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The cross-sections in Figures 1 and 2 depict the results of many geologic “events,” including deposition of sediments, erosion to form unconformities, movement of faults, and intrusion of magmas. Use the principles of superposition, cross-cutting relationships, inclusions and original horizontality to place these events in relative order.
Figure 1.
Label all of the unconformities marked X, Y, and Z on Figure 1 as disconformities, angular unconformities, or nonconformities (mark these directly on the figure).
What process does the stippled pattern around Units T, D, and the top of Unit B indicate?
Why does the top of Unit B show a stippled pattern, while Unit S does not?
List the events in Figure 1 in relative order. Start working from the oldest toward the youngest.
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Youngest event
Oldest event
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Figure 2
Label all of the unconformities on Figure 2 as disconformities, angular unconformities, or nonconformities.
List the events depicted in Figure 2 in their correct relative order.
Part 2—Absolute (radiometric) dating
Absolute dating of Earth materials is possible because unstable isotopes of elements decay to other isotopes over time. The decay process releases energy in the form of radioactivity, thus absolute dating is also known as radiometric dating. The time that it takes half of an unstable isotope to decay from the parent material to its daughter product is known as the half-life. This rate of decay is unique and constant for each radioactive isotope. Absolute dating relies on measuring the proportion of parent and daughter product to determine the number of half-lives that have elapsed. When does the clock start? The radioactive decay process starts when the mineral crystals in magma or metamorphic rocks cool to their blocking temperatures.
In order to better understand how radiometric dating works, we will work in groups to simulate the radioactive decay process using dice. Each group will be given a container containing 50 dice each representing an atom of the fictional radioactive element Statium. We will assume that at the beginning of our “experiment” the blocking temperature has been reached and all of the atoms in the container are the parent isotope (Statium). Over time these unstable atoms will decay to a stable daughter isotope.
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