12

Chapter 2 Internal Structure of Earth and Plate Tectonics

Learning Objectives

The surface of Earth would be much different—relatively smooth, with monotonous topography—if not for the active tectonic processes within Earth that produce earthquakes, volcanoes, mountain chains, continents, and ocean basins. In this chapter, we focus directly on the interior of Earth. Your goals in reading this chapter should be to

• understand the basic internal structure and processes of Earth. • know the basic ideas behind and evidence for the theory of plate tectonics. • understand the mechanisms of plate tectonics. • understand the relationship of plate tectonics to natural hazards.

Chapter Outline

2. Internal Structure of Earth and Plate Tectonics 2.1. Internal Structure of Earth

2.1.1. The Earth Is Layered and Dynamic 2.1.2. Continents and Ocean Basins Have Significantly Different Properties

2.2. How We Know about the Internal Structure of the Earth 2.2.1. What We Have Learned about Earth from Earthquakes

2.3. Plate Tectonics 2.3.1. Movement of the Lithospheric Plates

2.3.1.1. What Is a Plate? 2.3.1.2. Locations of Earthquakes and Volcanoes Define Plate Boundaries 2.3.1.3. Seafloor Spreading Is the Mechanism for Plate Tectonics 2.3.1.4. Sinking Plates Generate Earthquakes 2.3.1.5. Plate Tectonics Is a Unifying Theory

2.3.2. Types of Plate Boundaries A Closer Look 2.1: The Wonder of Mountains 2.3.3. Rate of Plate Motion

2.4. A Detailed Look at Sea Floor Spreading 2.4.1. Paleomagnetism

2.4.1.1. Earth’s Magnetic Field Periodically Reverses 2.4.1.2. What Produces Magnetic Stripes? 2.4.1.3. Why Is the Seafloor No Older than 200 Million Years?

2.4.2. Hot Spots 2.5. Pangaea and Present Continents 2.6. How Plate Tectonics Works: Putting It Together

13

2.7. Plate Tectonics and Hazards

Chapter Summary

Our knowledge concerning the structure of Earth’s interior is based on the study of seismology. Thus we are able to define the major layers of Earth, including the inner core, outer core, mantle, and crust. The uppermost layer of Earth is known as the lithosphere, which is relatively strong and rigid compared with the soft asthenosphere found below it. The lithosphere is broken into large pieces called plates that move relative to one another. As these plates move, they carry along the continents embedded within them. This process of plate tectonics produces large landforms, including continents, ocean basins, mountain ranges, and large plateaus. Oceanic basins are formed by the process of seafloor spreading and are destroyed by the process of subduction, both of which result from convection within the mantle.

The three types of plate boundaries are divergent (midoceanic ridges, spreading centers), convergent (subduction zones and continental collisions), and transform faults. At some locations, three plates meet in areas known as triple junctions. Rates of plate movement are generally a few centimeters per year.

Evidence supporting seafloor spreading includes paleomagnetic data, the configurations of hot spots and chains of volcanoes, and reconstructions of past continental positions.

The driving forces in plate tectonics are ridge push and slab pull. At present, we believe the process of slab pull is more significant than ridge push for moving tectonic plates from spreading centers to subduction zones.

Plate tectonics is extremely important in determining the occurrence and frequency of volcanic eruptions, earthquakes, and other natural hazards.

Answers to Review Questions:

1. What are the major differences between the inner and outer cores of Earth? (p. 27)

The inner core is solid with a thickness of more than 1300 km (808 mi) that is roughly the size of the moon but with a temperature about as high as the temperature of the surface of the sun. The inner core is believed to be primarily metallic, composed mostly of iron (about 90 percent by weight), with minor amounts of elements such as sulfur, oxygen, and nickel. Whereas the outer core is liquid with a thickness of just over 2000 km (1243 mi.) with a composition similar to that of the inner core.

 2. How are the major properties of the lithosphere different from those of the

asthenosphere? (pp. 27–32)

14

nto the asthenosphere.

The lithosphere includes the crust and part of the mantle, and the asthenosphere is located entirely within the mantle. The lithosphere is broken into large ridged pieces called lithospheric plates that move relative to one another (Figure 2.5a). Processes associated with the creation, movement, and destruction of these plates are collectively known as plate tectonics. The outer layer (or lithosphere) is approximately 100 km (approximately 62 mi.) thick and is stronger and more rigid than the deeper asthenosphere, which is a hot and slowly flowing layer of relatively low-strength rock.

3. What are the three major types of plate boundaries? (pp. 32–34)

There are three basic types of plate boundaries: divergent, convergent, and transform. These boundaries are zones that range from a few to hundreds of kilometers across. Plate boundary zones are narrower in ocean crust and broader in continental crust.

Divergent boundaries occur where new lithosphere is being produced and neighboring parts of plates are moving away from each other. Typically this process occurs at midocean ridges, and the process is called seafloor spreading.

Convergent boundaries occur where plates collide. They can be divided into three sub groups: Oceanic–Continental Boundary, Oceanic–Oceanic Boundary and Continental–Continental Boundary.

 Oceanic–Continental Boundary: When oceanic and continental plates converge, the oceanic plate must subduct beneath the continental plate

ecause the density of thick continental crust is too low to permit it to sink bi

enser subducts the less dense plate.

 Oceanic–Oceanic Boundary: When a convergent boundary forms

etween plates of oceanic lithosphere, the plate that is older, thicker, and bd Continental–Continental Boundary: When subduction brings two continents together, limited subduction may occur, but the buoyancy of continental crust eventually stops the subduction. The contraction of crust in the collision zone doubles the thickness of continental crust and creates high mountains. Slivers of oceanic crust are commonly uplifted in the mountain range and record the basin consumed by subduction prior to collision of the continents.

 Transform boundaries, or transform faults, occur where the edges of two plates slide past each other. Transform boundaries are generally found in two settings. Most are located on the sea floor offsetting ridge axes. Some occur within continents such as the San Andreas Fault in California.

15

4. What is the major process that is thought to produce Earth’s magnetic field? (p. 38)

Convection occurs in the iron-rich, fluid, hot outer core of Earth because of compositional changes and heat at the inner–outer core boundary. As more buoyant material in the outer core rises, it starts the convection. The convection in the outer core, along with the rotation of Earth that causes rotation of the outer core, initiates a flow of electric current in the core. This flow of current within the core produces and sustains Earth’s magnetic field.

5. Why has the study of paleomagnetism and magnetic reversals been important in understanding plate tectonics? (p. 39)

Earth’s magnetic field is sufficient to permanently magnetize some surface rocks. For example, volcanic rock that erupts and cools at mid-oceanic ridges becomes magnetized at the time it passes through a critical temperature. At that critical temperature, known as the Curie point, iron-bearing minerals (such as magnetite) in the volcanic rock orient themselves parallel to the magnetic field. This is a permanent magnetization known as thermoremnant magnetization.

The term paleomagnetism refers to the study of the magnetism of rocks at the time their magnetic signature formed. It is used to determine the magnetic history of Earth.

Marine geologists towed magnetometers, instruments that measure magnetic properties of rocks, from ships and completed magnetic surveys. The paleomagnetic record of the ocean floor is easy to read because of the fortuitous occurrence of the volcanic rock basalt (see Chapter 5) that is produced at spreading centers and forms the floors of the ocean basins of Earth. The rock is finegrained and contains sufficient iron-bearing minerals to produce a good magnetic record. The marine geologists’ discoveries were not expected. The rocks on the floor of the ocean were found to have irregularities in the magnetic field. These irregular magnetic patterns were called anomalies or perturbations of Earth’s magnetic field caused by local fields of magnetized rocks on the seafloor. The anomalies can be represented as stripes on maps. When mapped, the stripes form quasi-linear patterns parallel to oceanic ridges. The marine geologists found that their sequences of stripe width patterns matched the sequences established by land geologists for polarity reversals in land volcanic rocks. These magnetic anomalies on the sea floor added new evidence to support the theory of plate tectonics.

6. What are hot spots? (p. 40)

16

Hot spots are characterized by volcanic centers resulting from hot materials produced deep in the mantle (a mantle plume), perhaps near the core–mantle boundary. The partly molten materials are hot and buoyant enough to move up through mantle and overlying moving tectonic plates. An example of a continental hot spot is the volcanic region of Yellowstone National Park. Hot spots are also found in both the Atlantic and Pacific Oceans. If the hot spot is anchored in the slow-moving deep mantle, then, as the plate moves over a hot spot, a chain of volcanoes is produced. Perhaps the best example of this type of hot spot is the line of volcanoes forming the Hawaiian-Emperor Chain in the Pacific Ocean. Along this chain, volcanic eruptions range in age from present-day activity on the big island of Hawai’i (in the southeast) to more than 78 million years ago near the northern end of the Emperor Chain.

    7. What is the difference between ridge push and slab pull in the explanation of plate

motion? (p. 46)

Ridge push is a gravitational push, like a gigantic landslide, away from the ridge crest toward the subduction zone (the lithosphere slides on the asthenosphere). Slab pull results when the lithospheric plate moves farther from the ridge and cools, gradually becoming denser than the asthenosphere beneath it. At a subduction zone, the plate sinks through lighter, hotter mantle below the lithosphere, and the weight of this descending plate pulls on the entire plate, resulting in slab pull. Of the two processes, slab pull is the more influential of the driving forces. Calculations of the expected gravitational effects suggest that ridge push is of relatively low importance compared with slab pull.

Answers to Critical Thinking Questions:

1. Assume that the supercontinent Pangaea (see Figure 2.18*) never broke up. Now deduce how Earth processes, landforms, and environments might be different from how they are today with the continents spread all over the globe. Hint: Think about what the breakup of the continents did in terms of building mountain ranges and producing ocean basins that affect climate and so forth.

If Pangaea never broke up, Earth processes would continue to erode existing mountains with no new mountain building. The land would be nearly flat and covered with sediments from the erosion of the mountains. Ocean circulation

17

would remain the way it was back then, giving a warmer temperate climate known to the dinosaurs and not the circulation patterns we have today which gave us the ice ages. Mass extinctions are mostly the results of plate tectonics. If the plate tectonic process stopped, then life would probably have only gradual changes rather than abrupt changes we see in the geologic time scale.

* Textbook question states Figure 2.17, however, it should read 2.18.

Suggested Activities

1. Compare population density map with hazardous regions. Different types of hazards: coastal regions and regions that are within close proximity to fault zones and volcanoes.

2. Collect and discuss newspaper clippings of different hazards that occur around the world on a daily basis.

Additional Resources (media, film, articles, journals, web sites)

Print Resources Dealing with Natural Hazards

Abbott, P.L., 2012, Natural Disasters, 8th ed., McGraw Hill, Boston, 512 pp. Bryant, E.A., 1993, Natural Hazards, Cambridge University Press, Cambridge, 294 pp. Eldredge, N., 1998, Life in the Balance, Princeton University Press, Princeton, 224 pp. Erikson, J., 2001, Quakes, Eruptions, and Other Geologic Cataclysms, Revealing the

Earth’s Hazards, Facts on File Science Library, The Living Earth Series, New York, 310 pp.

Griggs, G.B., and Gilchrist, J.A., 1983, Geologic Hazards, Resources, and Environmental Planning, Belmont, CA, Wadsworth Publishing Co., 502 pp.

Keller, E.A., 2000, Environmental Geology, eighth ed., Prentice Hall, Englewood Cliffs, N.J., 562 pp.

Kusky, T.M., 2004, Encyclopedia of Earth Science, 528 pages, Facts on File, New York, ISBN 0816049734.

Kusky, T.M., 2003, Geological Hazards: A Sourcebook, an Oryx Book, Greenwood Press, Westport, Conn., 300 pp., ISBN 1-57356-469-9.

18

Mackenzie, F.T., and Mackenzie, J.A., 1995, Our Changing Planet: An Introduction to Earth System Science and Global Environmental Change, Prentice Hall, Englewood Cliffs, N.J., 387 pp.

Murck, B.W., Skinner, B.J., and Porter, S.C., 1997, Dangerous Earth: An Introduction to Geologic Hazards, John Wiley and Sons, New York, 300 pp.

Skinner, B.J., and Porter, B.J., 1989, The Dynamic Earth: An Introduction to Physical Geology, John Wiley and Sons, New York, 541 pp.

Nonprint Sources Dealing with Natural Hazards

http://edcwww.cr.usgs.gov/ EROS Data Center lists satellite images, land cover maps, elevation models, maps, and aerial photography useful for Natural Hazards Studies.

NASA’s web site on Natural Hazards: http://earthobservatory.nasa.gov/NaturalHazards/ NASA’s Earth Observatory lists satellite images of natural hazards, including dust, smoke, fires, floods, severe storms, and volcanoes.

USGS web site for Natural Hazards: http://www.usgs.gov/themes/hazard.html USGS activities in the hazards theme area deal with describing, documenting, and understanding natural hazards and their risks. The web page contains explanations of individual hazards, geographic distribution of hazards, and fact sheets on hazards. The site also has links describing USGS involvement in recent hazards.

http://www.accuweather.com/blogs/weathermatrix/ WeatherMatrix is a worldwide organization of over 3000 amateur and professional weather enthusiasts—meteorologists, storm chasers and spotters, and weather observers from all parts of the globe. WeatherMatrix was formerly the Central Atlantic Storm Investigators (CASI). Has frequently updated news about weather-related disasters.

http://www.colorado.edu/hazards/o/ This web site is the online version of the periodical, The Natural Hazards Observer. It contains features about various hazards and disasters. It also provides information of emergency management, research, politics, and education of natural disasters.

Organizations Dealing with Natural Hazards

Congressional Natural Hazards Work Group is a cooperative endeavor between a group of private and public organizations, whose goal is to develop a wider understanding within Congress of the value of reducing the risks and costs of natural disasters. The work group supports the effort of the Congressional Natural Hazards Caucus. Information on the Natural Hazards Caucus Work Group can be found at:

19

http://www.agiweb.org/gap/workgroup/resources.html. Some of the lead organizations include the American Meteorological Society and University Corporation for Atmospheric Research (http://www2.ucar.edu) and the National Science Foundation (http//www.nsf.gov).

Federal Emergency Management Agency FEMA 500 C Street SW Washington, D.C. 20472 202-646-4600 http://www.fema.gov FEMA is the nation’s premier agency that deals with emergency management and preparation, and issues warnings and evacuation orders when disasters appear imminent. FEMA maintains a web site that is updated at least daily and includes information on hurricanes, floods, fires, national flood insurance, and disaster prevention, preparation, and emergency management. Divided into national and regional sites. Also contains information on costs of disasters, maps, and directions on how to do business with FEMA.

U.S. Geological Survey U.S. Department of the Interior 345 Middlefield Road Menlo Park, CA 94025 650-329-5042 Also, offices in Reston, VA, Denver, CO http://www.usgs.gov/ The USGS is responsible for making maps of many of the different types of hazards discussed in this book, including earthquake and volcano hazards, tsunami, floods, landslides, and radon. The USGS National Landslide Information Center web site is http://landslides.usgs.gov/html_files/nlicsun.html.

National Oceanographic and Atmospheric Administration (NOAA) http://www.noaa.gov/ NOAA conducts research and gathers data about the global oceans, atmosphere, space, and sun, and applies this knowledge to science and service that touch lives of all Americans. NOAA’s mission is to describe and predict changes in Earth’s environment, and conserve and wisely manage the nation’s coastal and marine resources. NOAA’s strategy consists of seven interrelated strategic goals for environmental assessment, prediction, and stewardship. These include (1) advance short-term warnings and forecast services, (2) implement season to interannual climate forecasts, (3) assess and predict decadal to centennial change, (4) promote safe navigation, (5) build sustainable fisheries, (6) recover protected species, and (7) sustain healthy coastal ecosystems. NOAA runs a web site that includes links to current satellite images of weather hazards, issues warnings of current coastal hazards and disasters, and has an extensive historical and educational service.

20

The National Hurricane Center, http://www.nhc.noaa.gov/, is a branch of NOAA, and posts regular updates of hurricane paths and hazards.

The National Drought Mitigation Center http://www.drought.unl.edu/ The National Drought Mitigation Center helps people and institutions develop and implement measures to reduce societal vulnerability to drought. The NDMC, based at the University of Nebraska-Lincoln, stresses preparation and risk management rather than crisis management.