Reference:
Chapters 6, 8; Appendix A & B
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Unit 2:
Topic 2.1
 demonstrating an understanding of the processes and features used to establish relative and absolute time.
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 Many geologic events that Earth Scientists study occurred millions of years ago. The ages of these events can be determined in two different ways.
1) Relative Time
2) Absolute Time
 Both of these dating methods are needed to accurately record geologic time chronologically and to organize the geologic rock record.
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 Places events in a sequence of formation, but does not identify their actual date of occurrence.
 Relative time does not involve numbers, but rather a sequencing of events by comparing events.
 This method of dating can ’t tell us how long ago something happened, only that it followed one event and preceded another.
 Example; Students ages.
Text References: pages 7-8 & 218-228
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Relative Dating Principles Include:
1.
Principle (Law) of Superposition
2.
Principle of Original Horizontality
3.
Principle of Cross-Cutting Relationships
4.
Principle of Inclusions
5.
Fossil Succession ( index fossils)
6.
Unconformities
7.
Folding and Faulting
These dating principles will be discussed in detail in another presentation.
8.
Contact metamorphism
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 Identifies the actual date of an event, & pinpoints the exact time in history when something took place.
 It involves numbers to count or calculate from a known event.
 For example, the extinction of the dinosaurs occurred about 66 million years ago and the age of Earth is approximately 4.6 Billion years.
Text References: pages 228 - 235
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Absolute Dating Methods Include:
1.
Tree Rings - The age of a tree is found by counting the total number of rings.
2.
Varves - any sediment layer that shows a yearly cycle. Varves are often seen in glacial lakes dating back to the ice age.
3.
Radiometric Dating - calculating absolute ages of rocks and minerals that contain radioactive isotopes.
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While taking mud samples from the bottom of a glacial lake, a geologist discovers volcanic ash covered by 200 layers of sediment. These layers alternate between light and dark colour. How many years ago did the volcanic eruption occur?
(A) 25 (B) 50
Varve
(C) 100 (D) 200
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Which geologic principle states that in an undisturbed sedimentary sequence, the bottom layer is oldest?
(A) correlation
(C) superposition
(B) horizontality
(D) uniformitarianism
Which type of dating would be used to tell the age of a tree using tree rings?
(A) absolute
(C) radiometric
(B) isotope
(D) relative
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Question:
Explain how the principles of superposition and crosscutting relationships are applied when using relative dating.
 Relative dating determines the chronological order of events by comparing the events. Both principals of superposition and cross-cutting relationships are examples of relative dating.
 Superposition compares undisturbed horizontal rock layers, stating that a rock layer is older than layers above and is younger than layers below.
 Cross-cutting states that a rock unit or geological event
(fault) that cuts across other rock units, is younger than the rock units it cuts across.
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Take the time and complete the following questions . . .
(Solutions to follow)
Refer to the diagram to describe the relationship between superposition and relative time.
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Question:
Refer to the diagram to describe the relationship between superposition and relative time.
 Relative time is found by comparing one event with another and chronologically arranging them according to age.
 Relative ages of the layers in the diagram can be seen when referring to the Law of Superposition which states that the lowest layer is the oldest and the age progressively gets younger as you move toward the top of the rock sequence, assuming that the sequence of rock layers were not disturbed.
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Overview of Points covered:
Relative Time: Absolute Time:
 Does not identify their actual date of occurrence.
 Identifies the actual date of an event.
 Does not involve numbers, but rather a sequencing of events by comparing the events.
 Pinpoints the exact time in history when something took place.
 Can
’t tell us how long ago something happened, only that it followed one event and preceded another.
 Involves numbers to count or calculate from a known event.
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