First and Last Name Period
Geologic Laws of Relative Dating Activity
Introduction: Scientists assume a lot about Earth’s past. These assumptions are based on things that can be observed at present time. These assumptions are called the Geologic Laws. In this activity, you will explore the basics of geologic laws and by the end of this activity you should be able to explain three basic geologic laws.
The term relative dating, or relative time, means being able to give a time period to an event based on the events that came before and after. For example, each day in school, lunch comes after 3rd period, but before 4th period. This is an example of relative time.
Methods: With a partner, you will be looking at models to determine the “rules” of relative dating.
Procedure
- You both need to read along, discuss, agree and record the partner answer ontheir own loose leaf paper. Use clear and complete sentences to explain your thinking.
- Do your best work!
Model 1
1. Which book was laid down first?
2. Which book was laid down last?
3. Which book is the oldest book in the stack, in terms of how long it has been in the stack?
4. Which book is the youngest book in the stack, in terms of how long it has been in the stack?
5. Which book is older, book B or book C? Support your answer with evidence.
Read: There is a geologic law that states that in a regular, unchangedrock sequence, the oldest layer will be at the bottom of the sequence. This law is called the Law of Superposition.
Model 2
1. Which layer is the oldest layer?
2. Which layer is the youngest layer?
3. Looking at model 2, which rock layer is older, layer B or layer C? Support your answer with evidence from what you know about the law of superposition.
4. If a fossil were found in layer C, would it be older or younger than a fossil found in layer A? How do you know?
Read: Scientists assume that all rock was originally laid down in a horizontal manner. The Law of Original Horizontalitystates that layers of sediment are originally deposited horizontally under the action of gravity. Scientists now recognize that even sediments can be laid down at slight angles, but the idea is still the same. Scientists use this law to explain which layer is older in rock layers that have been tilted or bent. Go to the next page to exam Model 3 and learn more about this law.
Model 3
1. Which layer is the oldest layer?
2. Which layer is the youngest layer?
3. Looking at model 3, which rock layer(s) is flat? Which layer(s) is bent?
4. Looking at model 3, which rock layer is older, layer A or layer B? Support your answer with evidence.
5. Which geologic laws are being represented by this model? How do you know?
Read: Sometimes events such as earthquakes and volcanoes disrupt the regular rock strata (layers). Magma that intrudes into existing rock layers must be younger than the layers it has intruded into. Likewise, a fault (break in the crust) that breaks rock layers must be younger than the rocks it has broken. This is called the Law of Cross-Cutting Relationships. This law states that the geologic feature which cuts another is the younger of the two features. Look at the next page to examine Model 4 and learn more about this law.
Model 4
1. What do you think happened to the rock layers in Model 4?
2.Were the layers with the same letter formed at the same time? Support your answer with evidence based on what you have learned so far.
3.Which layer is the youngest? How do you know?
4. Which layer is the oldest? How do you know?
Model5
1. Looking at model 5, which layer is the youngest? Support your answer with evidence.
2. Which layer is the oldest? Support your answer with evidence.
3. Which Geologic Law does D represent? How do you know?
Putting it all together
Model 6
- What is the youngest rock layer? Explain using which Geologic law helped you to know.
- Is the extrusion older or younger than rock layer B? Explain using which Geologic law helped you to know.
- Is the fault older or younger than rock layer A? Explain using which Geologic law helped you to know.
4.How could a geologist use the fossil in rock layer B to date a rock
layer in another location?