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 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