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PLATE TECTONICS: A SCIENTIFIC
REVOLUTION UNFOLDS
2
INTRODUCTION
Plate Tectonics: A Scientific Revolution Unfolds covers the development of the Theory of Plate
Tectonics and discusses the characteristics of this theory. The chapter opens with a discussion of
Alfred Wegner’s hypothesis of continental drift, its supporting evidence, and its major criticisms.
The chapter then discusses the development of the Plate Tectonic Theory and the motions and
characteristics of transform, divergent and convergent boundaries. The chapter then discusses
modern evidence that confirms the theory, including ocean drilling, mantle plumes,
paleomagnetism, polar wandering, magnetic reversals, and seafloor spreading. The chapter ends
with a discussion of how plate motion is measured and an overview of the two hypothesized
mechanisms of plate motion through movements of the mantle.
CHAPTER OUTLINE
1.
2.
From Continental Drift to Plate Tectonics
a. Early geology viewed the oceans and continents as very old features with fixed
geographic positions
b. But researchers realized that Earth’s continents are not static; instead, they
gradually migrate across the globe
i. Create great mountain chains where they collide
ii. Create ocean basins where they split apart
c. Scientific Revolution
i. Reversal in scientific thought results in a very different model of processes on
Earth that act to deform the crust and create major structural features such as
mountains, continents, and oceans
ii. Began in 20th century with continental drift—the idea that continents were
capable of movement
iii. As more advanced, modern instruments came along, scientists evolved from
the ideas of continental drift to the theory
Continental Drift: An Idea Before Its Time
a. Challenged the long-held assumption that the continents and ocean basins had fixed
geographic positions
b. Set forth by Alfred Wegener in his 1915 book, The Origin of Continents and Oceans
c. Suggested that a single supercontinent (Pangea) consisting of all Earth’s landmasses
once existed
d. Further hypothesized that about 200 million years ago, this supercontinent began to
fragment into smaller landmasses that then “drifted” to their present positions over
millions of years.
e. Evidence
i. Similarity between the coastlines on opposite sides of the Atlantic Ocean led to
the hypothesis that they were once joined
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3.
4.
1. A very precise fit when the continental shelf boundary is considered the
edge of the continent
ii. Identical fossil organisms had been discovered in rocks from both South
America and Africa (Mesosaurus and Glossopteris)
1. Some type of land connection was needed to explain the existence of
similar Mesozoic age life forms on widely separated landmasses—no
evidence of this
2. Wegener asserted that South America and Africa must have been joined
during that period of Earth history
iii. Rocks found in a particular region on one continent closely match in age and
type those found in adjacent positions on the once adjoining continent
iv. Evidence of a glacial period that dated to the late Paleozoic in southern Africa,
South America, Australia, and India (near the equator)
1. A global cooling event was rejected by Wegener because during the
same span of geologic time, large tropical swamps existed in several
locations in the Northern Hemisphere
2. Can be explained by southern continents that were joined together and
located near the South Pole
The Great Debate
a. Main objections to Wegener’s hypothesis stemmed from his inability to identify a
credible mechanism for continental drift
i. Proposed that gravitational forces of the Moon and Sun that produce Earth’s
tides were also capable of gradually moving the continents across the globe
ii. Also incorrectly suggested that the larger and sturdier continents broke
through thinner oceanic crust, much like ice breakers cut through ice
b. Most of the scientific community, particularly in North America, either categorically
rejected continental drift or treated it with considerable skepticism
The Theory of Plate Tectonics
a. New technology post-WWII gave science evidence to support some of Wegener’s
ideas, and many new ideas
i. The discovery of a global oceanic ridge system that winds through all of the
major oceans
ii. Studies conducted in the western Pacific demonstrated that earthquakes were
occurring at great depths beneath deep-ocean trenches
iii. Dredging of the seafloor did not bring up any oceanic crust that was older than
180 million years
iv. Sediment accumulations in the deep-ocean basins were found to be thin, not
the thousands of meters that were predicted
b. Led to Theory of Plate Tectonics
i. The crust and the uppermost, and therefore coolest, part of the mantle
constitute Earth’s strong outer layer, known as the lithosphere
1. Lithosphere varies in thickness depending on whether it is oceanic
lithosphere or continental lithosphere
a. Oceanic crust thickest (100 km) in deep ocean basins, but
thinner along ridge system
b. Continental lithosphere averages 150 km thick, and may extend
to 200 km beneath stable continental interiors
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5.
2. The composition of the both oceanic and continental crusts affects their
respective densities
a. Oceanic crust is composed of rocks having a mafic (basaltic)
composition = higher density
b. Continental crust is composed largely of felsic (granitic) rocks =
lower density
ii. The asthenosphere (asthenos = weak, sphere = a ball) is a hotter, weaker
region in the mantle that lies below the lithosphere
1. Temperature and pressure put rocks very near their melting
temperature; causes rocks in asthenosphere to respond to forces by
flowing
2. The relatively cool and rigid lithosphere tends to respond to forces
acting on it by bending or breaking, but not flowing
3. Earth’s rigid outer shell is effectively detached from the asthenosphere,
which allows these layers to move independently
c. The lithosphere is broken into about two dozen segments of irregular size and
shape called plates that are in constant motion with respect to one another
i. Seven major plates: North American, South American, Pacific, African, Eurasian,
Australian-Indian, and Antarctic plates
ii. Intermediate-sized plates: Caribbean, Nazca, Philippine, Arabian, Cocos, Scotia,
and Juan de Fuca plates
iii. None of the plates are defined entirely by the margins of a single continent nor
ocean basin
d. Plates move as somewhat rigid units relative to all other plates
i. Most major interactions among them (and, therefore, most deformation) occur
along their boundaries
ii. Plates are bounded by three distinct types of boundaries, which are
differentiated by the type of movement they exhibit
1. Divergent plate boundaries (constructive margins)—where two plates
move apart, resulting in upwelling of hot material from the mantle to
create new seafloor
2. Convergent plate boundaries (destructive margins)—where two plates
move together, resulting in oceanic lithosphere descending beneath an
overriding plate, eventually to be reabsorbed into the mantle or possibly
in the collision of two continental blocks to create a mountain belt
3. Transform plate boundaries (conservative margins)—where two plates
grind past each other without the production or destruction of
lithosphere
iii. Divergent and convergent plate boundaries each account for about 40 percent
of all plate boundaries
iv. Transform faults account for the remaining 20 percent.
Divergent Plate Boundaries and Seafloor Spreading
a. Characteristics:
i. Most divergent plate boundaries are located along the crests of oceanic ridges
ii. Constructive plate margins—this is where new ocean floor is generated
iii. Two adjacent plates move away from each other, producing long, narrow
fractures in the ocean crust
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6.
iv. Hot rock from the mantle below migrates upward to fill the voids left as the
crust is being ripped apart
v. Molten material gradually cools to produce new slivers of seafloor
b. Oceanic Ridges and Seafloor Spreading
i. Ridges: elevated areas of the seafloor characterized by high heat flow and
volcanism
1. Including the Mid-Atlantic Ridge, East Pacific Rise, and Mid-Indian Ridge.
2. 2–3 km high, 1000–4000 km wide
3. Along the crest of some ridge segments is a deep canyon-like structure
called a rift valley
ii. Movement at ridges is called seafloor spreading
1. Typical rates of spreading average around 5 centimeters (2 inches) per
year
a. Slower along Mid-Atlantic Ridge; higher along East Pacific Rise
2. Generated all of Earth’s ocean basins within the past 200 million years
iii. Creation of ridges at areas of seafloor spreading
1. Newly created oceanic lithosphere is hot, making it less dense than
cooler rocks found away from the ridge axis
a. New lithosphere forms and is slowly yet continually displaced
away from the zone of upwelling.
b. Begins to cool and contract, thereby increasing in density, which
equals thermal contraction
c. It takes about 80 million years for the temperature of oceanic
lithosphere to stabilize and contraction to cease
2. As the plate moves away from the ridge, cooling of the underling
asthenosphere causes it to become increasingly more rigid
a. Oceanic lithosphere is generated by cooling of the asthenosphere
from the top down
b. The thickness of oceanic lithosphere is age-dependent; that is,
the older (cooler) it is, the greater its thickness
c. Oceanic lithosphere that exceeds 80 million years in age is about
100 kilometers thick: approximately its maximum thickness
c. Continental Rifting
i. Within a continent, divergent boundaries can cause the landmass to split into
two or more smaller segments separated by an ocean basin
1. Begins when plate motions produce opposing (tensional) forces that pull
and stretch the lithosphere.
2. Promotes mantle upwelling and broad upwarping of the overlying
lithosphere as it is stretched and thinned
3. Lithosphere is thinned, while the brittle crustal rocks break into large
blocks
4. The broken crustal fragments sink, generating an elongated depression
called a continental rift
5. Modern example of an active continental rift is the East African Rift
Convergent Plate Boundaries and Subduction
a. Total Earth surface area remains constant over time; this means that a balance is
maintained between production and destruction of lithosphere
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b.
c.
d.
e.
f.
i. A balance is maintained because older, denser portions of oceanic lithosphere
descend into the mantle at a rate equal to seafloor production
Convergent plate boundaries are where two plates move toward each other and the
leading edge of one is bent downward, as it slides beneath the other
Also called subduction zones, because they are sites where lithosphere is
descending (being subducted) into the mantle
i. Subduction occurs because the density of the descending lithospheric plate is
greater than the density of the underlying asthenosphere
ii. Old oceanic lithosphere is about 2 percent more dense than the underlying
asthenosphere, which causes it to subduct
iii. Continental lithosphere is less dense and resists subduction
Deep-ocean trenches are the surface manifestations produced as oceanic
lithosphere descends into the mantle
i. Large linear depressions that are remarkably long and deep
ii. Example: Peru–Chili trench along West Coast of South America
The angle at which oceanic lithosphere subducts depends largely on its age and,
therefore, its density
i. When seafloor spreading occurs near a subduction zone, the subducting
lithosphere is young and buoyant which, results in a low angle of descent
ii. Older, very dense slabs of oceanic lithosphere typically plunge into the mantle
at angles approaching 90 degrees
Types of convergence:
i. Oceanic–Continental Convergence: Oceanic crust converges with continental
crust
1. The buoyant continental block remains “floating”; the denser oceanic
slab sinks into the mantle
2. When a descending oceanic slab reaches a depth of about 100 kilometers
(60 miles), melting is triggered within the wedge of hot asthenosphere
that lies above it
a. Water contained in the descending plates acts as “wet” rock in a
high-pressure environment and melts at substantially lower
temperatures than does “dry” rock of the same composition.
b. Partial melting: the wedge of mantle rock is sufficiently hot that
the introduction of water from the slab below leads to some
melting
3. Being less dense than the surrounding mantle, this hot mobile material
gradually rises toward the surface
4. Examples include Andes of South Amercia and Cascade Range of North
America
ii. Oceanic—Oceanic Convergence: oceanic crust converges with oceanic crust
1. One slab descends beneath the other, initiating volcanic activity by the
same mechanism that operates at all subduction zones
2. Volcanoes grow up from the ocean floor, rather than upon a continental
platform
3. Will eventually build a chain of volcanic structures large enough to
emerge as islands = volcanic island arc
4. Examples include the Aleutian, Mariana, and Tonga islands
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7.
8.
iii. Continental-Continental Convergence—continental crust converges with
continental crust
1. The buoyancy of continental material inhibits it from being subducted
2. Causes a collision between two converging continental fragments
3. Folds and deforms the accumulation of sediments and sedimentary
rocks along the continental margins
4. Result is the formation of a new mountain belt composed of deformed
sedimentary and metamorphic rocks that often contain slivers of oceanic
crust
5. Example is the Himalayas created by collision of Indian and Asian
continental landmasses
Transform Plate Boundaries
a. Where plates slide horizontally past one another without the production or
destruction of lithosphere
b. Most transform faults are found on the ocean floor where they offset segments of
the oceanic ridge system
c. Transform faults are part of prominent linear breaks in the seafloor known as
fracture zones
i. Include both the active transform faults as well as their inactive extensions into
the plate interior
ii. Active transform faults lie only between the two offset ridge segments and are
generally defined by weak, shallow earthquakes
iii. Trend of these fracture zones roughly parallels the direction of plate motion at
the time of their formation
d. Transform faults also transport oceanic crust created at ridge crests to a site of
destruction
e. Most transform fault boundaries are located within the ocean basins; however, a
few cut through continental crust
i. Example is San Andreas fault of North America—the Pacific plate is moving
toward the northwest, past the North American plate
Testing the Plate Tectonics Model
a. Ocean Drilling
i. The Deep Sea Drilling Project (1968–1983) sampled the seafloor to determine
its age
ii. Showed that the sediments increased in age with increasing distance from the
ridge
1. Supported the seafloor-spreading hypothesis: youngest crust would be
found at the ridge axis (where it is produced), oldest crust would be
found adjacent to the continents
iii. Thickness of ocean-floor sediments provided additional verification of seafloor
spreading
1. Sediments are almost entirely absent on the ridge crest and that
sediment thickness increases with increasing distance from the ridge
iv. Reinforced the idea that the ocean basins are geologically young because no
seafloor with an age in excess of 180 million years was found
b. Mantle Plumes and Hot Spots
i. Mapping volcanic islands and seamounts (submarine volcanoes) of Hawaiian
Islands to Midway Islands revealed several linear chains of volcanic structures
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ii. Radiometric dating of this linear structure showed that the volcanoes increase
in age with increasing distance from the “big island” of Hawaii
1. Youngest volcanic island in the chain (Hawaii) rose from the ocean floor
less than one million years ago, Midway Island is 27 million years old,
and Detroit Seamount, near the Aleutian trench, is about 80 million
years old
iii. A cylindrically shaped upwelling of hot rock, called a mantle plume, is located
beneath the island of Hawaii
1. Hot, rocky plume ascends through the mantle, the confining pressure
drops, which triggers partial melting
2. The surface manifestation of this activity is a hot spot, an area of
volcanism, high heat flow, and crustal uplifting that is a few hundred
kilometers across
3. As the Pacific plate moved over a hot spot, a chain of volcanic structures
known as a hot-spot track was built
iv. Supports ideas that plates move over the asthenosphere, which means that age
of each volcano indicates how much time has elapsed since it was situated over
the mantle plume
c. Paleomagnetism
i. Rocks that formed thousands or millions of years ago and contain a “record” of
the direction of the magnetic poles at the time of their formation
1. Earth’s magnetic field has a north and south magnetic pole that today
roughly align with the geographic poles
2. Some naturally occurring minerals are magnetic and are influenced by
Earth’s magnetic field (e.g., magnetite)
3. As the lava cools, these iron-rich grains become magnetized and align
themselves in the direction of the existing magnetic lines of force
4. They act like a compass needle because they “point” toward the position
of the magnetic poles at the time of their formation
ii. Apparent Polar Wandering
1. The magnetic alignment of iron-rich minerals in lava flows of different
ages indicates that the position of the paleomagnetic poles have changed
through time
a. Magnetic North Pole has gradually wandered from a location
near Hawaii northeastward to its present location over the Arctic
Ocean
b. Evidence that either the magnetic North Pole had migrated, an
idea known as polar wandering, or that the poles remained in
place and the continents had drifted beneath them
2. If the magnetic poles remain stationary, their apparent movement is
produced by continental drift.
a. Studies of paleomagnetism show that the positions of the
magnetic poles correspond closely to the positions of the
geographic poles
b. When North America and Europe are moved back to their predrift positions, their apparent wandering paths coincide
c. Evidence that North America and Europe were once joined and
moved relative to the poles as part of the same continent
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9.
10.
iii. Magnetic Reversals and Seafloor Spreading
1. Over periods of hundreds of thousands of years, Earth’s magnetic field
periodically reverses polarity
a. Lava solidifying during a period of reverse polarity will be
magnetized with the polarity opposite that of volcanic rocks
being formed today
i. Normal polarity—rocks with same polarity as present
magnetic field
ii. Reverse polarity—rocks with opposite polarity of
present magnetic field
b. Magnetic time scale established by radiometric dating
techniques on magnetic polarity of hundreds of lava flows
2. Magnetic surveys of the ocean showed alternating stripes of high- and
low-intensity magnetism that represent the polarity of the magnetism of
Earth
a. Magma along a mid-ocean ridge “records” the current polarity of
Earth
b. As the two slabs move away from the ridge, they build a pattern
of normal and reverse magnetic stripes
3. Magnetic stripes exhibit a remarkable degree of symmetry in relation to
the ridge axis, thus supporting seafloor spreading
How Is Plate Motion Measured?
a. Geologic Evidence
i. An average rate of plate motion can be calculated from the radiometric age of
an oceanic crust sample and its distance from the ridge axis where it was
generated
ii. Combine age data with paleomagnetism data to get maps of age of the seafloor
iii. Show us that the rate of seafloor spreading in the Pacific basin must be more
than three times greater than in the Atlantic
iv. Fracture zones are inactive extensions of transform faults, and therefore
preserve a record of past directions of plate motion
b. Measuring Plate Motion From Space
i. Data from GPS (Global Positioning System) establish the rate of movement of
plates using repeated measurements over many years
ii. GPS devices have also been useful in establishing small-scale crustal
movements such as those that occur along faults in regions known to be
tectonically active
c. How Does Plate Motion Affect Plate Boundaries?
i. Because of plate motion, the size and shape of individual plates are constantly
changing
ii. Another consequence of plate motion is that boundaries also migrate
iii. Plate boundaries can also be created or destroyed in response to changes in the
forces acting on the lithosphere
What Drives Plate Motions?
a. Some type of convection, where hot mantle rocks rise and cold, dense oceanic
lithosphere sinks is the ultimate driver of plate tectonics
b. Forces that drive plate motion
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i. Slab pull: subduction of cold, dense slabs of oceanic lithosphere is a major
driving force of plate motion
ii. Ridge push: gravity-driven mechanism results from the elevated position of the
oceanic ridge, which causes slabs of lithosphere to “slide” down the flanks of
the ridge
iii. Ridge push appears to contribute far less to plate motions than slab pull
iv. Mantle drag
1. Enhances plate motion when flow in the asthenosphere is moving at a
velocity that exceeds that of the plate
2. Resist plate motion when the asthenosphere is moving more slowly than
the plate, or in the opposite direction
c. Models of Plate-Mantle Convection
i. Convective flow is the underlying driving force for plate movement
ii. Mantle convection and plate tectonics are part of the same system
iii. Convective flow in the mantle is a major mechanism for transporting heat away
from Earth’s interior
iv. Two models:
1. Whole-Mantle Convection (Plume Model)
a. Cold oceanic lithosphere sinks to great depths and stirs the
entire mantle
b. Suggests that the ultimate burial ground for subducting slabs is
the core-mantle boundary
c. Downward flow is balanced by buoyantly rising mantle plumes
that transport hot material toward the surface
d. Two kinds of plumes : narrow tubes and giant upwellings
2. Layer Cake Model
a. Mantle has two zones of convection—a thin, dynamic layer in the
upper mantle and a thick, larger, sluggish one located below
b. Downward convective flow is driven by the subduction of cold,
dense oceanic lithosphere
c. These subducting slabs penetrate to depths of no more than
1000 kilometers (620 miles)
d. The lower mantle is sluggish and does not provide material to
support volcanism at the surface
e. Very little mixing between these two layers is thought to occur
LEARNING OBJECTIVES/FOCUS ON CONCEPTS
Each statement represents the primary learning objective for the corresponding major heading
within the chapter. After completing the chapter, students should be able to:
2.1
Discuss the view that most geologists held prior to the 1960s regarding the geographic
positions of the ocean basins and continents.
2.2
List and explain the evidence presented by Wegener to support his continental drift
hypothesis.
2.3
Discuss the two main objections to the continental drift hypothesis.
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2.4
List the major differences between Earth’s lithosphere and its asthenosphere, and explain
the importance of each in the plate tectonic theory.
2.5
Sketch and describe the movement along a divergent plate boundary that results in the
formation of new oceanic lithosphere.
2.6
Compare and contrast the three types of convergent plate boundaries and name a location
where each type can be found.
2.7
Describe the relative motion along a transform fault boundary and be able to locate
several examples on a plate boundary map.
2.8
List the evidence used to support the plate tectonics theory and briefly describe each.
2.9
Describe two methods researchers employ to measure relative plate motion.
2.10
Summarize what is meant by plate-mantle convection and explain two of the primary
driving forces for plate motion.
TEACHING STRATEGIES
Muddiest Point: In the last 5 minutes of class, have students jot down the points that were most
confusing from the day’s lecture, and what questions they still have. Or provide a “self-guided”
muddiest point exercise, using the Clicker PowerPoints and website questions for this chapter.
Review the answers, and cover the unclear topics in a podcast to the class or at the beginning of the
next lecture.
The following are fundamental ideas from this chapter that students have the most difficulty
grasping and activities to help address these misconceptions and guide learning.
A. Movement of Plates
• Students have many misconceptions about plate motion. These may include: only
continents move, oceans are stationary, plate movement is imperceptible on a human
timeframe, the size of Earth is gradually increasing over time because of seafloor
spreading, plate tectonics started with the breakup of Pangea, and tectonic plates drift
in oceans of melted magma just below the surface of Earth. As you discuss plate
tectonics, integrate imagery, graphics, and animations to help students visualize the
processes involved (see Teacher Resources in the following section)
• Isostasy Animation http://www.geo.cornell.edu/hawaii/220/PRI/isostasy.html
i. This interactive animation allows students to visualize how continental and
oceanic crust “float” on the mantle. In the menu along the bottom, enter a liquid
density of 3.3 g/cm3, the average density of the asthenosphere—this will stay the
same. Then, enter the thickness and density of oceanic crust (5 kilometers thick,
density of 3.0 g/cm3). Record the height of the block above the liquid—you will
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•
•
•
•
•
•
have to subtract the block height from the block root value. Do the same for
continental crust (50 kilometers thick, density of 2.7 g/cm3).
ii. Then, ask students: Which sits higher above the liquid surface? Which sits lower?
Why? Use this as a lead-in to tectonics—if plates can move up and down
(buoyancy) in the asthenosphere, might they also move back and forth? Why? This
is plate tectonics—plates moving laterally across the asthenosphere.
Hot Spot Model Activity
i. (Supplies: metal pan, spray bottle of water, about 1 cup of sugar, a candle or
tealight, lighter/matches). Spray a disposable metal pan with water, then add a
thin layer of sugar. Have one student hold the lit candle stationary beneath the pan
of sugar. Have another student slowly move the pan in one direction over the
candle. Students should see “islands” of molten sugar form on the surface as the
pan (plate) moves over the candle (hotspot).
ii. (Supplies: blank overhead and overhead pens) One student is the “hotspot” (pen),
another is the “plate” (overhead). Ask the “plate” student to move the “plate” to
the NW (like the Pacific plate) while the “hotspot” student holds the pen
stationary on the overhead. Result is a linear chain created on the moving plate.
Tracking Tectonic Plates Activity
http://serc.carleton.edu/NAGTWorkshops/intro/activities/28504.html
Subduction Zone Earthquake Activity
http://serc.carleton.edu/introgeo/demonstrations/examples/subduction_zone_earthq
uakes.html
Nannofossils Reveal Seafloor Spreading Truth Activity
http://www.oceanleadership.org/wp-content/uploads/2009/08/Nannofossils.pdf
You Try It: Plate Tectonics
http://www.pbs.org/wgbh/aso/tryit/tectonics/shockwave.html
Sea-Floor Spreading Activity
http://oceanexplorer.noaa.gov/edu/learning/player/lesson02/l2la2.htm
B. Characteristics of Plates and Boundaries
• Students have difficulty understanding relationships between geologic processes and
plate boundaries until they can clearly visualize and analyze their relationships.
• Discovering Plate Boundaries Activity
http://plateboundary.rice.edu/intro.html
• A similar activity on plate boundaries using Google Earth:
http://serc.carleton.edu/NAGTWorkshops/structure/SGT2012/activities/63925.html
• NOAA Mid-Ocean Ridge Activity
http://www.montereyinstitute.org/noaa/lesson02/l2la1.htm
• NOAA Earthquakes and Plates Activity
http://www.montereyinstitute.org/noaa/lesson01/l1la2.htm
C. Paleomagnetism
• The ideas of paleomagnetism are often difficult for students to grasp. Again,
visualizations are key here.
• Paleomagnetism Assignment
http://www.lcps.org/cms/lib4/VA01000195/Centricity/Domain/685/Paleomagnetism
%20Activity.pdf
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•
•
Magnetic Reversals Activity
https://www.msu.edu/~tuckeys1/highschool/earth_science/magnetic_reversals.pdf
A Model of Seafloor Spreading Activity
http://www.ucmp.berkeley.edu/fosrec/Metzger3.html or
http://www.geosociety.org/educate/LessonPlans/SeaFloorSpreading.pdf
TEACHER RESOURCES
Web Resources
•
This Dynamic Earth http://pubs.usgs.gov/gip/dynamic/dynamic.html
•
Teaching Plate Tectonics With Illustrations http://geology.com/nsta/
•
Continents on the Move www.pbs.org/wgbh/nova/ice/continents/
•
GPS—Measuring Plate Motions
http://www.iris.edu/hq/files/programs/education_and_outreach/aotm/14/1.GPS_Backgro
und.pdf
Animations and Interactive Maps
•
This Dynamic Planet Interactive Map http://nhbarcims.si.edu/ThisDynamicPlanet/index.html
•
Plate Tectonics Animations http://www.ucmp.berkeley.edu/geology/tectonics.html
•
Exploring Our Interactive Planet Interactive Mapping Tool
http://www.dpc.ucar.edu/VoyagerJr/intro.html
•
Plate Motion Simulations
http://sepuplhs.org/middle/iaes/students/simulations/sepup_plate_motion.html
•
Imagery, Maps, Movies, and References on Plate Tectonics
http://www.ig.utexas.edu/research/projects/plates/
Maps and Imagery
•
USGS Real-Time Earthquake Map. Use this real-time map to make connections between
plate boundaries and the locations of earthquakes on Earth.
http://earthquake.usgs.gov/earthquakes/map/
•
Global Volcanism Map. Use this map to make connections between plate boundaries and the
locations of volcanoes on Earth.
http://www.volcano.si.edu/world/find_regions.cfm.
•
Plate Tectonics Articles, Theory, Plate Diagrams, Maps, and Teaching Ideas
http://geology.com/plate-tectonics/
•
Imagery, Maps, Movies, and References on Plate Tectonics
http://www.ig.utexas.edu/research/projects/plates/
•
Plate Tectonic Movement Visualizations
http://serc.carleton.edu/NAGTWorkshops/geophysics/visualizations/PTMovements.html
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•
GPS Time Series Map of Plate Motions
http://sideshow.jpl.nasa.gov/post/series.html
ANSWERS TO QUESTIONS IN THE CHAPTER:
CONCEPT CHECKS
2.1 FROM CONTINENTAL DRIFT TO PLATE TECTONICS
1. Prior to the 1960s, most geologists thought the oceans and continental landmasses were in
fixed geographic positions, and had been for most of geologic time.
2. North American geologists were most opposed to the continental drift hypothesis because
much of the evidence for this idea came from unfamiliar areas to North American geologists
(Africa, South America, and Australia).
2.2 CONTINENTAL DRIFT: AN IDEA BEFORE ITS TIME
1. The first line of evidence that the continents were once connected was the jigsaw puzzle-like fit
of the coastlines of South America and Africa.
2. The discovery of the fossil remains of Mesosaurus in both South America and Africa, but
nowhere else, supports the continental drift hypothesis because this was a small aquatic
freshwater reptile that would not have been capable of making a crossing of the Atlantic
Ocean. Further, had the Mesosaurus actually been able to make that trip, the fossil remains of
the species would be much more widely distributed on each continent.
3. The prevailing view, in the early 20th century, of how land animals migrated over vast ocean
expanses included rafting, transoceanic land bridges, and island stepping. These scientists
looked for evidence of such features on the seafloor to refute hypotheses of continental drift.
4. Wegener accounts for the existence of glaciers in the southern landmasses at a time when
areas in North America, Europe, and Asia supported lush tropical swamps by suggesting that
the southern continents were joined together and located near the South Pole to provide the
conditions necessary for large glaciations. At the same time, the Northern continents were
located nearer the equator, an area conducive to the formation of great tropical swamps.
2.3 THE GREAT DEBATE
1. The two aspects of continental drift most objectionable to Earth scientists were (1) his
inability to provide a credible mechanism for continental drift and (2) his incorrect suggestion
that larger and sturdier continents could break through thinner oceanic crust.
2.4 THE THEORY OF PLATE TECTONICS
1. Following WWII, oceanographers were able to produce much better pictures of the seafloor
through advances in the technology of marine tools. From these studies, oceanographers
discovered the large oceanic ridge system winding through all of Earth’s major oceans.
2. The lithosphere consists of the uppermost mantle and overlying crust, and is a strong, rigid
layer. The lithosphere contains the plates. The asthenosphere is a weaker region of the upper
mantle; this is an area where pressures and temperatures are high enough that the rocks are
near their melting points and capable of flowing.
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3. The seven major lithospheric plates include: the North American, South American, Pacific,
African, Eurasian, Australian-Indian, and Antarctic plates.
4. The three types of plate boundaries are convergent, divergent and transform. At convergent
boundaries, plates move towards one another. At divergent boundaries, plates move away
from one another. And at transform boundaries, plates slide past one another.
2.5 DIVERGENT PLATE BOUNDARIES AND SEAFLOOR SPREADING
1. At divergent boundaries, two plates move away from one another. These boundaries are the
location of new oceanic crust, as hot rock from the mantle migrates upward to fill the void of
the diverging plates. Divergent boundaries are also called constructive plate margins due to
this creation of new rock.
2. The average rate of seafloor spreading in modern oceans is about 5 cm (2 inches) per year.
The Mid-Atlantic Ridge spreads much slower than average, at a rate of 2 cm (0.7 inches) per
year and the East Pacific Rise spreads much more quickly than average, at a rate of 15 cm (6
inches) per year.
3. The oceanic ridge system is characterized by an elevated ridge created by hot, newly formed
oceanic crust (hot rock is less dense than cool rock). At the axis of the ridge, a rift valley
develops—a deep, canyon-like structure representing the active area of spreading. Away from
the ridge, rock is cooler (and thus denser) and sits topographically lower than the ridge itself.
This cool rock is thicker as the underlying asthenosphere is cooler and more rigid. As the rock
moves away from the ridge, it also slowly accumulates sediment from the deep ocean basin.
4. Continental rifting occurs where a continental landmass is split into segments, in a similar
manner to mid-ocean ridge divergence. This occurs in areas where plate motions create
opposing forces on the lithosphere, pulling continental rock apart. In this process, the
lithosphere is thinned and crustal rocks break into large blocks, creating a central downdropped rift valley. This thinning and stretching also promotes mantle upwelling and broad
areas of upwarped lithosphere on either side of the divergence.
2.6 CONVERGENT PLATE BOUNDARIES AND SUBDUCTION
1. The balance is maintained along convergent margins where older, denser oceanic lithosphere
descends into the mantle at a rate equal to seafloor oceanic lithosphere production.
2. A continental volcanic arc is created where oceanic lithosphere converges with continental
crust—at an oceanic-continental convergent plate boundary. These volcanic arcs are
characterized by thickened continental crust (from ascending magma) as well as volcanic
mountains. Examples include the Andes Mountains of South America and the Cascade Range of
the northwest United States.
A volcanic island arc forms where two slabs of oceanic lithosphere converge—at an oceanicoceanic convergent plate boundary. These volcanic arcs are generally located 100–300 km
from a deep ocean trench. Volcanic island arcs are comprised of many volcanic cones
underlain by oceanic crust 20–35 km thick. Examples include the Aleutian, Mariana, and
Tonga islands.
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3. Deep ocean trenches are one of the surface features of continental-oceanic and oceanicoceanic convergent plate boundaries. Trenches are long, linear, deep areas of the seafloor—
the depth of the trench is dependent on the angle at which the oceanic crust subducts; this
angle is dependent on the age and density of the oceanic crust. Younger, less dense oceanic
crust creates a less-deep trench than older, denser oceanic crust. The deepest trenches are
found in the Western Pacific Ocean, where very old oceanic crust descends into the mantle.
4. Due to its mineralogy, oceanic lithosphere is more dense than continental lithosphere.
Continental crust, therefore, tends to be buoyant upon the mantle, and thus remains floating at
convergent margins. Because of its high density, the oceanic lithosphere has a greater
tendency to sink into the mantle where slabs of lithosphere meet.
5. The Himalayan Mountains are a classic example of surface features created by continentalcontinental convergent plate boundaries. When two slabs of continental lithosphere converge,
their buoyancy prevents either from being subducted. Thus, a collision between the two slabs
occurs, folding and deforming rocks of the plate boundaries. This collision causes the crust to
buckle and fracture, shorten horizontally and thicken vertically, creating large,
topographically high, mountain ranges.
2.7 TRANSFORM PLATE BOUNDARIES
1. Along a transform plate boundary, two plates slide horizontally past one another without the
production or destruction of lithosphere.
2. Transform boundaries are created where two plates move horizontally past one another and
are characterized by deep, vertical faults parallel to the plate boundary. In contrast, divergent
and convergent boundaries are characterized by motion perpendicular to the boundary.
Transform boundaries are characterized by earthquake activity, but volcanism is absent at
these boundaries. In contrast, divergent and convergent boundaries are characterized by
volcanic activity as their motions promote crustal melting.
2.8 TESTING THE PLATE TECTONICS MODEL
1. The oldest sediments recovered by deep-ocean drilling are 180 million years in age. These are
much younger than the oldest continental rocks, which are mostly hundreds of millions of
years in age, with some as much as 4 billion years in age.
2. The Hawaiian Islands get older to the northwest, with Hawaii being about 0.7 million years old
and Midway Island being about 27 million years old. Assuming hot spots remain fixed, the
Pacific plate was moving northwest while the Hawaiian Islands were forming. The chain that
includes Suiko Seamount gets older to the north; therefore, the Pacific plate was moving north
as the Suiko Seamount formed.
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3. Sedimentary cores drilled from the ocean floor provided age-distance relationships to support
the concept of seafloor spreading. Sediment age increases with distance from a divergent plate
boundary. The thickness of ocean sediments, as revealed by drilling cores, reveals that
sediments are thinnest near the spreading center, and become thicker with distance from the
ridge. This supports seafloor spreading because new crust formed at ridges would have less
time to accumulate sediment than old crust far from the ridge.
4. High- and low-intensity magnetic stripes on the seafloor provided further evidence for seafloor
spreading. As magma cools and solidifies at a spreading center (oceanic ridge), the magnetic
minerals of the magma align with Earth’s existing magnetic field. Therefore, these minerals
act as recorders of past polarity—the high-intensity stripes are regions where the crust
exhibits normal polarity, the low-intensity stripes represent regions where the crust exhibits
reverse polarity. Looking at the seafloor magnetic pattern, we see a pattern of stripes
(polarity) that is a mirror image on either side of the ridge.
2.9 HOW IS PLATE MOTION MEASURED?
1. Transform faults create the offsets of the mid-ocean ridge systems and are aligned parallel to
the direction of spreading. Scientists can measure these transform faults to determine the
direction of spreading. Further, inactive transform faults (fracture zones) that extend from the
ridge crest can also preserve a record of past directions of plate motion.
2. On Figure 2.32, rate of motion is indicated by the length of the red arrows; those arrows that
are longer indicate higher rates of motion. The three plates with the highest motion are the
Pacific plate, the Nazca plate, and the Australian-Indian plate.
2.10 WHAT DRIVES PLATE MOTIONS?
1. Slab pull is driven by cold, dense slabs of oceanic lithosphere sinking (subducting) into the
warm, less dense asthenosphere. Ridge push is gravity-driven; because the ridge is elevated
from the surrounding ocean floor, slabs of lithosphere slide down the flanks of the ridge.
Evidence from extensive subduction zones of the Pacific, Nazca, and Cocos plates suggest that
slab pull has a greater contribution to plate motion.
2. The whole-mantle convection model suggests that cold oceanic lithosphere sinks to the coremantle boundary and stirs the entire mantle. Hot mantle plumes (large and small) buoyantly
rise from the core-mantle boundary to the surface, balancing the downward flow of cold
lithosphere.
3. Whole-mantle convection stirs the entire mantle, from the surface to the core-mantle
boundary. This type of convection is characterized by slabs of cold oceanic lithosphere that
sink to the core-mantle boundary, and rising plumes of hot mantle materials from the coremantle boundary.
The layer cake model, in contrast, involves two mostly disconnected layers—an upper layer
driven by descending slabs of cold oceanic lithosphere and a sluggish lower layer that carries
heat upward with little mixing with the upper layer.
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EYE ON EARTH
EOE #1 GULF OF CALIFORNIA
1. The Gulf of California was opened by a divergent plate boundary—the East Pacific Rise.
2. The Colorado River flows into the northern end of the Gulf of California.
3. The inland sea shown in the satellite image is the Salton Sea.
EOE #2 RED SEA VOLCANIC ISLANDS
1. The new volcanic island shown was produced by the divergent boundary of the Red Sea
Rift.
2. The diverging plates of the Red Sea Rift are the African and Arabian Plates.
3. These plates are moving away from each other.
GIVE IT SOME THOUGHT
1.
a.
The observation that continents, especially South America and Africa, led Alfred
Wegener to develop his continental drift hypothesis.
b. The continental drift hypothesis was rejected by the majority of the scientific
community because Wegner could not identify a credible mechanism for continental
drift.
c. Yes, Wegner followed the basic principles of scientific inquiry. He developed a
hypothesis, a tentative explanation of his observations. He then collected data and
observations to support his hypothesis (matching fossils on different continents,
mountain ranges, fit of the continents, evidence of cold climates in tropical areas).
However, his data did not hold up under the critical testing necessary for scientific
inquiry because some of the evidence did not support continental drift, and because
technological advances allowed for a deeper understanding of the mechanisms of drift.
2.
a. A. oceanic-continental convergence, B. oceanic-oceanic convergence, C. continentalcontinental convergence
b. Volcanic island arcs form on oceanic crust at oceanic-oceanic convergent boundaries.
c. Volcanoes are absent where two continental blocks collide because the low density of
continental crust prevents either block from subducting into the mantle. No
subduction means no melting of crust, and therefore no magma for volcanoes.
d. Oceanic-oceanic convergent boundaries are different from oceanic-continental
boundaries in the types of crust involved. Oceanic-oceanic convergent boundaries are
the convergence of two oceanic plates, while oceanic-continental convergence is the
convergence of oceanic crust with continental crust. In oceanic-oceanic convergence,
volcanoes grow up from the ocean floor, whereas in oceanic-continental convergence,
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volcanoes rise from a continental platform.
Oceanic-oceanic convergent boundaries are similar to oceanic-continental boundaries
in that they both involve plates converging, they are both characterized by volcanic
activity on the over-riding plate, and they both create subduction zones characterized
by deep trenches.
3. This idea is not consistent with the Theory of Plate Tectonics for several reasons. One,
California represents a section of continental crust—we know that continental crust has a low
density, and thus is buoyant in the asthenosphere. This buoyancy would prevent sinking IF this
was a convergent boundary. However, and more importantly, California sits on a transform
plate boundary—a boundary where two plates slide past one another with no creation nor
destruction of crustal material. Portions of California west of the San Andreas fault are slowly
moving northwest as part of the Pacific plate and the movement is mostly horizontal. Far in
the geologic future, this portion of California may eventually arrive in Alaska or the Aleutian
Islands, but this would occur millions of years from now at the current rate of movement.
4.
a. Five portions of plates are shown.
b. Assuming that creation of lithosphere at the ocean ridge and destruction of
lithosphere at the subduction zone are equal, continents A and B are staying an equal
distance from each other. Because continent C is surrounded by a diverging oceanic
ridge, it is moving away from continents A and B.
c. Continents A and B both have a subduction zone along their boundary. Subduction
zones are characterized by volcanic activity as the descending slab triggers melting of
the mantle.
d. Volcanic activity might be triggered on continent C if a mantle plume were located
beneath the continent.
5. The large size of Martian shield volcanoes suggests a very long-lived source of magma. On
Earth, the motion of the Pacific Plate continues to move the plate over the hotspot, creating
new volcanoes and extinguishing the source of the older volcanoes. On Mars, perhaps plate
motion was much slower or even nonexistent, allowing for extensive building of volcanic
shields.
6. If both had been spreading at the same rate, the pattern of stripes for the two locations would
be identical; representing changes in Earth’s magnetic field over time. On Spreading Center B,
older seafloor has similarly sized stripes as those of Spreading Center A. However, newer
stripes near the ridge are narrower than those of Spreading Center A. This suggests a change
in rate of Spreading Center B at some point in the geologic past—the spreading is now slower
than that of the past.
7. In Pangea, Australia and Amercia were closer to one another geographically as part of one
large supercontinent. Therefore, similar fossil species may have existed throughout the
supercontinent. As Pangea broke apart, the Americas and Australia moved away from one
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another, effectively separating species on each landmass. Evolutionary theory suggests, then,
that those species that were once similar may have changed over geologic time.
8. Density differences at convergent boundaries create the processes and features of these
boundaries. A trench is formed where more dense oceanic crust subducts beneath less dense
oceanic or continental crust. Further, this density drives the slab to subduct into the
asthenosphere, thus triggering melting that creates volcanism at the surface. This volcanism is
driven by density differences of hot, buoyant magma and cool, relatively less dense lithosphere.
Density also drives hot spot formation—hot, buoyant magma plumes rise upward through
relatively less dense lithosphere. Density is key in the formation of large mountain ranges at
convergent continental-continental plate boundaries—continental crust has a low density,
and thus resists subducting. Folding and deformation of the leading edges of this buoyant crust
create very high mountain ranges at these boundaries.
9.
a. London, on the Eurasian plate, and Boston, on the North American plate, are currently
moving apart as a result of plate motion.
b. Honolulu, on the Pacific plate, and Beijing, on the Eurasian plate, are currently moving
closer as a result of plate motion.
c. Boston and Denver are on the same plate, and therefore are presently not moving in
respect to one another.
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