Terms you should be able to define and concepts you should understand:
Relative dating
Principle of Horizontality
Principle of Superposition
Inclusion
Unconformity
Cross-cutting
Not on this test:
Radiometric dating (see lecture notes)
Half-life (see lecture notes)
===================================================================
Figure one illustrates a geological cross section, a side view, of the rocks beneath the
surface of a hypothetical region. Use this figure to answer the two questions below the
figure.
Figure 1. Geological block diagram of a hypothetical region showing igneous intrusive features and
sedimentary rocks.
1. Of the two sequences of rocks, A-D and E-G, which were disturbed by crustal
movement after its deposition, A-D or E-G? What principal did you apply to
arrive at your conclusion?
2. Apply the principle of superposition to determine the relative ages of the
undisturbed sequence of sedimentary rocks. List the oldest rock first.
Oldest _________________________________________Youngest
Inclusions
Inclusions are pieces of one rock unit that are contained within another unit (Figure 2).
The rock mass adjacent to the one containing the inclusions must have been there first in
order to provide the rock garments. Therefore, the rock containing the inclusions is the
younger of the two.
Refer to Figure 2, below, to answer the next two questions.
3. Identify and label the inclusions in the figure
4. Of the two rocks, B and C, which rock is older?
Figure 2. Geologic block diagram showing sedimentary rocks.
Unconformities
As long as continuous sedimentation occurs at a particular place, there will be an
uninterrupted record of the material and fossils. However, if the sedimentation process is
suspended by an emergence of the area from below sea level, then no sediment will be
deposited and an erosion surface will develop. The result is that no rock record will exist
for a part of geological time. Such a gap in the rock record is termed an unconformity. An
unconformity is typically shown on a cross-sectional diagram by a wavy line. Three types of
unconformities are:
Sedimentary over eroded sedimentary rock
Sedimentary over eroded igneous or metamorphic rock
Sedimentary over angular (tilted, folded) sedimentary rock
5. Identify and label an example of an angular
unconformity in Figure 1. (above)
Principle of Cross-Cutting Relationships
Whenever a fault or intrusive igneous rock cuts through an existing feature, it is younger
than the structure it cuts. For example, if a granite dike cuts through a sandstone layer,
the sandstone had to be there first and, therefore, is older than the dike. Figure 3 (see next
page) illustrates a geological cross section showing sedimentary rocks (A,B,D,E,F, and
G), an igneous intrusion feature called a dike (C), and a fault (H). Use this figure to
answer the following 4 questions.
6. Is the igneous intrusion, C, younger or older than sedimentary rocks B and D?
7. Is fault H younger or older than the sedimentary beds A-E?
8. Is the relative age of fault H younger or older than the sedimentary layer F?
9. Did the fault occur before or after the igneous intrusion? Explain how you arrived
at your answer.
Figure 3. Geological block diagram of a hypothetical area showing an igneous intrusion (C), a fault (H),
and sedimentary rocks.
Refer to Figure 1.
10. Is the igneous intrusion H younger or older than rock layer E and younger or older
than stratum D?
11. What evidence supports the conclusion that the igneous intrusive feature called a
sill, C, is more recent than both of the rock layers B and D and older than the
igneous intrusion H?
Applying Relative Dating Techniques
Geologists often apply several of the techniques of relative dating when investigating the
geologic history of an area. Use Figure 4 to complete the questions below.
12. Which of the structures or layers are obviously igneous rocks?
13. Which structure is a fault?
14. Identify the unconformities in the figure.
15. Is rock layer I older or younger than layer J? What principle did you apply to
determine your answer?
16. Is the fault older or younger than rock layer I? What principle did you apply to
determine your answer?
17. Is the stratum K older or younger than layers A and B. What two principles did
you apply to determine your answer?
18. Is the age of intrusion L older or younger than layers J, I, H, G, and F?
19. List the entire sequence of events, in order from oldest to youngest, by writing the
appropriate letter in the space provided on the figure.
20. Explain why it was difficult to place the fault in a specific position among the
sequence of events.
Figure 4. Geological block diagram of a hypothetical area showing igneous intrusive features, a fault, and
sedimentary rocks.
=====================================================================
The following absolute age problems are NOT on Prof. Horn’s Test 2
Applying Radiometric Dates
When used in conjunction with relative dates, radiometric dates help geologists refine
their interpretations of the geological history of an area.
In Figure 4, assume that the rock layers
H and I can be dated with radioactive minerals of known half-lives (Why is this a poor
assumption?).
21. An analysis of a sample of rock from layer H indicates an equal proportion of
parent isotope and daughter produced from the parent. The half-life of the parent
is known to be 425 million years.
a. What percent of the original parent has decayed to the daughter product?
b. How many half-lives of the parent isotope have elapsed since rock H
formed?
c. What is the numerical age of the rock layer H?
The analysis of a sample of rock from layer I indicates its age to be 400 million years.
22. How many years long is the interval of time represented by the unconformity that
separates rock layer H from layer I? Explain how you arrived at your answer.
23. Is the fault older or younger than 400 million years? How did you arrive at your
answer?
24. What is the approximate maximum numerical age of the intrusion L?
Complete the following general statement describing the numerical ages of rock layers G,
F, and E.
25. All of the rock layers are (younger/older) than ________ million years.