Study Guide for Chapter 18

# Study Guide for Chapter 18 - Chapter 18 Answers to the...

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Chapter 18 Answers to the Review Questions 1. Absolute dating involves a numerical age measurement in actual time units, like thousands or millions of years. Relative dating involves placing sequences of rocks, geological features, and events in the correct order in which they occurred, without necessarily knowing their absolute ages. 2. The law of superposition is the idea or notion that beds in a sequence of horizontal, sedimentary strata become younger upward in the sequence. In other words, younger strata are deposited over older strata. A feature that truncates or cuts across another geologic feature is the younger of the two. This is known as the principle of cross- cutting relationships. For example, a dike of basalt injected into a crack in sedimentary strata is younger than the strata. 3. The principle of original horizontality states that, in general, stratification in sedimentary beds was horizontal when the beds were deposited. 4. (a) Is fault A older or younger than the sandstone layer? Fault A cuts the sandstone layer so the fault is younger. (b) Is dike A older or younger than the sandstone layer? Dike A also crosscuts the sandstone layer so the dike is younger. (c) Was the conglomerate deposited before or after fault A? Fault A stops at the base of the conglomerate; thus the conglomerate layer truncates the fault and is younger than the fault. (d) Was the conglomerate deposited before or after fault B? The conglomerate is cut and displaced by fault B; thus fault B is younger. (e) Which fault is older, A or B? The faults do not cross, but the relationship between the faults and the conglomerate proves that fault A is older than fault B. (f) Is dike A older or younger than the batholith? Dike A does not cut the batholith so other relationships must be used. Dike B clearly cuts the batholith; the sill fed by dike B is crosscut by dike A, proving that dike A is younger than dike B and younger than the batholith. 5. A depositional contact or unconformity would be proven if detrital rock and mineral grains from the granite were found in the sandstone. Also the granite just below the contact might show reddish discoloration or other evidences of having been weathered before the sandstone was deposited. Bedding in the sandstone will be parallel or nearly

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Study Guide for Chapter 18 - Chapter 18 Answers to the...

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