Key Concept Review (Answers to in-text “Concept Checks”)
Chapter 3
1. Wegener believed that the heavy continents were slung toward the equator on the
spinning Earth by a centrifugal effect. He was wrong in that belief, but correct in
sensing that continents were moving.
2. Where were the tracks through old seabed that the migrating continents might leave?
What forces were strong enough to propel continents over long distances?
3. 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. Density is usually expressed in grams per cubic centimeter (g/cm3).
5. No one has yet sampled below Earth’s outermost layer, the crust.
6. 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.
7. The P wave (or primary wave), is a compressional wave similar in behavior to a
sound wave. Rapidly pushing and pulling a very flexible spring (like a Slinky)
generates P waves. The S wave (or secondary wave) is a shear wave like that seen in
a rope shaken side to side.
8. The P and S waves from Alaska were very large and easily detected at great
distances. When correlated with the frequency, intensity, and phase characteristics of
the waves, this information helped to confirm the models of Earth layering.
9. Earth’s layers are classified by composition and by physical properties.
Understanding the physical properties of the layers is important in understanding
tectonic processes.
10. Lithosphere includes crust (oceanic and continental) and rigid upper mantle down to
the asthenosphere. Note that the rigid sandwich of crust and upper mantle—the
lithosphere—floats on (and is supported by) the denser deformable asthenosphere.
11. The outer core is thought to be liquid.
12. 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. Because granite is comparatively light, the top of the
continent can float high above sea level.
13. Much of the heat inside Earth results from the decay of radioactive elements.
14. Some of this internal heat journeys toward the surface by conduction – the same
process that makes a frying pan’s handle hot.
15. Understand Earth’s age was hampered by an interpretation of the biblical account of
Creation. At the end of the eighteenth century most European natural scientists
believed in a young Earth, one that had formed only about 6,000 years ago. Since
many of the processes observed on Earth had clearly operated for much greater spans
of time, confusion was understandable.
16. 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
1|Page
demanded explanation, and researchers redoubled their efforts to discover the links
after the conclusion of the Second World War.
17. 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.
18. 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.
19. Have a go at your own synthesis, and then compare your drawing with Figure 3.37.
20. The three types of plate boundaries that result from these interactions are called
divergent, convergent, and transform boundaries, depending on their sense of
movement.
21. Ocean basins are formed at divergent plate boundaries; island arcs form, continents
collide, and crust recycles at convergent plate boundaries; crust fractures and slides at
transform plate boundaries.
22. 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.
23. A subducting plate's periodic downward lurches cause earthquakes and tsunami.
24. 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.
25. 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.
26. Hot spots are one of the surface expressions of plumes of magma rising from
relatively stationary sources of heat in the mantle. My favorite hot spot is Iceland,
but the island of Hawaii is a close second.
27. 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.
28. Subduction guarantees young seafloors. Consider the Pacific: New seabed is made at
the east Pacific rise, transits to the northwest, and disappears in the trenches seaward
of the Aleutian Islands and Japan. This process takes less than 200 million years, so
the seabed and its sediments are always young.
29. You might live on a terrane. If you live in western North America, check Figure
3.35.
2|Page
30. A review of the bulleted list in this section will provide food for thought. What other
questions come to mind?
31. 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!
3|Page