic volcanic eruption under a glacier
courtesy BBC News
Unifying Theory
• A unifying theory is one that helps
– explain a broad range of diverse observations
– interpret many aspects of a science on a
grand scale
– and relate many seemingly unrelated
phenomena
• Plate tectonics is a unifying theory for
geology.
Three phases of the theory:
• Continental drift, considered by many but
investigated by Alfred Wegener
• Seafloor Spreading, discovered in the
1960s by Harry Hess and others– showed
why the continents appear to be moving
• Theory of Plate Tectonics: still being
tested and developed
What Is the Driving Mechanism of
Plate Tectonics?
• In addition to a thermal convection system,
– some geologists think that movement may be aided
by
– “slab-pull”
• the slab is cold and
dense and pulls the
plate
– “ridge-push”
• rising magma pushes
the ridges up
• and gravity pushes the
oceanic lithosphere
away from the ridge
and toward the trench
Plate Tectonics
• Plate tectonic theory is based on the
simple model that
– the lithosphere is rigid
– it consists of oceanic and continental crust
with upper mantle
– it consists of variable-sized pieces called
plates
– with plate regions containing continental
crust
• up to 250 km thick
– and plate regions containing oceanic crust
• up to 100 km thick
Plate Map
Numbers represent average rates of relative movement, cm/yr
What Is the Driving Mechanism of Plate
Tectonics?
• Most geologists accept some type of
convective heat system hypothesis
– as the basic cause
– of plate motion
• In one possible
model,
– thermal convection
cells
– are restricted to the
asthenosphere
Plate Movement
• Plates carrying both ocean and
continental crust move over hotter and
weaker semiplastic asthenosphere
• Movement of the plates
– results from some type of heat-transfer
system within the asthenosphere
• As plates move over the asthenosphere
– they separate, mostly at oceanic ridges
– they collide, in areas such as oceanic
trenches
– they move past each other
There are 7 types of plate boundaries
Divergent (2 types)
1. Continental: extensional forces move
continental crust apart. Faulting,
stretching, and thinning of crust occurs.
Magma may intrude.
2. Oceanic:
spreading ridges, occur beneath the ocean
Faulting and thinning of crust occurs. Lava
flows onto the ocean floor.
new oceanic lithosphere is forming.
Magma intrudes into faults or erupts as lava
Lavas are high temperature, low viscosity, basaltic
Convergent Boundaries (3)
3. Continent - continent
When two continental plates collide, each is relatively
buoyant and similar in density. Minor subduction may
occur but typically continental crust is deformed, pushed
together and
uplifted forming thick crustal mountain ranges.
Example: India and Asia – Himalayan Mountains
4.
Ocean - continent
When ocean crust converges with continental crust,
the more dense ocean crust subducts into a deep trench
beneath the less dense continental crust.
As the oceanic “slab” moves down into the mantle, it
melts at about 100 km depth. The melted rock or magma
moves up within the crust and eventually erupts forming
a continental volcanic arc.
Example: Cascades (Oregon & Washington State, U.S.)
Convergent Boundaries cont’d
5. Ocean – Ocean: When ocean crust coverges with ocean crust,
both are similar density. Typically, the colder, older, more
dense ocean crust subducts. The subducting ocean slab melts
at about 100 km depth and forms an underwater volcano.
When this volcano reaches a height above sea level, a volcanic
island arc forms.
•
Convergent boundaries are characterized by explosive volcanism,
rocks typically andesite and felsic composition, earthquakes that are
high magnitude and varying in depth from shallow to deep. Tsunamis
may originate from shallow subduction zone earthquakes.
•
Mountain ranges at convergent boundaries form generally linear belts
or curved arcs.
Metamorphism is common as pressure, heat and/or hydrothermal
fluids are present.
Mineral deposits are associated with mountain building and
subduction zones.
•
•
The process of subduction accommodates the new crust formed at
divergent boundaries and keeps the earth’s size constant.
Transform plate boundaries (2)
6. Mid Ocean Ridge transform faults: connect
two oceanic ridge segments
– and are marked by perpendicular fracture zones
causing a discontinuous pattern along the ridge
7. Continental transform faults: characterized
by strike-slip faults as two plates “slide” past each
other.
Transform faults are characterized by build-up of stress
and strain and inevitable shallow earthquakes which
can be of significant magnitude
Divergent Boundaries
1. Continental
Divergent
occur during the
early stages
of continental
breakup
the crust is
initially
elevated,
stretched
and
thinned
1. cont’d Contintal “Rift Valley” forms
magma typically
intrudes into the fractures
and flows onto the valley floor
• East African Rift Valley
Narrow Sea
– the continental
crust eventually
breaks
– a narrow linear sea
is formed,
– separating two
continental blocks
– Examples:
• Red Sea
• Gulf of
California
Ocean
•
As a newly created narrow sea
– continues to spread,
– it may eventually become
– an expansive ocean basin
Modern Divergence
View looking down the Great
Rift Valley of Africa.
Little Magadi
soda lake
Ancient Rifting
• These Triassic fault basins
– mark the zone of rifting
– between North America and
Africa
sill
Palisades of
Hudson River
– They contain
thousands of
meters of
continental
sediment
– and are riddled
with dikes and
2. Divergent Mid Ocean Ridge
North America
Europe
Atlantic
Ocean
basin
South America
Africa
Elevation map of Mid Atlantic Ridge
Divergent Boundaries
• Mid Ocean Ridges also have
– high heat flow
– and basaltic flows or pillow lavas
• Pillow lavas
have
– a distinctive
bulbous shape
resulting from
underwater
eruptions
Convergent Plate Boundaries
Fig. 2.8
Keller
Convergent Boundary:
Continent-Continent Boundary
• When 2 continents collide
– the continental lithosphere cannot subduct
• Its density is
too low,
– although one
continent
may partly
slide under
the other
Continent-Continent Boundary
• When the 2 continents collide
– they weld together at a continent-continent plate
boundary, (suture zone)
– An mountain belt forms on the continent
• Earthquakes
occur here
Continental-Continental Boundary
Himalayas in central Asia
– Earth’s youngest and highest mountain system
– resulted from collision between India and Asia
– began 40 to 50 million years ago
– and is still continuing
Himalayas
Mt. Everest
Himalays
•
From its summit to the top of the Yellow
Band, about 8,600 m above sea level, the
top of Mount Everest consists of the
Qomolangma Formation,. It consists of
grayish bedded limestone with dolomite
and siltstone. There are finely
fragmented fossils of trilobites, crinoids,
and ostracods – marine organisms.
4. Oceanic-Continental Boundary
• Formation of Continental Volcanic Arc:
4. Oceanic-Continental Boundary
• Where the Nazca plate in the Pacific Ocean is
subducting under South America
– the Peru-Chile Trench marks subduction site
– and the Andes Mountains are the volcanic arc
Andes
Mountains
5. Oceanic-Oceanic Boundary
subduction complex has volcanic island arc,
deep trench and back arc basin
Japan Island Arc
5.
•
•
•
•
Volcanic Island Arc
Forearc composed of slices of folded and faulted sediments
and oceanic lithosphere
scraped off the descending plate
Back arc caused by stretching and thinning allowing magma to
reach the surface beneath a shallow sea
Sea of Japan
6. Transform Boundaries
• Movement results in
– zone of intensely shattered rock
– numerous shallow earthquakes
• The majority of
transform faults
– connect two oceanic
ridge segments
– and are marked by
fracture zones
fracture
zone
6.Transform Boundaries
• Other kinds of
transform plate
boundaries
– connect two
trenches
– or connect a ridge to
a trench
– or even a ridge or
trench to another
transform fault
• Transforms can also
extend into
continents
7. Transform Boundaries
connect other plate boundary types
Continental: San Andreas Fault, California
• separates the Pacific plate from the
North American plate
– connects ridges in
• Gulf of California
• with the Juan de Fuca and Pacific
plates
– Many of the earthquakes
in California result from
movement along this fault
What Is the Driving Mechanism
of Plate Tectonics?
• In a second model, the entire mantle is
involved in thermal convection.
• In both models,
– spreading ridges mark the
rising limbs of neighboring
convection cells
– trenches occur where the
convection cells descend
back into Earth’s interior
Plate Movement at Hot Spot
Map of Plate Boundaries
and Rate of Plate Motion
Fig. 2.4a
Map of Volcanoes and Earthquakes
Fig. 2.4 b
Map and cross section of Hawaiian Ridge and
Emporer Seamounts
Fig. 2.16 a
Fig. 2.16 b
Hot Spots and Mantle Plumes
These are NOT plate boundaries!
• Hot spots are locations where
– stationary columns of magma
– originating deep within the mantle,
• called mantle plumes
– slowly rise to the surface
• Mantle plumes remain stationary
• although some evidence suggests they may
move
• When plates move over them
– hot spots leave trails
• of extinct, progressively older volcanoes
• called aseismic ridges
• which record the movement of the plates
Hot Spots and Mantle Plumes
• Example: Emperor SeamountHawaiian Island chain
Age
increases
plate
movement
How Is Plate Motion Determined?
• Rates of plate movement can be
calculated in several ways
– Sediment
• determine the age of sediment* that is
immediately above any portion of oceanic crust
• divide the distance from the spreading ridge by
the age
• gives average rate of movement relative to the
ridge
* LEAST ACCURATE METHOD relies on incomplete fossil
record
Plate Movement Measurements
– Seafloor magnetic anomalies
• measure the distance of the magnetic anomaly in
seafloor crust from the spreading ridge
• divide by the age of the anomaly
– The present
average rate
of movement,
relative
motion, and
the average
rate of motion
in the past
can be
determined.
Plate Position Reconstruction
• Reconstructing plate positions
– to determine the plate and continent positions at the
time of an anomaly
– move the anomaly back to the spreading ridge
• Since subduction
destroys oceanic
crust
• this kind of
reconstruction
cannot be done
earlier than the
oldest oceanic
crust
Plate Movement Measurements
• Satellite-laser ranging
– bounce laser beams from a station on one plate
– off a satellite, to a station on another plate
– measure the elapsed time
– measure the elapsed time at a later date.
– use the difference in elapsed times to calculate
the rate of movement between the two plates
• Hot spots
– determine the age of rocks and their distance from a
hot spot
– divide the distance by the age
– this gives the motion relative to the hot spot so
– (possibly) the absolute motion of the plate
Plate Tectonics and the
Distribution of Natural Resources
• Plate movements influence the formation
and distribution of some natural resources
such as
– Petroleum – folded & faulted plate
boundaries form “traps” for oil and gas
– natural gas
– some mineral deposits related to volcanism
• Metal resources related to igneous and
associated hydrothermal activity
include
– Copper, gold, lead
Summary
• Plate tectonic theory
replaced the idea of continental “drift”
– became widely accepted by the 1970s
– because of overwhelming evidence supporting it
– Especially the new theory of sea floor spreading
• and because it provides a powerful explanation and
becomes the unifying theory for
–
–
–
–
–
–
volcanism,
earthquake activity,
mountain building,
global climate changes,
distribution of the world’s biota
and distribution of resources
Alfred Wegener and the
Continental Drift Hypothesis
• German
meteorologist
• Credited with
hypothesis of
continental drift:
he studied
continental glaciers
and iceberg
movement in
Greenland as a
model for earth’s
crust
Reasons to propose that the
continents were moving
•
•
•
•
•
Fit of the continents like a puzzle
Fossil evidence
Glacial evidence
Patterns of mountain ranges
Apparent polar wandering
Early Ideas
about Continental Drift
• Another scientist,
Edward Suess (Austrian, late 1800s)
noted Late Paleozoic plant fossils
Glossopteris flora found on many
continents
• This plant
• Grows in subpolar climates
but is found today
in warmer areas
Suess (cont’d)
Evidence for glaciation
– in rock sequences of
• India
• Australia
• South Africa
• South America
• He proposed the name Gondwanaland
(or Gondwana)
– for a supercontinent composed of these
modern continents
– It was located near the South Pole
Alfred Wegener and the
Continental Drift Hypothesis
• He proposed Pangaea: a supercontinent
– Greek for “all land”
– showing the breakup of Pangaea
– Using geologic, paleontologic, and
climatologic evidence
Wegener’s Evidence
• Shorelines of continents fit together
– matching rock types: marine, nonmarine
and glacial
– from Pennsylvanian to Jurassic age
– for all five Gondwana continents
• including Antarctica
• Mountain ranges and glacial deposits
– match up when continents are united
– into a single landmass
Jigsaw-Puzzle Fit of Continents
• Continental Fit
Jigsaw-Puzzle Fit of Continents
• Matching mountain
ranges
• Matching glacial
evidence
Matching Fossils
The Perceived Problem with
Continental Drift
– There was no suitable mechanism to explain
– how continents could move over Earth’s
surface
• Interest in continental drift only revived in
mid 1900s
– new evidence from studies of Earth’s magnetic
field and oceanographic research
– ocean basins as geologically young features
Earth’s Magnetic Field
• Earth as a
giant dipole
magnet
– magnetic poles
essentially
coincide
– with the
geographic
poles
– and may result
from different
rotation speeds
– of outer core
and mantle
Magnetic Field Varies
• Strength and orientation of the magnetic
field varies
– weak and horizontal at the equator
– strong and vertical at the poles
Paleomagnetism
• Paleomagnetism is
–
–
–
–
–
A remanent magnetism recorded in rocks
Recording the direction of Earth’s magnetic field
Recording the strength of the field over time
Recording the latitude of the rocks
All at the time of the rock’s formation
When magma cools
–
–
–
–
below the Curie point temperature 770o C
magnetic iron-bearing minerals align
with Earth’s magnetic field
For example, magnetite
When iron sediments settle in liquid water (i.e.
oceans, lakes, they can record the same
history
Polar Wandering
• In 1950s, research
revealed
– that paleomagnetism
of ancient rocks
showed
– orientations different
from the present
magnetic field
• Two different “paths” were
recorded in N. America and
Europe
• The best explanation
– is stationary poles
– and moving
continents
Magnetic Reversals
• Earth’s present magnetic field: Normal
– with magnetic north near the north geographic pole
– and magnetic south near the south geographic pole
• At various times in the past, Reversed
– with magnetic south near the north geographic pole
– and magnetic north near the south geographic pole
• a series of magnetic reversal is recorded in newly lava
on the continent or sea floor
continent – alternating layers of reversed orientation
sea floor – pattern or “stripes” of reversals parallel to
the Mid Ocean Ridge
Magnetic Reversals
• Measuring paleomagnetism and
dating continental lava flows led
to
– the realization that magnetic
reversals existed
– the establishment of a magnetic
reversal time scale
Mapping Ocean Basins:
a modern discovery
• Ocean mapping revealed
– a ridge system
– more than 65,000 km long (over 40,000 mi
– the most extensive mountain range in the
world
• The Mid-Atlantic Ridge
– is the best known part of the system
– and divides the Atlantic Ocean basin
– in two nearly equal parts
Atlantic Ocean Basin
Mid-Atlantic Ridge
Seafloor Spreading Hypothesis
Harry Hess, 1962
– Continents and oceanic crust move
together
– Seafloor separates at oceanic ridges
• where new crust forms from upwelling and
cooling magma, and
• the new crust moves laterally away from the
ridge
– The mechanism that drives seafloor
spreading is thermal convection cells in
the mantle
• hot magma rises from mantle to form new crust
• cold crust subducts into the mantle at oceanic
trenches, where it is heated and recycled
Evidence for Hess’s Hypothesis
• In addition to mapping mid-ocean ridges,
– ocean research also revealed
– magnetic anomalies on the sea floor
• A magnetic anomaly is a deviation
– from the average strength
– of Earth’s Magnetic field
Confirmation of Hess’s
Hypothesis
• The magnetic anomalies were
discovered to be rougly parallel to
and symmetrical with the oceanic ridges
Evidence for Plate Tectonics
• Magnetic anomaly
• Presence of volcanic mid ocean ridges
• Age of the ocean crust
– Vs. age of the continental crust
• Symmetrical Patterns
• “Disappearance” of sea floor at edges of some
continents
• Linear mountain ranges of active volcanoes –
continental volcanic arcs and volcanic island
arcs
Oceanic Crust Is Young
• Seafloor spreading theory indicates that
– oceanic crust is geologically young
because
– it forms during spreading
– and is destroyed during subduction,
therefore it does not survive as long as
continental crust that does not subduct
• Radiometric dating confirms
– the oldest oceanic crust is less than 180
million years old
• whereas oldest continental crust
– is 3.96 billion yeas old
Age of Ocean Basins
Historical Geologists:
Recognizing Ancient
Convergent Boundaries
• How can former subduction zones
be recognized in the rock record?
– Andesitic magma erupted,
• forming island arc volcanoes and continental volcanoes
– The subduction complex results in
• a zone of intensely deformed rocks between the trench and
the area of igneous activity
– Sediments and submarine rocks
• are folded, faulted and metamorphosed
• making a chaotic mixture of rocks termed a mélange
– Slices of oceanic lithosphere may be accreted
• to the continent edge and are called ophiolites
Ophiolite
•
Ophiolites consist of layers
– representing parts of
the oceanic crust and
upper mantle.
•
The sediments include
– graywackes
– black shales
– Cherts
•
Ophiolites are key to
detecting old subduction
zones