1. Dating of Rocks
Dinosaur national monument
October 2012

2. Layers of the Grand Canyon

3. Age
Relative age: establishes order from youngest to oldest, but does not give exact time frames
Absolute age: establishes the date of an event
First, a couple of terms about age. What is relative age? It is establishing the order from youngest to oldest, but without giving exact time frames. You were born and are now listening to this video. How long apart were these events?
[advance] absoluate age establishes the date of an event, within the limits of the technology used at least.

4. We Place Geologic Events in Relative Order
By application of a set of geological “rules”
Uniformitarianism
Superposition
Original Horizonality
Cross-cutting Relationships
Inclusions
We apply these principles to determine the relative age of the layers of a sedimentary rock formation.

5. Uniformitarianism
Uniformitarianism: the processes we can observe today are the same processes that happened millions or billions of years ago
Waves crashing against the shore
Wind blowing sand eroding stone

6. Superposition
For an undisturbed rock sequence, the oldest rocks are at the bottom and each successive layer is younger than the layer beneath

7. Original Horizontality
Original horizontality: sedimentary rocks are deposited in nearly horizontal or horizontal layers (just like your laundry)
The principle of original horizonality points out that sedimentary layers are, more or less, horizontal when they form. By the time we see them, they may no longer be horizontal – perhaps do to an earthquake or two, but when they formed, at least, they were flat.

8. Original horizontality: as it was formed – it was flat!
The principle of original horizonality points out that sedimentary layers are, more or less, horizontal when they form. By the time we see them, they may no longer be horizontal – perhaps do to an earthquake or two, but when they formed, at least, they were flat.
Of course, it may no longer be flat!

9. Cross-cutting Relationships
Cross-cutting relationships: an intrusion or fault is younger than the rock it cuts through
You have to bake the cake before you can cut through it, right?

10. Cross-Cutting Relationships
Which is older? Which is younger? Why?

11. Cross-Cutting Relationships
Which is older? Which is younger? Why?

12. Inclusions
Any material in a layer has to be younger than the material it is embedded in
When particles of loose rock are incorporated into the newly deposited top layer, these clasts are called inclusions.

13. Unconformity
Unconformity: some of the rock record is missing

14. Correlation
Matching of geologic or fossil record in different places in the world

15. But that’s another story…

16. Absolute Dating
Radioactive decay- the process in which a radioactive isotope breaks down into a stable isotope of a different atom
Radiometric dating- a method of determining age by estimating percentages of a radioactive (parent) isotope and a stable (daughter) isotope
Half-life- the time needed for half of a sample of a radioactive substance to decay