Answers to STUDY BREAK Questions
Essentials 5th
Chapter 3
EARTH STRUCTURE AND PLATE TECTONICS
1. What force did Wegener believe was responsible for the movement of continents?
He believed "centrifugal force" (inertia, actually) and tidal drag was the motive power
for continental drift.
2. What were the greatest objections to Wegener’s hypothesis?
He was incorrect in believing "centrifugal force" and tidal drag was the motive power
for continental drift. The continental tracks that would have proven his theory -- trailing
scars left on the seabed by the movement of continents -- were never found. He also
assumed that the continents had split only once, while we now know that the process of plate
tectonics is a lengthy cycle, with lithospheric plates suturing and splitting over great spans of
time. (The cycle has been called the Wilson Cycle in honor of the insights of Canadian
geologist John Tuzo Wilson.)
Wegener’s main contributions were to draw attention to the diverse bits of evidence
suggesting continents were once together, and to stimulate geophysical study of the
underpinnings of continents.
3. What do we mean when we say something is dense?
Something is said to be dense if it weighs a lot per unit of volume. Density is an
expression of the relative heaviness of a substance.
4. How is density expressed (units)?
Density is usually expressed in grams per cubic centimeter (g/cm3).
5. How can seismic waves be used to “see” inside Earth?
Seismic waves form in two types: surface waves and body waves. Surface waves
can sometimes be seen as an undulating wave-like motion in the ground. Surface waves
cause most of the property damage suffered in an earthquake. Body waves (P waves and S
waves) are less dramatic, but they are useful for analyzing Earth’s interior structure.
The time of transit through Earth, the changes in the “sound” of the waves (analogous
to our hearing difference in the treble or bass in our stereo systems), the attenuation of the
waves, and the later arrival of faint echos all can be used to analyze the interior.
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Answers to STUDY BREAK Questions
Essentials 5th
6. List the Earth’s internal layers by physical characteristics.
Different conditions of temperature and pressure prevail at different depths, and these
conditions influence the physical properties of the materials subjected to them. The behavior
of a rock is determined by three factors: temperature, pressure, and the rate at which a
deforming force (stress) is applied. This behavior, in turn, determines how (and if) rocks will
move. We can classify the internal layers by physical properties as follows:
The lithosphere -- the Earth's cool, rigid outer layer -- is about 100 – 200 kilometers
(60 - 125 miles) in thickness. It is comprised of the brittle continental and oceanic crusts and
the uppermost cool and rigid portion of the mantle.
The asthenosphere is the thin, hot, slowly-flowing layer of upper mantle below the
lithosphere. Extending to a depth of about 350 – 650 kilometers (220 - 400 miles) the
asthenosphere is characterized by its ability to deform plastically under stress.
The lower mantle extends to the core. Though it is hotter than the asthenosphere, the
greater pressure at this depth probably prevents it from flowing.
The core is divided into two parts: the outer core is a viscous liquid with a density
about 4 times that of the crust, the inner core a solid with a maximum density of about 6
times crustal material.
As we saw in the chapter, recent research has shown that slabs of Earth's relatively
cool and solid surface -- its lithosphere -- float and move independently of one another over
the hotter, partially molten asthenosphere layer directly below. The physical properties of
each make this possible, so classification by physical properties is more useful in explaining
plate tectonics.
7. What is the relationship between crust and lithosphere? Between lithosphere and
asthenosphere?
Lithosphere includes crust (oceanic and continental) and rigid upper mantle down to
the asthenosphere. The velocity of seismic waves in the crust is much different from that in
the mantle. This suggests differences in chemical composition, or crystal structure, or both.
The lithosphere and asthenosphere have different physical characteristics: the lithosphere is
generally rigid, but the asthenosphere is capable of slow plastic movement. Asthenosphere
and lithosphere also transmit seismic waves at different speeds.
8. Why is Earth’s interior still hot? Shouldn’t it have cooled off by now?
Much of the heat inside Earth results from the decay of radioactive elements. Some
of this internal heat journeys toward the surface by conduction.
9. How can continents be supported high above sea level?
A continent floats above sea level because the lithosphere gradually sinks into the
deformable asthenosphere until it has displaced a volume of asthenosphere equal in mass to
the continent’s mass.
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Answers to STUDY BREAK Questions
Essentials 5th
10. How did a careful plot of earthquake locations affect the discussion of the Theory of
Continental Drift (as it was first called)? What about the jigsaw-puzzle-like fit of continents
around the Atlantic?
The jigsaw-puzzle fit of continents around the Atlantic and the distinctly non-random
distribution of earthquakes stimulated vigorous discussion in geological circles. Hugo
Benioff’s plots of earthquake activity surrounding the Pacific Ring of Fire demanded
explanation, and researchers redoubled their efforts to discover the links after the conclusion
of the Second World War.
11. How did an understanding of radioactive decay and radiometric dating and influence the
debate?
Radiometric dating allowed rock sequences to be dated and their relative positions
through time determined. Radiometric studies also solidified understanding of Earth’s age,
assuring researchers that Earth was indeed older than 6,000 years and that time was sufficient
for large-scale seafloor spreading.
12. What were the key insights that Hess and Wilson brought to the discussion?
Hess (and Dietz) suggested that new seafloor develops at the Mid-Atlantic Ridge (and
the other newly discovered ocean ridges) and then spreads outward from this line of origin.
Continents would be carried along by the same forces that cause the ocean to grow. This
motion could be powered by convection currents. In 1965 John Tuzo Wilson integrated the
ideas of continental drift and seafloor spreading into the overriding concept of plate tectonics.
13. Can you outline – in very simple terms – the action of Earth’s crust described by the
Theory of Plate Tectonics?
Have a go at your own synthesis, and then compare your drawing with Figure 3.12.
14. What kinds of plate boundaries exist? Can you tell what happens at each provide
examples?
The three types of plate boundaries that result from these interactions are called
divergent, convergent, and transform boundaries, depending on their sense of movement.
15. About how fast do plates move?
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Answers to STUDY BREAK Questions
Essentials 5th
Though spreading speeds can reach a rate of 18 centimeters (7 inches) a year along
parts of the Pacific plate, most plates move more slowly, about 3 centimeters (1.3 inches)
each year.
16. What causes most earthquakes and volcanoes?
A subducting plate's periodic downward lurches cause earthquakes and tsunami.
Volcanoes often occur over a subducting plate or at divergent plate boundaries where the
crust is stressed and thinned.
17. Is Earth’s magnetic field a constant? That is, would a compass needle always point
north?
Earth’s magnetic field reverses at irregular intervals of a few hundred thousand years.
In a time of reversal a compass needle would point south instead of north, and any particles
of magnetic material falling below their Curie points in fresh seafloor basalt at a spreading
center would be imprinted with the reversed field.
18. How can Earth’s magnetic field be “frozen” into rocks as they form?
A compass needle points toward the magnetic north pole because it aligns with
Earth’s magnetic field (Figures 3.25 and 3.26). Tiny particles of an iron-bearing magnetic
mineral called magnetite occur naturally in basaltic magma. When this magma erupts at midocean ridges, it cools to form solid rock. The magnetic minerals act like miniature compass
needles. As they cool to form new seafloor, the magnetic minerals’ magnetic fields align
with Earth’s magnetic field. Thus the orientation of Earth’s magnetic field at that particular
time becomes frozen in the rock as it solidifies. Any later change in the strength or direction
of Earth’s magnetic field will not significantly change the characteristics of the field trapped
within the now-solid rocks.
19. Can you explain the matching magnetic alignments seen south of Iceland (Figure 3.27)?
The alternating magnetic stripes represent rocks with alternating magnetic polarity—
one band having normal polarity (magnetized in the same direction as today’s magnetic field
direction), and the next band having reversed polarity (opposite from today’s direction).
Researchers realized that the pattern of alternating weak and strong magnetic fields was
symmetrical because freshly magnetized rocks born at the ridge are spread apart and carried
away from the ridge by plate movement.
20. How does the long chain of Hawaiian volcanoes seem to confirm the Theory of Plate
Tectonics?
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Answers to STUDY BREAK Questions
Essentials 5th
The northern Pacific contains an “assembly line” chain of islands which extends from
the old eroded volcanoes of the Emperor Seamounts to the still-growing island of Hawaii.
The Pacific Plate is moving northwest relative to a mantle plume anchored in the mantle
below.
21. Earth is 4,600 million years old, and the ocean nearly as old. Why is the oldest ocean
floor so young -- rarely more than 200 million years old?
The light, ancient granitic continents ride high in the lithospheric plates, rafting on the
moving asthenosphere below. In subduction, heavy basaltic ocean floor (and its overlying
layer of sediment) plunges into the mantle at a subduction zone to be partially remelted, but
the light granitic continents ride above, too light to subduct. The subducting plate may be
very slightly more dense than the upper asthenosphere on which it rides, and so is pulled
downward into the mantle by gravity. Because the ocean floor itself acts as a vast "conveyor
belt" transporting accumulated sediment to subduction zones where the seafloor sinks into
the asthenosphere, no marine sediments (or underlying crust) are of great age. The ocean
floor is recycled; the continents just jostle above the fray. Figure 3.22, showing accreting
terranes, demonstrates this nicely.
22. Do you live on a terrane?
Changes are good that you live on a terrane. If you live in North America, check
Figure 3.35.
23. Can you suggest areas for future research in plate tectonics?
A review of the bulleted list in this section will provide food for thought. What other
questions come to mind?
24. In your opinion, how has an understanding of plate tectonics revolutionized geology?
It’s difficult to underestimate the effect our understanding of plate tectonics has had
on all areas of science. Coal in the Antarctic? Latitudinal variations in the Australian Barrier
Reef? Similar fossils across separated continents? The relative youth of the seabed?
Earthquake distribution (and prediction)? It’s hard to know where to stop!
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