Earth science 14th edition tarbuck solutions manual

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Chapter

7

Plate Tectonics: A Scientific Revolution Unfolds

Plate Tectonics: A Scientific Revolution Unfolds opens with a discussion of historical views about the placement of continents and ocean basins. It continues by examining the lines of evidence that Alfred Wegener used in the early 1900s to support his original continental drift hypothesis. This evidence included the fit of the continents, fossils, rock types, structural similarities between continents, and paleoclimates. Also presented are the main objections to Wegener’s ideas, particularly the lack of a driving mechanism.

Following a brief overview, the theory of plate tectonics is examined in detail. The distinction between Wegener’s idea of continental drift and the science of plate tectonics is addressed. The movement of lithospheric plates and different types of plate boundaries are examined extensively. This chapter looks at the features associated with different types of plate boundaries and how these features shape Earth’s surface.

Ages and distribution of ocean-basin sediments, hot spots, and paleomagnetism are used to provide additional support for plate tectonics. The chapter closes with comments about the driving mechanism of plate tectonics.

FOCUS ON CONCEPTS After reading, studying, and discussing the chapter, students should be able to: 7.1

Discuss the view that most geologists held prior to the 1960s regarding the geographic positions of the ocean basins and continents.

7.2 List and explain the evidence Wegener presented to support his continental drift hypothesis.

7.3 Discuss the two main objections to the continental drift hypothesis.

7.4 List the major differences between Earth’s lithosphere and asthenosphere and explain the importance of each in the plate tectonics theory.

7.5 Sketch and describe the movement along a divergent plate boundary that results in the formation of new oceanic lithosphere.

7.6 Compare and contrast the three types of convergent plate boundaries and name a location where each type can be found.

7.7 Describe the relative motion along a transform plate boundary and locate several examples on a plate boundary map.

7.8 Explain why plates such as the African and Antarctic plates are getting larger, while the Pacific plate is getting smaller.

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

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

7.11 Summarize what is meant by plate–mantle convection and explain two of the primary driving forces of plate motion.

TEACHING PLATE TECTONICS Plate tectonics is an important topic in geology. It can be more challenging to teach some later book chapters, especially volcanoes, earthquakes, mountain building, and the ocean floor before having established a good foundation in plate tectonics.

• When you introduce plate tectonics, you may find that many or most students are at least somewhat familiar with the fact that the continents once fit together and have since moved. It is important to

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stress, however, that this idea has been widely accepted only recently. This is a good opportunity to remind students that while Earth is very old and people have made observations about the Earth for a long time, geology itself is a relatively young science.

When discussing Wegener and his hypothesis along with the theory of plate tectonics, this is a good time to revisit or discuss the scientific method with students. You can discuss the difference between a hypothesis and a theory and how a hypothesis might become a theory. The evolution of the continental drift hypothesis to the plate tectonic theory is a nice illustration of the scientific method at work.

Mantle upwelling is part of the mantle convection process. You can demonstrate convection with a hot light bulb and a piece of paper cut into a swirl shape. Explain that similar to in the mantle, the warmer substance moves upwards where the cooler substance subsides. Alternately, you can describe the example of a boiling pot of water to relate this topic to something with which students are already familiar or bring in a lava lamp to show this process.

The use of animations that show historical continent locations and future projections of continent locations is helpful for visualizing how the surface of the Earth has changed over time due to plate tectonics. See Additional Resources.

Animations of motion at the three different types of plate boundaries are helpful supplements to the illustrations in the text. They allow students to see how these plates move relative to each other.

After introducing the three types of plate boundaries, have students draw them in their notebooks along with the relative motions of each without referencing anything you have discussed. It will help them reinforce these boundary types and how they differ.

When you talk about subduction, it is a good time to revisit the topic of igneous rocks and remind students of the composition of basalt and of granite. Remind students that the denser basalts will be more likely to subduct, and you can relate this back to mineral content. It keeps students from forgetting earlier course concepts and makes the topic more cohesive by tying earlier units into newer ones.

If you don’t mind food in your classroom, Oreos or similar sandwich style cookies can be used to show plate motion and plate boundary types. Break the top portion of the cookie in half. It will then slide about on the semi-solid filling layer. This is especially good for illustrating transform fault motion.

Print off several copies of a world map with plate boundaries and plate names on it. If you can print this on a large piece of paper, it works a bit better. Cut apart the map at plate boundaries. Distribute one “plate” to each student and have them find other “plates” to create a complete map. This gives students an idea of how many plates exist and where they are found relative to each other.

Although earthquakes and volcanoes are prominent plate boundary features, save in-depth discussions of these two topics for the chapters dedicated to them. It is important to mention them in the context of plate tectonics, but tell students you will discuss them in more detail later.

CONCEPT CHECK ANSWERS Concept Check 7.1 1. Briefly describe the view held by most geologists regarding the ocean basins and continents prior to the 1960s.

Ocean basins and continents had fixed geographic positions and were very old.

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

North American geologists were the least receptive because most of the evidence was from Africa, South America, and Australia; these continents were unfamiliar to these geologists.

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Concept Check 7.2 1. What was the first line of evidence that led early investigators to suspect that the continents were once connected?

Similarity of continental coastlines and jigsaw-like fit of the continents.

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.

Mesosaurus was a freshwater reptile and could not have migrated across the ocean. In addition, there is no evidence of any bridge or other connector between these two continents.

3. Early in the twentieth century, what was the prevailing view of how land animals migrated across vast expanses of open ocean?

Lower sea level and land bridges, rafting, and island stepping stones.

4. How did Wegener account for the existence of glaciers in the southern landmasses at a time when areas in North America, Europe, and Asia supported lush tropical swamps?

Pangaea; southern continents were one landmass situated about the South Pole.

Concept Check 7.3 1. What two aspects of Wegener’s continental drift hypothesis were objectionable to most Earth scientists?

Gravitational force of the Sun and Moon moved the continents the way they moved the tides and the idea that continents plowed through the oceanic crust.

Concept Check 7.4 1. What major ocean floor feature did oceanographers discover after World War II? Mid ocean ridge.

2. Compare and contrast the lithosphere and the asthenosphere.

The lithosphere is the hard outer portion of the Earth’s crust. The asthenosphere lies beneath and is more ductile, allowing the lithospheric plates to “float” and be mobile.

3. List the seven largest lithospheric plates.

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

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

• Divergent – plates move apart.

• Convergent – plates move towards each other.

• Transform – plates slide laterally past each other.

Concept Check 7.5 1. Sketch or describe how two plates move in relation to each other along divergent plate boundaries.

Plates move apart, with new rock being formed between them due to magmatic upwelling.

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

5 cm (2 in) per year.

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3. List four facts that characterize the oceanic ridge system.

Long, continuous underwater mountain chain, divergent plate boundary, marked by formation of new basaltic rock, rift valleys along the crest.

4. Briefly describe the process of continental rifting. Where is it occurring today?

Mantle plumes beneath continental crust generate magmatic upwelling and forces that pull the crust apart, generating a new rift valley. The rift valley eventually widens to create a new ocean basin. It is occurring today in East Africa.

Concept Check 7.6 1. Explain why the rate of lithosphere production roughly balances with the rate of lithosphere destruction.

Subduction at convergent boundaries, which destroys old lithosphere, occurs at the same rate as seafloor spreading and new rock generation at divergent plate boundaries.

2. Compare a continental volcanic arc and a volcanic island arc.

A continental volcanic arc occurs on the edge of a continental land mass near an ocean-continent convergent plate boundary. These volcanoes tend to produce more felsic to intermediate magma. A volcanic island arc occurs at ocean-ocean plate convergent plate boundaries and produces an island chain in the ocean; these volcanic islands are basaltic.

3. Describe the process that leads to the formation of deep-ocean trenches.

When two oceanic plates collide, one is subducted. As one plate descends beneath the other, a trench is formed; deeper trenches tend to form where older, denser basaltic ocean floor can descend very deeply beneath the other plate, often at very steep angles.

4. Why does oceanic lithosphere subduct, while continental lithosphere does not?

Oceanic lithosphere is denser than continental lithosphere. Continental lithosphere is too buoyant to allow for subduction.

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

As two continental plates collide, the crust buckles, thickens vertically, and fractures. This leads to the uplift of major mountain ranges such as the Himalayas.

Concept Check 7.7 1. Sketch or describe how two plates move in relation to each other along a transform plate boundary.

The plates slide laterally past each other. See Figure 7.21 for a diagram.

2. Differentiate between transform faults and the other types of plate boundaries.

Transform faults do not generate new lithosphere or destroy old lithosphere. The movement involves two plates sliding past each other rather than moving apart or coming together.

Concept Check 7.8 1. What two plates are growing in size? Which plate is shrinking?

The African and Antarctic plates are growing and the Pacific plate is shrinking.

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

The Atlantic.

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3. Briefly describe some major changes to the globe when we extrapolate present-day plate movements 50 million years into the future.

North America’s Baja Peninsula and parts of southern California will be closer to Alaska. Africa will collide with Eurasia, closing the Mediterranean Sea. Australia will migrate towards the equator, North and South America will separate, and the Atlantic Ocean basin will be larger.

Concept Check 7.9 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 ocean sediments are 180 million years old, while the oldest continental rocks are more than 4 billion years old.

2. Assuming that hot spots remain fixed, in what direction was the Pacific plate moving while the Hawaiian islands were forming? When Suiko Seamount was forming?

The Pacific plate was moving northwest when the Hawaiian islands were forming and moving north when the Suiko Seamount was forming.

3. How do sediment cores from the ocean floor support the concept of seafloor spreading?

Sediment cores can be radiometrically dated, and their distance from a spreading center recorded. Data have shown that older cores are further from the mid-ocean ridges while the youngest rocks and sediments are near the ridges.

4. Describe how Fred Vine and D.H. Mathews related the seafloor-spreading hypothesis to magnetic reversals.

They found that new rock made from cooling magma magnetizes itself in the current direction of Earth’s magnetic polarity. They found lateral symmetry in magnetic patterns on either side of mid-ocean ridges, showing that these separated stripes were formed at the same time at the mid-ocean ridges.

Concept Check 7.10 1. What do transform faults that connect spreading centers indicate about plate motion?

These transform faults are aligned parallel to the direction of spreading. Measuring their alignments carefully will reveal the direction of plate movement.

2. Refer to Figure 7.35 and determine which three plates appear to exhibit the highest rates of motion.

The Pacific plate, the Australian-Indian plate, and the Nazca plate.

Concept Check 7.11 1. Describe slab pull and ridge push. Which of these forces appears to contribute more to plate motion?

As old, dense lithosphere subducts, the slab is pulled deep into the mantle by gravity and destroyed. The elevation of mid-ocean ridges allows new lithosphere to slide down; this is ridge push. Slab pull seems to be the dominant force.

2. What role are mantle plumes thought to play in the convective flow in the mantle?

Mantle plumes are thought to originate deep within the mantle, close to the core. The heat from the bottom of these plumes rises through the mantle, generating convective flow.

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3. Briefly describe the two models proposed for mantle-plate convection.

• Whole-mantle convection – also called the plume model. Subducted lithosphere descends to the coremantle boundary and this is balanced by buoyantly rising mantle plumes that move hot material to the surface.

• Layer cake model – has two zones of convection; one is a thin, dynamic upper mantle layer and one is slower and thicker in the lower mantle. Slow convection in the lower level carries heat upward but the upper layer is what generates surface volcanism.

GIVE IT SOME THOUGHT ANSWERS 1. After referring to the section in the Introduction titled “The Nature of Scientific Inquiry,” answer the following: a. What observation led Alfred Wegener to develop his continental drift hypothesis? b. Why did the majority of the scientific community reject the continental drift hypothesis? c. Do you think Wegener followed the basic principles of scientific inquiry? Support your answer. a. The puzzle-like fit of the coastal outlines of South America and Africa. b. The continental drift hypothesis was rejected by the scientific community because Wegener lacked a credible mechanism to explain why the continents were moving or drifting. Also, he incorrectly proposed that the continents broke through the thinner oceanic crust although no evidence existed to support this idea. c. Yes, overall Wegener followed the basic principles of scientific inquiry. He proposed evidence to support his various explanations although not all of his evidence supported his conclusions.

2. Referring to the accompanying diagrams that illustrate the three types of convergent plate boundaries, complete the following: a. Identify each type of convergent boundary. b. On what type of crust do volcanic island arcs develop? c. Why are volcanoes largely absent where two continental blocks collide? d. Describe two ways that oceanic–oceanic convergent boundaries are different from oceanic– continental boundaries. How are they similar? a. A = ocean to continent convergence, B = ocean to ocean convergence, C = continent to continent convergence b. Oceanic crust c. Volcanoes are absent at continent to continent convergent boundaries because there is no subduction and therefore no mechanism to produce partial melting of mantle rocks. d. Ocean to ocean convergent boundaries are different from ocean to continent convergent boundaries in that the resulting volcanism occurs on the ocean floor rather than on land and the composition of the volcanoes between the two would be somewhat different due to the interaction of continental rather than oceanic crust. They are similar in that both of them produce volcanism, they both involve subduction zones, and both are characterized by earthquakes.

3. Some predict that California will sink into the ocean. Is this idea consistent with the theory of plate tectonics? Explain.

California will not necessarily sink into the ocean, but the continued motion of the Pacific plate will separate a portion of California from the North American plate along the San Andreas Fault system. The broken segment will then move with the Pacific plate as a large island since it is composed of continental crust rather than oceanic.

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4. Refer to the accompanying hypothetical plate map to answer the following questions: a. How many portions of plates are shown? b. Are continents A, B, and C moving toward or away from each other? How did you determine your answer? c. Explain why active volcanoes are more likely to be found on continents A and B than on continent C. d. Provide at least one scenario in which volcanic activity might be triggered on continent C. a. five b. Continents A, B, and C are moving away from each other because of the divergent boundary that occurs between all three of them. c. Active volcanoes are found on both A and B because each of them has an oceanic plate that is subducting underneath the continental plate. d. Continent C does not have volcanoes because it does not have a plate boundary involving subduction or rifting. Hot spot activity or continental rift could perhaps produce volcanoes in the future.

5. Volcanoes, such as the Hawaiian chain, that form over mantle plumes are some of the largest shield volcanoes on Earth. However, several shield volcanoes on Mars are gigantic compared to those on Earth. What does this difference tell us about the role of plate motion in shaping the Martian surface?

The extremely large volcanoes on Mars suggests that either the tectonic plates were moving really slowly when the volcanism occurred or perhaps Mars was lacking tectonic plates and the large volcanoes resulted from hot, stationary plumes inside of Mars.

6. Imagine that you are studying seafloor spreading along two different oceanic ridges. Using data from a magnetometer, you produced the two accompanying maps. From these maps, what can you determine about the relative rates of seafloor spreading along these two ridges? Explain.

Along the first ridge, the plate movements were apparently steady and relatively fast (as evidenced by the narrow magnetic stripes). On the second ridge, the movements are relatively slower over the most recent geologic times, but the plates were apparently moving faster at some point further back in time.

7. Australian marsupials (kangaroos, koala bears, etc.) have direct fossil links to marsupial opossums found in the Americas. Yet the modern marsupials in Australia are markedly different from their American relatives. How does the breakup of Pangaea help to explain these differences (see Figure 7.24)?

The break-up of Pangaea some 200 million years ago allowed for the separation of once-joined landmasses and the isolation of common organisms to undergo a long period of evolutionary development. The resulting organisms today share a common ancestry because of their common origins on Pangaea, but the long period of time has allowed for distantly related groups to develop their own unique characteristics.

8. Density is a key component in the behavior of Earth materials and is especially important in understanding key aspects of plate tectonics. Describe three different ways that density and/or density differences play a role in plate tectonics.

Density or density differences in plate tectonics play a key role in 1) the subduction of more dense oceanic crust underneath continental crust; 2) the rising of hotter mantle material at divergent boundaries and the sinking of colder lithospheric plates at subduction zones, thus creating the “conveyor belt” model that somehow drives the plate tectonics engine; and 3) the collision and uplift of continental crust at continent to continent convergent boundaries due to lower densities of continental rocks.

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9. Refer to the accompanying map and the pairs of cities below to complete the following: (Boston, Denver), (London, Boston), (Honolulu, Beijing) a. List the pair of cities that is moving apart as a result of plate motion. b. List the pair of cities that is moving closer as a result of plate motion. c. List the pair of cities that is presently not moving relative to each other. a. London, Boston b. Honolulu, Beijing c. Boston, Denver

EXAMINING THE EARTH SYSTEM ANSWERS 1. The changing positions of the continents and the redistribution of land and water over Earth’s surface have had a significant impact on Earth’s atmosphere, hydrosphere, and biosphere through time. Atmospheric and oceanic circulations are interrelated systems driven by heat energy from the Sun. As continents moved about, the distribution of heat energy over Earth’s surface varied, which, in turn, caused changes in global wind patterns and ocean circulation. Different atmospheric and oceanic circulation patterns produced changes in temperature, precipitation, storm tracks, and global climates in general. Furthermore, when the continents were assembled into large landmasses, their size and location produced climates much different from today.

Life on Earth was greatly affected by the distribution of landmasses and the resulting climates. At times landmasses may have been arid and may have promoted the evolution of certain adaptable species. Then, as a consequence of plate tectonics, the landmass may have split and/or changed global position and become tropical, forcing extinctions and/or adaptations. A good example of the impact of plate tectonics on life is found in the unique species that currently inhabit Australia.

2. Assume that plate tectonics did not cause the breakup of the supercontinent Pangaea. Use the accompanying map of Pangaea to describe how the climate (atmosphere), vegetation and animal life (biosphere), and geologic features (geosphere) would be different from the conditions that exist today in the areas currently occupied by the cities of Miami, Florida; Chicago, Illinois; New York, New York; and your college campus location.

Miami, FL: Miami would be situated on the equator, warmer, and perhaps a tropical rainforest. Without the breakup of Pangaea, Miami would be landlocked and continental rather than having beaches and a coast.

Chicago, IL: Chicago would be far removed from any moisture sources (the Atlantic Ocean and Gulf of Mexico do not exist) and therefore rather arid. Its location in the subtropics would probably result in warm temperatures.

New York, NY: New York would be part of present northwest Africa and, like Chicago, inland from any sources of moisture, dry, and subtropical. Pre-Pangaea continental collisions would have formed the Appalachian Mountains.

Your college campus: (answers will vary)

ADDITIONAL RESOURCES DVDs and Movies • •

How the Earth Was Made (2008) Narrated by Alec Baldwin. History Channel, 1 hour 34 minutes.

Inside Planet Earth (2008) Narrated by Patrick Stewart. Discovery Channel, 2 hours.

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Earth Revealed, Episode 5: The Birth of a Theory (1992) Annenberg Media, 30 minutes. Available on DVD or for free streaming video on demand from http://www.learner.org/resources/series78.html Earth Revealed, Episode 6: Plate Dynamics (1992) Annenberg Media, 30 minutes. Available on DVD or for free streaming video on demand from http://www.learner.org/resources/series78.html Planet Earth, Episode 1: The Living Machine (1986) WQED/Pittsburgh in association with the National Academy of Sciences, 1 hour. Discusses plate tectonics. Available on DVD or for free streaming video on demand from http://www.learner.org/resources/series49.html Plate Tectonics. Science Channel, 2 minutes. Dr. Neal Driscoll shows California coastline features to Bill Nye. Available for free streaming from http://www.sciencechannel.com/tv-shows/greatestdiscoveries/videos/100-greatest-discoveries-plate-tectonics.htm When Earth Erupts: Glaciers vs. Tectonics. Science Channel, 2 minutes. Exploration on New Zealand’s South Island. Available for free streaming from http://www.sciencechannel.com/tvshows/greatest-discoveries/videos/when-earth-erupts-glaciers-vs-tectonics-video.htm Peaceful Easy Obduction. Musical discussion of plate tectonic motion performed by Dr. Richard Alley of Penn State University. 5 minutes. http://www.youtube.com/watch?v=f_BWRlx6VpA

Websites • • • •

Plate Tectonics Animations.

From the University of California, Berkeley. http://www.ucmp.berkeley.edu/geology/tectonics.html Mountain Maker, Earth Shaker. From PBS. Interactive plate tectonics activity that allows manipulation of tectonic plates. http://www.pbs.org/wgbh/aso/tryit/tectonics/# Thirteen different plate tectonics animations. From the U.S. Geological Survey and National Park Service. You can choose the one that best suits your needs. http://www2.nature.nps.gov/geology/ usgsnps/animate/pltecan.html Plate Tectonic Movement Visualizations. From Carleton College. A collection of various visualizations and descriptions of each. http://serc.carleton.edu/NAGTWorkshops/geophysics/ visualizations/PTMovements.html

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