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The Earth Through Time, 10th Edition
by Harold L. Levin
CHAPTER 7—PLATE TECTONICS UNDERLIES ALL EARTH
HISTORY
CHAPTER OUTLINE FOR TEACHING
I.
Seismic Waves
A. Body Waves
1. Primary (P-waves)
2. Secondary (S-waves)
B. Surface Waves
1. Love
2. Rayleigh
II. Earth’s Interior
A. Main Discontinuities in Earth
1. Mohorovičić (30-40 km)
2. Gutenberg (2,900 km)
B. Earth’s Core
1.
2.
3.
4.
5.
Detected by P- and S-wave shadow zones
Average density 10.7 g/cm 3
Composed mainly of Fe and Ni
Radius: 3500 km
Inner core (solid) and outer core (liquid)
C. Earth’s Mantle
1. Stony composition (4.5 gm/cm 3)
2. Peridotite (including olivines and pyroxenes)
3. Upper mantle features
a. low velocity zone
b. asthenosphere (plastic layer)
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D. Earth’s Crust
1. Oceanic crust (basaltic; 3.0 gm/cm3)
2. Continental crust (granitic; 2.7 gm/cm 3)
a. thickest crust (35 km)
b. floats according to isostasy
III. Crustal Structures
A. Faults
1. Normal (gravity)
2. Reverse (including thrust faults)
3. Lateral (strike-slip faults)
B. Folds
1. Anticlines and synclines
2. Domes and basins
3. Monoclines
IV. Plate Tectonics
A. Older Concept of Continental Drift
1. Ancient supercontinents—later fragmented and dispersed
a. Gondwanaland (Suess’ idea of the 19th century)
b. Pangea (Wegener’s idea of 1915)
2. Evidence cited for drifting
a. geologic and geographic fit
b. climatic evidence
c. fossil distribution (Glossopteris, Mesosaurus)
d. rock sequences
B. Discoveries about Paleomagnetism (1950s-1960s)
1. Magnetic poles susceptible to switching, N-S
2. Rocks preserve weak remnant magnetism from time of formation
a. themal
b. depositional
3. Apparent polar-wandering paths
C. Basic Concept of Modern Plate Tectonics
1. Integration of ideas: unifying concept
a. continental drift
b. sea-floor spreading
c. paleomagnetic data
2. Mechanism of movement: plates float on asthenosphere
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3. Plate boundaries
a. divergent (sea-floor spreading, rifting)
b. convergent (trenches, folded mountains, subduction zones)
c. transform (great lateral faults)
4. Wilson cycles: opening and closing of an ocean basin on time scale of hundreds of
millions of years
5. Driving mechanism: competing theories
a. mantle convection—induced drag on the lithosphere (thermal plumes)
b. “ridge-push” and “slab-pull”
6. Effect of mantle’s thermal plumes
a. doming of overlying plate
b. three-armed crustal rifts
7. Test of plate tectonics theory
a. sea-floor studies of Hess (1960s)
b. paleomagnetism of seafloor (Vine, 1963)
c. sea-floor ages
d. sediment cores from drilling ships
e. laser measurements of crustal motion from space
f. seismic and gravity evidence
g. ages of hot spot volcanoes like the Hawaiian islands
8. Hot spots: island chains and seamounts
9. Exotic terrains (far-traveled microcontinents)
Answers to Discussion Questions
1. Drilling from the North Pole to Earth’s center would penetrate oceanic crust, lithosphere,
asthenosphere, upper mantle, lower mantle, outer core, and inner core.
2. The three major categories of seismic waves are primary, secondary, and surface. Primary
(P) waves travel fastest at 4 to 5 km/sec by pushing particles in directions parallel to direction
of propagation. Secondary (S) waves travel 1 to 2 km/sec by moving particles at right angles
to the direction of propagation. Surface waves are large-motion waves that travel through the
outer crust resembling outward-moving concentric water waves.
3. An S-wave shadow zone indicates that the Earth has a liquid outer core as liquid absorbs Swaves thus creating a shadow zone.
4. A seismic discontinuity is a narrow zone within Earth where there is an abrupt change in
velocity or direction of body waves. The Gutenberg and Mohorovičić discontinuities mark
boundaries between major internal zones (crust-lithosphere and lower mantle-outer core
boundaries, respectively).
5. Eroded anticlines and domes tend to expose the oldest rocks of the folded strata near their
centers, whereas eroded synclines and basins tend to expose their youngest strata near their
centers.
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6. The average continental crustal composition is exemplified by granite; the oceanic crust by
basalt. Peridotite, an ultramafic rock, exemplifies the mantle.
7. P-waves are the only sort of body waves typically detected at a site 180 o from the epicenter
of an earthquake.
8. The main types of faults are normal, reverse, and lateral. Compressional forces create
reverse faults (including a low-angle variety called thrust fault). Tensional forces tend to
create normal faults (including oblique faults with rotational movement).
9. A gravity anomaly is the difference between the observed value of Earth’s gravity at any point
on the Earth and the computed theoretical value. Negative gravity anomalies occur where
there is excess low-density rock below the surface, for example at deep-sea trenches and
mid-ocean ridges.
10. Folds are bent rock strata in the Earth’s crust. The principal kinds of folds are anticline,
dome, syncline, basin, and monocline.
11. Alfred Wegener might have cited the following evidence in support of his ideas on continental
motion: geological similarities of India, Africa, and South America; the geographic fit of the
continents; similar climates of parts indicated in widely separated areas; evidence that some
areas at low latitude had once been at a pole; tropical fossils and rock types occurring today
at high latitude; and the distribution of key fossil species that could only be explained by
continental linkage in the past.
12. Oceanic volcanoes issue basaltic lavas; continental volcanoes (adjacent to subduction
zones) issue basaltic and andesitic lavas. The difference in lavas is due to the origin of the
liquid rock, which is partially melted upper mantle in the instance of basalt and partially
melted down-going plate in the instance of rhyolite-andesite.
13. The Himalayan Mountains resulted from the collision of the Indo-Australian plate (with India
riding on the northern tip) with the southern margin of the Eurasian plate. The San Andreas
fault is a sliding boundary created by the lateral passage of the North American plate and the
Pacific plate. The Dead Sea and the Red Sea are two areas of a three-arm rift that is splitting
the African and Eurasian plates apart.
14. According to plate tectonics, new sea-floor material is continually added at the mid-ocean
ridges and destroyed continually at the oceanic trenches.
15. The leading edge of the North American plate is located along the western coast of North
America (California to Alaska) and the trailing edge is located along the mid-ocean ridge in
the North Atlantic.
16. Paleomagnetism is the weak magnetic character of rock acquired at the time of formation. In
igneous rocks, it is acquired when the rock cools below the Curie point, 578 o C. In
sedimentary rock, it is acquired upon settling of magnetic grains to seafloor. The magnetic
inclination and declination can be measured via magnetometer to indicate vectors toward the
ancient pole. These vectors would parallel lines of force in Earth’s past field. Validation of
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plate tectonics has come through confirmation of sea-floor spreading (“magnetic stripes”) and
assisting in latitudinal reconstruction of continental positions in past.
17. Ancient rocks are confined to the continents because the lighter continental rocks are not
subducted and re-melted during plate tectonics.
18. Exotic terrains will have rock types and fossil contents that are significantly different from
directly adjacent areas. In the Appalachians, a terrain which showed southern ocean faunas
would be a likely alien terrain. A micro-continent-derived terrain would consist of continental
rock, probably granitic rock and sedimentary rock, whereas an island-arc terrain would have
a volcanic rock composition mainly.
19. Variations of magnetization of seafloor show parallel, symmetrically arranged “stripes” on the
seafloor about ridge axes. Thus, they show continuous spreading of the ridges while Earth’s
magnetic field changed over time.
20. The alignment and age distribution of volcanic islands in the Pacific Ocean shows
progressive plate movement over the site of a stationary mantle plume’s upwelling of basaltic
magmas (a hot spot).
21. a
22. d
23. b
CHAPTER ACTIVITIES
Student activities for in-depth learning:
1. On Earth, plate tectonics has been a dynamic process for hundreds of millions of years. Take
a look at the “plate tectonics movie” on the University of California, Berkeley, Museum of
Paleontology web page (http://www.ucmp.berkeley.edu/geology/tectonics.html). Do this by
clicking on “animated GIFs” for the last 750 million years. Play the movie. As time passes, the
continents seem to move around. Why is this so? What is “pushing” the continents around and
why? Why do the continents appear to move around over time?
2. Using the fault-motion animations at http://www.iris.edu/gifs/animations/faults.htm observe the
differences between a normal and a reverse fault and the differences and similarities (if any)
between a strike-slip and an oblique slip fault. Record your observations on how these pairs of
faults are different and how they are similar. What kind of forces: compression or extension
would be involved in forming these faults. (Observe the video and see for yourself.)
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