Chapter 8: Measuring Geological Time Questions & Answers
What are the features of a useful index fossil?
A useful index fossil must have survived for a relatively short period (e.g., around a million years), and also should have a wide distribution so that it can be used to correlate rocks from different regions.
Based on your answer to question 4, what can you say about the age of layer C in the figure above?
Layer C must be between 5,000 y and 275 Ma.
What type of unconformity exists between layer C and rock B?
The granite (F) was eroded prior to deposition of C, so it's likely that layer B was also eroded at the same time. If so, that makes the boundary between C and B a disconformity.
Figure 19.29 shows a geological cross-section. The granitic rock F at the bottom is the rock that you estimated the age of in the exercise in 19.4, Radiometric Dating with Potassium-Argon. A piece of wood from layer D has been sent for radiocarbon dating and the result was 0.55 14C remaining. How old is layer D?
The granitic rock F has been dated to 175 Ma. The wood in layer D is approximately 5,000 years old, so we can assume that layer D is no older than that, although it could be as much as a few hundred years younger if the wood was already old when it got incorporated into the rock.
We cannot use magnetic chronology to date anything older than ~780,000 years. Why?
The last magnetic reversal was 780,000 years ago, so all rock formed since that time is normally magnetized and it isn't possible to distinguish older rock from younger rock within that time period using magnetic data
What type of unconformity exists between layer C and rock F?
The unconformity between layer C and rock F is a nonconformity.
How did William Smith apply the principle of faunal succession to determine the relative ages of the sedimentary rocks of England and Wales?
William Smith was familiar with the different diagnostic fossils of the rocks of England and Wales and was able to use them to identify rocks of different ages.
A granitic rock contains inclusions (xenoliths) of basalt. What can you say about the relative ages of the granite and the basalt?
Xenoliths of basalt within a granite must be older than the granite according to the principle of inclusions Xenoliths: a piece of rock within a an igneous rock that is not derived from the original magma but has been introduced from elsewhere, especially by the surrounding country rock.
Explain the differences between a) disconformity and paraconformity; and b) nonconformity and angular unconformity
a. At both disconformities and paraconformities the beds above and below are parallel, but at a disconformity there is clear evidence of an erosion surface (the lower layers have been eroded). b. A nonconformity is a boundary between sedimentary rocks above and non-sedimentary rocks below while an angular unconformity is a boundary between sedimentary rocks above and tilted and eroded and sedimentary layers below.
