1. We know Earth is 4.6 Billion years old
BUT HOW DO WE KNOW?

2. How do we know Earth’s age?
Most thought Earth was 6,000 years old and all geologic features had formed at the same time
James Hutton
theorized the same forces changing Earth’s surface now were working in the past
uniformitarianism – geologic processes that occurred in the past can be explained by current geologic processes

3. Relative Age
Layers of rock, called strata, show the sequence of events that took place
relative age - age of an object in relation to the ages of other objects
Relative age indicates one layer is older or younger than another layer not the rock’s age in years

4. What are the principles used to study Earth to figure out its age?

5. Law of Superposition
law of superposition – layers on bottom are older than layers on top if undisturbed
Study sedimentary rocks to determine relative age of rocks

6. Principle of Original Horizontality
Principle of original horizontality - undisturbed sedimentary rocks will remain in horizontal layers
Nonhorizontal layers were tilted or deformed by movements after the layers formed.

7. Unconformities
unconformity - break in the geologic record created when rock layers are eroded or when sediment is not deposited for a long period of time

8. Types of Unconformities
Nonconformity - stratified rock rests upon unstratified rock (igneous/metamorphic)
Angular unconformity - boundary between a set of tilted layers and a set of horizontal layers.
Disconformity – layer is uplifted w/o tilting/folding and eroded. Sediments continue to deposit.

10. What is the law of superposition?
Agenda for Tuesday Nov 30th
Finish notes
Relative age activity

11. Cross-Cutting Relationships
law of crosscutting relationships - principle that a fault or body of rock is younger than any other body of rock that it cuts through
A fault is a break or crack in Earth’s crust where rocks shift their position
An intrusion is a mass of igneous rock that forms when magma is injected into rock

13. Relative Age
How old an object is in relation to other objects, not actual age
If we do not know the actual age, how do we know Earth is 4.6 Billion Years Old?

14. Absolute Age
absolute age - numeric age of an object or event

15. Determining Absolute Age
rate of erosion – useful for geologic features that formed within the past 10,000 to 20,000 years
rates of deposition – estimate the average rate of sediment deposition
Use data collected over a long period of time
varve - layer of sand and silt that deposited annually in a lake
near ice sheets or glaciers

16. Radiometric Dating
radiometric dating – comparing radioactive (parent) isotopes to stable (daughter) isotopes
Radioactive isotopes have nuclei that emit particles and energy at a constant rate
regardless of surrounding conditions

17. Half Life
half-life – length of time it takes for half of the atoms to decay
Years it takes to decay = half life
The time required for half to decay is always the same

20. Carbon 14
Carbon dating (radiocarbon dating) is a variety of radioactive dating which is applicable only to matter which was once living
Half life of carbon is 5730 years

21. Compare and Contrast Relative and Absolute Age.
Agenda for Wednesday Dec 1st
Radiometric Dating activity
Relative Age of Rock Layers
Review for Final

22. How Old are these rocks?
Figure out how old the rock layers are (relative age)
Which came first
Remember our Laws/Principles
Law of Superposition
Principle of Original Horizontality
3 Unconformities
Law of Cross Cutting Relationships

27. Radiometric Dating Problems
A sample contains 20g of an isotope that has a half life of 1000 years. How much will be left after 2 half lives?
20g at 0
10 g at 1 half life
5 g at 2 half lives