plate tectonics: a scientific revolution …c. tectonic processes deform earth’s crust to create...

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Copyright © 2016 Pearson Education, Inc.

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 tectonics 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. From Continental Drift to Plate Tectonics

a. Early geology viewed the oceans and continents as very old features with fixed geographic

positions

b. 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. Tectonic processes deform Earth’s crust to create major structural features on Earth

d. Scientific revolution

i. Began in 20th century with continental drift hypothesis—the idea that continents

were capable of movement

ii. As more advanced, modern instruments came along, scientists evolved from the

ideas of continental drift to the theory

2. Continental Drift: An Idea Before Its Time

a. Set forth by Alfred Wegener in his 1915 book, The Origin of Continents and Oceans

i. Challenged the long-held assumption that the continents and ocean basins had fixed

geographic positions

ii. Suggested that a single supercontinent (Pangea) consisting of all Earth’s landmasses

once existed

iii. Further hypothesized that about 200 million years ago, this supercontinent began to

fragment into smaller landmasses

iv. These landmasses “drifted” to their present positions over millions of years.

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b. Evidence: The Continental Jigsaw Puzzle

i. Similarity between the coastlines on opposite sides of the Atlantic Ocean led to the

hypothesis that they were once joined

1. A very precise fit when the continental shelf boundary is considered the

edge of the continent

c. Evidence: Fossils Matching Across the Seas

i. Identical fossil organisms had been discovered in rocks from both South America

and Africa (Mesosaurus and Glossopteris)

ii. Some type of land connection was needed to explain the existence of similar

Mesozoic age life forms on widely separated landmasses

iii. Wegener asserted that South America and Africa must have been joined, and closer

to the South Pole during that period of Earth’s history

d. Evidence: Rock Types and Geologic Features

i. 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

ii. Similar evidence found in mountain belts that terminate at one coastline and

reappear on landmasses across the ocean

iii. When these landmasses are positioned as they were about 200 million years ago, as

shown in Figure 2.6B, the mountain chains form a nearly continuous belt

e. Evidence: Ancient Climates

i. Evidence of vast ice sheets covering extensive portions of the Southern Hemisphere

and India that presently lie in subtropic and tropical climates

ii. 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

iii. Can be explained by southern continents that were joined together and located near

the South Pole

1. At the same time, this geography places today’s northern continents nearer

the equator and accounts for the tropical swamps that generated the vast

coal deposits

f. As compelling as this evidence may have been, 50 years passed before most of the

scientific community accepted the concept of continental drift

3. The Great Debate

a. Rejection of the Drift Hypothesis

i. Main objections to Wegener’s hypothesis stemmed from his inability to identify a

credible mechanism for continental drift

ii. 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

iii. 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

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4. The Theory of Plate Tectonics

a. New technology post-World War II 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. Rigid lithosphere overlies weak asthenosphere

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 kilometers) in deep ocean basins, but

thinner along ridge system

b. Continental lithosphere averages 150 kilometers thick, and may

extend to 200 kilometers beneath stable continental interiors

2. The composition of 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. Earth’s major plates

i. 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

ii. None of the plates are defined entirely by the margins of a single continent nor

ocean basin

iii. Seven major plates: North American, South American, Pacific, African, Eurasian,

Australian-Indian, and Antarctic plates

iv. Intermediate-sized plates: Caribbean, Nazca, Philippine, Arabian, Cocos, Scotia, and

Juan de Fuca plates

v. Several smaller microplates

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d. Plate movement

i. Plates move as somewhat rigid units relative to all other plates

ii. Most major interactions among them (and, therefore, most deformation) occur

along their boundaries

iii. Plates are bounded by three distinct types of boundaries differentiated by the type

of movement they exhibit

1. Divergent plate boundaries—where two plates move apart, resulting in

upwelling of hot material from the mantle to create new seafloor

2. Convergent plate boundaries—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—where two plates grind past each other

without the production or destruction of lithosphere

iv. Divergent and convergent plate boundaries each account for about 40 percent of all

plate boundaries

v. Transform faults account for the remaining 20 percent.

5. Divergent Plate Boundaries and Seafloor Spreading

a. Most divergent plate boundaries are located along the crests of oceanic ridges

i. Constructive plate margins—this is where new ocean floor is generated

ii. Two adjacent plates move away from each other, producing long, narrow fractures

in the ocean crust

iii. Hot rock from the mantle below migrates upward to fill the voids left as the crust is

being ripped apart

iv. Molten material gradually cools to produce new slivers of seafloor that forms

between the spreading plates = spreading centers

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 kilometers high, 1000–4000 kilometers 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

2. Slower along Mid-Atlantic Ridge; higher along East Pacific Rise

3. 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.

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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 underlying

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 the 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

6. 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

i. A balance is maintained because older, denser portions of oceanic lithosphere

descend into the mantle at a rate equal to seafloor production

b. 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

c. 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

d. 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–Chile trench along west coast of South America

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e. 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

f. 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 America 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

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

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7. 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

8. How Do Plates and Plate Boundaries Change?

a. The size and shape of individual plates are constantly changing

i. African and Antarctic plates are continually growing in size as new lithosphere is

added to their margins

ii. Pacific plate is being consumed along its flanks faster than it is growing, so

diminishing in size

b. Boundaries of plates also migrate

i. Peru–Chile trench migrating westward due to westward drift of South American

Plate relative to Nazca plate

c. Plate boundaries can be created or destroyed in response to changes in the forces acting

on the lithosphere

d. The breakup of Pangea

i. Using modern tools geologists have recreated the steps in the breakup of this

supercontinent, an event that began about 180 million years ago

ii. Important consequence of Pangaea’s breakup was the creation of a “new” ocean

basin: the Atlantic

iii. Splitting of the supercontinent did not occur simultaneously along the margins of

the Atlantic

1. First split developed between North America and Africa, began between

200 million and 190 million years ago

2. By 130 million years ago, the South Atlantic began to open near the tip of

what is now South Africa

a. Led to the separation of Africa and Antarctica and sent India on a

northward journey

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3. 50 million years ago, India collided with Asia and created the Himalayans

and the Tibetan Highlands

4. During past 20 million years,

a. Arabia has rifted from Africa to form the Red Sea

b. Baja California has separated

e. Plate tectonics in the future

i. Geologists have extrapolated present-day plate movements into the future

1. Along the San Andreas Fault, Los Angeles and San Francisco will pass each

other in about 10 million years, and in about 60 million years the Baja

Peninsula will begin to collide with the Aleutian Islands

2. Africa may continue northward and collide with Eurasia, resulting in the

closing of the Mediterranean

3. North and South America will begin to separate, while the Atlantic and

Indian Oceans will continue to grow, at the expense of the Pacific Ocean

ii. A few geologists have even speculated on the nature of the globe 250 million years

in the future

1. Atlantic seafloor will eventually become old and dense enough to form

subduction zones around much of its margins

2. Atlantic will close, and collision of Americas with Eurasian–African

landmasses will form the next supercontinent

3. Dispersal of Pangaea will end when the continents reorganize into the next

supercontinent

iii. Projections must be viewed with skepticism, as many assumptions must be correct

for the events to unfold as hypothesized

9. Testing the Plate Tectonics Model

a. Evidence: Ocean Drilling

i. The Deep Sea Drilling Project (1968–1983) sampled the seafloor to determine

its age

1. Showed that the sediments increased in age with increasing distance from

the ridge

2. 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

ii. 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

iii. 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. Evidence: 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

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. Evidence: 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 has 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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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 the same polarity as present

magnetic field

ii. Reverse polarity—rocks with the 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

10. How Is Plate Motion Measured?

a. Geologic measurement of plate motion

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. Shows 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 Global Positioning System (GPS) 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

iii. GPS measurements have also confirmed small-scale crustal movements such as

those along faults in tectonically active areas

11. 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

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

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iii. Ridge push appears to contribute far less to plate motions than slab pull

iv. Mantle drag resists 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

v. Geologists continue to debate the nature of convective flow of the mantle

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 Summarize 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 Wegener presented to support his continental drift hypothesis.

2.3 Summarize the two main objections to the continental drift hypothesis.

2.4 List the major differences between Earth’s lithosphere and its asthenosphere, and explain the

importance of each in the plate tectonics theory.

2.5 Sketch and describe the movement along a divergent plate boundary that results in the

formation of new oceanic lithosphere.

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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 Explain why plates such as the African and Antarctic plates are increasing in size, while the

Pacific plate is decreasing in size.

2.9 List and explain the evidence used to support the plate tectonics theory.

2.10 Describe two methods researchers use to measure relative plate motion.

2.11 Describe plate-mantle convection and explain two of the primary driving forces of 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

a. 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)

b. 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 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.

http://www.geo.cornell.edu/hawaii/220/PRI/isostasy.html

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c. Hot Spot Model Activity

i. (Supplies: metal pan, spray bottle of water, about one cup of sugar, a candle or

tea light, lighter/matches). Spray a disposable metal pan with water, and

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.

d. Tracking Tectonic Plates Activity

http://serc.carleton.edu/NAGTWorkshops/intro/activities/28504.html

e. Subduction Zone Earthquake Activity

http://serc.carleton.edu/introgeo/demonstrations/examples/subduction_zone_ear

thquakes.html

f. Nannofossils Reveal Seafloor Spreading Truth Activity

http://www.oceanleadership.org/wp-content/uploads/2009/08/Nannofossils.pdf

g. You Try It: Plate Tectonics

http://www.pbs.org/wgbh/aso/tryit/tectonics/shockwave.html

h. Sea-Floor Spreading Activity

http://oceanexplorer.noaa.gov/edu/learning/player/lesson02/l2la2.htm

B. Characteristics of Plates and Boundaries

a. Students have difficulty understanding relationships between geologic processes

and plate boundaries until they can clearly visualize and analyze their relationships.

b. Discovering Plate Boundaries Activity

http://plateboundary.rice.edu/intro.html

c. A similar activity on plate boundaries using Google Earth:

http://serc.carleton.edu/NAGTWorkshops/structure/SGT2012/activities/63925.html

d. NOAA Mid-Ocean Ridge Activity

http://www.montereyinstitute.org/noaa/lesson02/l2la1.htm

e. NOAA Earthquakes and Plates Activity


C. Paleomagnetism

a. The ideas of paleomagnetism are often difficult for students to grasp. Again,

visualizations are key here.

b. Magnetic Reversals Activity

https://www.msu.edu/~tuckeys1/highschool/earth_science/magnetic_reversals.pdf

c. A Model of Seafloor Spreading Activity

http://www.ucmp.berkeley.edu/fosrec/Metzger3.html or

http://www.geosociety.org/educate/LessonPlans/SeaFloorSpreading.pdf

http://serc.carleton.edu/NAGTWorkshops/intro/activities/28504.html

http://serc.carleton.edu/introgeo/demonstrations/examples/subduction_zone_earthquakes.html

http://serc.carleton.edu/introgeo/demonstrations/examples/subduction_zone_earthquakes.html

http://www.oceanleadership.org/wp-content/uploads/2009/08/Nannofossils.pdf

http://www.pbs.org/wgbh/aso/tryit/tectonics/shockwave.html

http://oceanexplorer.noaa.gov/edu/learning/player/lesson02/l2la2.htm

http://plateboundary.rice.edu/intro.html

http://serc.carleton.edu/NAGTWorkshops/structure/SGT2012/activities/63925.html



https://www.msu.edu/~tuckeys1/highschool/earth_science/magnetic_reversals.pdf

http://www.ucmp.berkeley.edu/fosrec/Metzger3.html

http://www.geosociety.org/educate/LessonPlans/SeaFloorSpreading.pdf

33


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_Background.pdf

Animations and Interactive Maps

This Dynamic Planet Interactive Map http://nhb-arcims.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.volcanodiscovery.com/earthquake-monitor.html

Plate Tectonics Articles, Theory, Plate Diagrams, Maps, and Teaching Ideas

http://geology.com/plate-tectonics/

Imagery, Maps, Movies, and References on Plate Tectonics


Plate Tectonic Movement Visualizations

http://serc.carleton.edu/NAGTWorkshops/geophysics/visualizations/PTMovements.html

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. Briefly describe the view held by most geologists regarding the ocean basins and

continents prior to the 1960s. 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.

http://pubs.usgs.gov/gip/dynamic/dynamic.html

http://geology.com/nsta/

http://www.pbs.org/wgbh/nova/ice/continents/

http://www.iris.edu/hq/files/programs/education_and_outreach/aotm/14/1.GPS_Background.pdf

http://nhb-arcims.si.edu/ThisDynamicPlanet/index.html

http://www.ucmp.berkeley.edu/geology/tectonics.html

http://www.dpc.ucar.edu/VoyagerJr/intro.html

http://sepuplhs.org/middle/iaes/students/simulations/sepup_plate_motion.html


http://earthquake.usgs.gov/earthquakes/map/

http://www.volcanodiscovery.com/earthquake-monitor.html

http://geology.com/plate-tectonics/


http://serc.carleton.edu/NAGTWorkshops/geophysics/visualizations/PTMovements.html

http://sideshow.jpl.nasa.gov/post/series.html

34


2. What group of geologists was the least receptive to the continental drift hypothesis?

Why? North American geologists were most opposed to the continental drift hypothesis

because much of the evidence for this idea came from areas unfamiliar to North American

geologists (Africa, South America, and Australia).

2.2 CONTINENTAL DRIFT: AN IDEA BEFORE ITS TIME

1. What was the first line of evidence that led early investigators to suspect that the

continents were once connected? 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. Explain why the discovery of the fossil remains of Mesosaurus in both South America

and Africa, but nowhere else, supports the continental drift hypothesis. 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. Early in the twentieth century, what was the prevailing view of how land animals

migrated across vast expanses of open ocean? 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. How did Wegener account for evidence of glaciers in the southern landmasses at a

time when areas in North America, Europe, and Asia supported lush tropical

swamps? 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. Describe two aspects of Wegener’s continental drift hypothesis that were

objectionable to most Earth scientists. The two aspects of continental drift most

objectionable to Earth scientists were (1) his inability to provide a credible mechanism for

continental drift (he proposed that gravitational forces of the Moon and Sun that produce

Earth’s tides were capable of gradually moving continents across the globe) and (2) his

incorrect suggestion that larger and sturdier continents could break through thinner

oceanic crust.

2. What analogy did Wegner use to describe how the continents moved through the

ocean floor? Larger and sturdier continents broke through thinner oceanic crust, much as

icebreakers cut through ice.

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2.4 THE THEORY OF PLATE TECTONICS

1. What new findings about the ocean floor did oceanographers discover after World War

II? Following World War II, 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. Compare and contrast Earth’s lithosphere and asthenosphere. 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.

3. List the seven largest lithospheric plates. The seven major lithospheric plates include: the

North American, South American, Pacific, African, Eurasian, Australian-Indian, and Antarctic

plates.

4. List the three types of plate boundaries and describe the relative motion along each.

The three types of plate boundaries are convergent, divergent, and transform. At convergent

boundaries, plates move toward 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. Sketch or describe how two plates move in relation to each other along divergent plate

boundaries. 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. What is the average rate of seafloor spreading in modern oceans? The average rate of

seafloor spreading in modern oceans is about 5 centimeters (2 inches) per year. The Mid-

Atlantic Ridge spreads much slower than average, at a rate of 2 centimeters (0.7 inches) per

year and the East Pacific Rise spreads much more quickly than average, at a rate of 15

centimeters (6 inches) per year.

3. List four features that characterize the oceanic ridge system. 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. Briefly describe the process of continental rifting. Where is it occurring today?

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

36


lithosphere is thinned and crustal rocks break into large blocks, creating a central down-

dropped rift valley. This thinning and stretching also promotes mantle upwelling and broad

areas of upwarped lithosphere on either side of the divergence. Today, continental rifting is

occurring in the East African Rift Zone.

2.6 CONVERGENT PLATE BOUNDARIES AND SUBDUCTION

1. Explain why the rate of lithosphere production is roughly equal to the rate of

lithosphere destruction. 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. Why does oceanic lithosphere subduct, while continental lithosphere does not? Due to its

mineralogy, oceanic lithosphere is denser 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.

3. What characteristic of a slab of oceanic lithosphere leads to the formation of deep-

ocean trenches instead of shallow trenches? Deep ocean trenches are one of the surface

features of continental-oceanic and oceanic–oceanic 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 shallow trench than older, denser

oceanic crust, which creates deeper, more steeply sloped trenches. The deepest trenches are

found in the Western Pacific Ocean, where very old oceanic crust descends into the mantle.

4. What distinguishes a continental volcanic arc from a volcanic island arc? 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 oceanic-

oceanic convergent plate boundary. These volcanic arcs are generally located 100–300

kilometers from a deep ocean trench. Volcanic island arcs are comprised of many volcanic

cones underlain by oceanic crust 20–35 kilometers thick. Examples include the Aleutian,

Mariana, and Tonga islands.

5. Briefly describe how mountain belts such as the Himalayas form. The Himalayan

Mountains are a classic example of surface features created by continental-continental

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.

37


2.7 TRANSFORM PLATE BOUNDARIES

1. Sketch or describe how two plates move in relation to each other along a transform

plate boundary. Along a transform plate boundary, two plates slide horizontally past one

another without the production or destruction of lithosphere.

2. List two characteristics that differentiate transform faults from the two other types of

plate boundaries. 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 HOW DO PLATES AND PLATE BOUNDARIES CHANGE?

1. Name two plates that are growing in size. Which plate is shrinking in size? The African

and Antarctic plates are growing in size because they are mostly bounded by divergent

boundaries where new ocean floor is created. The Pacific Plate is shrinking in size because it is

being consumed into the mantle along much of its boundaries, and at a faster rate than it is

growing along the East Pacific Rise.

2. What new ocean basin was created by the breakup of Pangaea? The Atlantic Ocean basin

was created by the breakup of Pangaea, first between North America and Africa (180 million

years ago) and later in the Southern Ocean (130 million years ago).

3. Briefly describe changes in the positions of the continents if we assume that the plate

motions we see today continue 50 million years into the future. In North America, the

portion of Southern California to the west of the San Andreas fault will slide northward and

begin to collide with the Aleutian Islands. Africa will continue on a northward path, continuing

to collide with Eurasia and closing the Mediterranean Ocean. North and South America will

continue to separate, growing the Atlantic and Indian Oceans, and the Pacific Ocean will

continue to grow smaller.

2.9 TESTING THE PLATE TECTONICS MODEL

1. What is the age of the oldest sediments recovered using deep-ocean drilling? How do

the ages of these sediments compare to the ages of the oldest continental rocks? 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.

38


2. How do sedimentary cores from the ocean floor support the concept of seafloor

spreading? 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.

3. Assuming that hot spots remain fixed, in what direction was the Pacific plate moving

while the Hawaiian Islands were forming? 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.

4. Describe how Fred Vine and D. H. Matthews related the seafloor-spreading hypothesis

to magnetic reversals. Because of seafloor spreading, this strip of magnetized crust would

gradually increase in width. When Earth’s magnetic field reverses polarity, any newly formed

seafloor having the opposite polarity would form in the middle of the old strip. Gradually, the

two halves of the old strip would be carried in opposite directions, away from the ridge crest.

Subsequent reversals would build a pattern of normal and reverse magnetic striping on either

side of the ridge crest.

2.10 HOW IS PLATE MOTION MEASURED?

1. What do transform faults that connect spreading centers indicate about plate motion?

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. Refer to Figure 2.35 and determine which three plates appear to exhibit the highest

rates of motion. On Figure 2.35, 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.

2.11 WHAT DRIVES PLATE MOTIONS?

1. Describe slab pull and ridge push. Which of these forces appears to contribute more to

plate motion? 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 suggests that slab pull has a greater contribution to plate motion.

39


2. Briefly describe the two models of mantle–plate convection. 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 core–mantle 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.

3. What geologic processes are associated with the upward and downward circulation in

the mantle? The whole–mantle convection model suggests that cold oceanic lithosphere sinks

to the core–mantle 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.

EYE ON EARTH

EOE 2.1 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.

EOE 2.2 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.

GIVE IT SOME THOUGHT

1.

a. The observation that continents, especially South America and Africa, fit together like

pieces of a puzzle 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.

40


2.

a. A. oceanic–continental convergence, B. oceanic–oceanic convergence, C. continental-

continental 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,

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 overriding 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 or

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.

41


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 are 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 America 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

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 is responsible for subduction at convergent boundaries—older, colder, and denser

oceanic crust will subduct beneath younger, warmer, less dense crust. At divergent boundaries,

less dense magma rises to the surface to fill the void between diverging plates. Continental

plates resist subduction due to their relatively low density.

9. Hot spot volcanic chains are created as a hot, rocky plume ascends through the mantle, and

partially melts in the asthenosphere (decompression melting), and erupts at the surface in

association with uplift and crustal warping. Hot spots are not associated with plate

boundaries. Island arc volcanoes form at the site of oceanic–oceanic convergence where

subducting oceanic lithosphere triggers melting in the mantle, producing magma that rises to

the surface and erupts as volcanoes on the seafloor.

10.

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.

plate tectonics: a scientific revolution …c. tectonic processes deform earth’s crust to create...

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