Understanding Geologic Time and Unconformities
Geologic Time: An unconformity is a buried surface of erosion separating younger strata above from older strata below. Which of the following best characterizes an angular unconformity? Tilted strata lie below the unconformity; bedding in younger strata above is parallel to the unconformity. In the late 1700s, James Hutton published his important work titled Theory of the Earth. Which of the following is not a very long-lived radioactive isotope? C-14. Catastrophism was a philosophy that was first expounded by the early Greeks. The subdivision of the geologic time scale that represents the longest time span is called a(n) era. The ratio of parent to daughter isotopes in a radioactive decay process is 0.40. How many half-lives have elapsed since the material was 100% parent atoms? Between 1 and 2. Consider the names of the eras in the geologic time scale. What is meant by “zoic”? Life; living things. Who is credited with formulating the doctrine of uniformitarianism? Hutton. Which of the following geologic observations would not bear directly on working out the sequence of geologic events in an area? The feldspar and quartz contents of a granite. What fundamental concept states that in a horizontal sequence of conformable sedimentary strata, each higher bed is younger than the bed below it? The law of superposition. What of the following refers to the investigative process by which geologists identify and match sedimentary strata and other rocks of the same ages in different areas?
a. Super matching
b. Correlation
c. Strata indexing
d. Cross-access dating
13. The ____________ is the idea or concept that ancient life forms succeeded each other in a definite, evolutionary pattern and that the contained assemblage of fossils can determine geologic ages of strata?
a. Principle of cross correlation
b. Law of fossil regression
c. Law of correlative indexing
d. Principle of faunal succession
14. Which of the following is an essential characteristic of an index fossil?
a. The organism lived only in specific environments such as beaches or estuaries
b. The organism only lived for a short period of geologic time
c. The fossils are exceptionally abundant and well preserved
d. The fossils occur in deep-water marine sediments, but the organism actually lived in the sunlit, surface layer of the ocean
15. By applying the law of superposition, _______ dates can be determined.
a. Conventional
b. Radiometric
c. Relative
d. Both relative and radiometric
16. The era of “ancient life” is the _______ era.
a. Cenozoic
b. Paleozoic
c. Mesozoic
d. Precambrian
17. About 88 percent of geologic time is represented by the time span called the __________.
a. Paleozoic
b. Precambrian
c. Mesozoic
d. Phanerozoic
18. What is the age of the Earth accepted by most scientists today?
a. 6.4 billion years
b. 4.5 million years
c. 4.5 billion years
d. 6.4 million years
19. The era known as the “age of mammals” is the __________.
a. Precambrian
b. Paleozoic
c. Mesozoic
d. Cenozoic
20. The half-life of carbon-14 is about 6000 years. Assume that a sample of charcoal formed by burning of living wood 15,000 years ago. How much of the original carbon-14 would remain today?
a. More than one-half
b. Between one-fourth and one-eighth
c. Between one-half and one-fourth
d. Between one-half and one-third
21. Visualize five horizontal sedimentary strata exposed in a cliff or canyon wall identified by consecutive numbers, 1 being the lowest bed and 5 being the highest. Which of the following statements concerning the strata are true?
a. Bed 5 is the oldest
b. Beds 1 and 3 are older than bed 4
c. Bed 4 is older than bed 2
d. Bed 3 is older than beds 2 and 4
22. Which of the following denotes the divisions of the geologic time scale in correct order of decreasing lengths of time beginning with the longest time interval and ending with the shortest?
a. Eon, era, epoch, period
b. Era, period, epoch, eon
c. Eon, epoch, period, era
d. Eon, era, period, epoch
23. Assume that man’s recorded history can be stretched back to 4600 years before the present. This is approximately what fraction of geologic time?
a. One ten-thousandth
b. One millionth
c. One billionth
d. One hundred-thousandth
24. A worm would stand a poor chance of being fossilized because __________.
a. Worms have been rare during the geologic past
b. Worms have no hard parts
c. Worms contain no carbon-14
d. All of these
25. ____________ is an erosional contact between tilted, older strata below and horizontal, younger strata above.
a. Inverse bedding
b. An angular unconformity
c. A disconformity
d. Cross cutting
26. Who made the first clear statement of the law of superposition? When?
a. John Wesley Powell, 19th century
b. William Smith, 18th century
c. John Stuart Priestly, 19th century
d. Nicolaus Steno, 17th century
27. Sandstone strata and a mass of granite are observed to be in contact. Which of the following statements is correct geologically?
a. The sandstone is younger if the granite contains sandstone inclusions
b. The sandstone is younger if it shows evidence of contact metamorphism
c. The granite is older if the sandstone contains pebbles of the granite
d. The granite is older if it contains inclusions of sandstone
28. Assume that you have just examined several flat-lying sedimentary layers. After much study, you determine that there is a considerable span of time for which no sedimentary rock layer exists at this site. You have just discovered a(n) __________.
a. Angular unconformity
b. Series of conformable strata
c. Disconformity
d. Example of cross-cutting relationships