RELATIVE DATING
INTRODUCTION
Suppose you take a burro ride deep into the Grand Canyon. As you take the precarious trail,
you notice clear differences in the rock layers. Some layers exhibit a particular color
whereas others have unique fossils. What can rock layers reveal to scientists?
The study and comparison of exposed rock layers in various parts of the Earth led scientists
in the early 19th century to propose that the rock layers could be correlated from place to
place. Locally, physical characteristics of rocks can be compared and correlated. On a larger
scale, even between continents, fossil evidence can help in correlating rock layers. The law
of superposition, which states that in an undisturbed horizontal sequence of rocks, the oldest
rocks will be on the bottom, with successively younger rocks on top of these, helps
geologists correlate rocks around the world. This also means that fossils found in the lower
levels of a sequence of layered rocks represent the oldest record of life.
By correlating fossils from various parts of the world, scientists are able to give relative ages
to particular rock layers. This is called relative dating. Relative dating tells scientists if a
particular rock layer is older or younger than another. This would also mean that fossils
found in the deepest layer of rocks would represent the oldest forms of life. If certain fossils
are found only in particular layers of rock, they may be useful as index fossils in determining
the age of unknown rock layers. By using the information from rock formations in various
parts of the world and correlating the studies, scientists have been able to construct the
geologic time scale. This time scale divides the vast amount of Earth history into sections
based on geologic events such as mountain building and biologic events such as extinction.
MATERIALS
card set A, card set B, fossil samples, geologic time scale poster
PROCEDURE
Card set A
1.
2.
3.
4.
Spread the cards on the table.
Let the cards represent rock layers and the letters represent fossils.
Suppose the card labeled “TC” formed first and is therefore oldest.
Determine the next card by comparing letters that overlap.
For example, the next card should have either a “T” or a “C”.
5. Sequence the remaining cards by using the same process.
6. When you finish you should have a row of cards with “TC” being oldest.
Card set A analysis
1. List the cards from oldest to youngest without repeating letters.
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2. How do you know that “fossil” X is older than “fossil” M?
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3. Explain why “fossil” D is the same age as “fossil” M in rock layer DM.
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4. Explain why “fossil” D in rock layer OXD is older than “fossil” D in layer DM.
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Card set B
1. Examine the second set of cards with sketches of fossils. Each card represents a
particular rock layer with a collection of fossils in that particular rock layer.
2. The oldest rock layer is marked with the letter “M”. The letters on the
other cards have no significance and should not be used in sequencing.
3. Find a rock layer that has at least one of the fossils you found in the oldest rock layer.
This rock layer would be younger as indicated by the appearance of new fossils.
4. Sequence the remaining cards by using the same process.
5. When you finish you should have a row of cards with “M” being oldest.
Card set B analysis
1. Use the letters to sequence the cards from oldest to youngest.
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2. Which fossil organisms could possibly be used as index fossils?
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Explain.
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3. Which fossil organisms would probably not be used as index fossils?
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Explain why not. __________________________________________________
4. State the law of superposition.
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5. How does this activity relate to the law of superposition?
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6. Relative dating is occasionally more difficult because rock layers have been
disturbed. What forces could cause a disturbance of rock layers?
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Application
1. Examine the fossil samples. What were these fossilized organisms?
Use card set B to help identify the fossilized organisms.
2. When did the fossilized organisms live?
Use the geologic time scale poster to determine the geologic period and time frame.
common name poster name geologic period time frame(mya)
Billingsella
Hydnoceras
Ceratites