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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
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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
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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)
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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
because the density of thick continental crust is too low to permit it to sink
into the asthenosphere.
Oceanic–Oceanic Boundary: When a convergent boundary forms
between plates of oceanic lithosphere, the plate that is older, thicker, and
denser subducts the less dense plate.
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.
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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)
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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 presentday 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
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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.
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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:
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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.
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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.
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