1. Catastrophism was the prevailing philosophy that guided peoples' explanations of Earth prior to the time of Hutton. Catastrophists believed that Earth's landscape had been shaped primarily by great catastrophes. The philosophy of uniformitarianism, which was born with the writings of James Hutton in the late 18th century, stated that the same processes that mold Earth today shaped it in the past. Thus, in order to understand the past, one must understand the present. Catastrophism was a natural outgrowth of peoples' view that Earth was created in 4004 B.C. (a figure derived from biblical interpretation). The creation of features of the natural landscape had to be tied to this short age. In contrast, the acceptance of uniformitarianism meant accepting a very long history for Earth, for most processes take a great deal of time to create and destroy features.
2. Numerical dates indicate the number of years ago that events took place. Relative dating yields a sequence of events but does not tell us how long ago something took place.
3. The law of superposition is a basic principle of relative dating. It states that, in an undeformed sequence of sedimentary rocks (or surface-deposited igneous rocks), each bed is older than the one above and younger than the one below. When faults or igneous intrusions cut through other rocks, they are assumed to be younger than the features they cut. This is a statement of cross- cutting relationships.
4. The principle of original horizontality, which states that layers of sediment are generally deposited in a
nearly horizontal position.
5. (a) Fault A is younger than the sandstone layer.
(b) Dike A is younger than the sandstone layer.
(c) after
(d) before
(e) Fault A is older than fault B.
(f) Dike A is younger than the batholith.
6. Check to see if either rock mass contains inclusions of the other. The rock layer containing the inclusions would be the younger of the two.
7. Angular unconformities consist of folded or tilted sedimentary layers overlain by more flat-lying strata. In a disconformity, the layers are essentially parallel on either side of the gap. When older metamorphic or intrusive igneous rocks are overlain by younger sedimentary strata, a nonconformity exists.
8. Correlation involves matching rocks of similar age in different areas. By correlating rocks from one place to another, a more comprehensive view of geologic history is obtained.
9. Two conditions that improve an organism's chances of being preserved as a fossil are rapid burial and the possession of hard parts.
10. Fossils are excellent indicators of time. Rocks can often be dated because of the presence of index fossils or fossil assemblages.
11. Fossils are very useful environmental indicators.
12. The atomic number of the stable daughter product is 82, and the atomic mass number is 208.
13. With careful sample collection and laboratory procedures, the radiometric methods consistently give accurate, reliable, absolute ages. The rate at which radioactive isotopes decay is constant and can be measured accurately. Also, no other method can be applied to all of geologic time. Fossils are accurate and reliable for Phanerozoic sedimentary rocks but are not found in most igneous and metamorphic rocks and are very rare in Precambrian rocks.
14. If the ratio is 1:3, then two half-lives have transpired, making the rock 20,000 years old.
15. (a) The fraction of geologic time represented by recorded history is 1/1,000,000.
(b) The percentage of geologic time represented by abundant fossil evidence is 12%.
16. The major subdivisions of the geologic time scale, from longest to shortest are eon, era, period, and epoch. Life forms (fossils) were the primary basis for differentiating the eras.
17. In general, sedimentary rocks do not contain minerals that are both suitable for dating and that crystallized when the bed was deposited. One exception would be feldspar or mica grains in volcanic ash deposited at the time of the eruption. Minerals such as glauconite crystallize as sedimentary grains but contain large quantities of non-radiogenic daughter element, making an age determination imprecise. Sedimentary rocks are usually dated by examining their relations to igneous rocks.