1. Age Dating of Rocks

2. Question:
How do we know how old rocks are?

3. Two types of dating
Relative Age Dating – approximately how old rocks are
Found by using the age of things around it
Absolute Age Dating – exactly how old rocks are
Found using radiometric dating

4. Laws of Relative Age Dating
Relative dating – used to determine the order of events and the relative age by examining the position of rocks in a sequence.
Example: if bedrock is broken by a fault, then the fault must be younger because it cuts through the rocks

5. Relative Age Dating
This is called the Law of Cross Cutting Relationships – If something cuts across a layer of rock it must be younger than the rock.

6. Laws of Relative Age Dating
The Law of Superposition –
In an undisturbed layer of rock, the oldest rocks are on the bottom and the rocks become younger toward the top.
Sediments are deposited in horizontal layers, so the oldest one would be on the bottom
Sometimes layers flip, so geologists must use other clues to determine if this has happened.

7. Laws of Relative Age Dating
Principle of Original Horizontality – rocks are deposited in horizontal layers

8. Laws of Relative Age Dating
Unconformities – gaps in the rock record caused by missing rock layers
Form when layers are eroded away
Also when time goes by without any new deposition

9. Principle of Uniformitarianism – earth processes that are occurring today have been occurring since the beginning of the earth.

10. Three types of Unconformities
Angular Unconformity – horizontal layers of sedimentary rocks are tilted and uplifted, where they are eroded and weathered. Younger sediments are then deposited horizontally on top of the tilted layers

11. Three types of Unconformities
Disconformity – layers missing between two beds were an erosional surface is present

12. Three types of Unconformities
Nonconformity – sedimentary rock layers above igneous or metamorphic rocks

13. Correlating layers – finding the same rock layer at two separate locations and matching them up. How?
Index fossils – similar, rare fossils found in one layer at multiple locations
Rock correlation – matching similar rock types

14. Absolute Dating
Absolute Dating – process that uses the properties of radioactive decay in rocks to determine their exact age

15. Absolute Dating Radioactive decay – unstable isotopes break down
A neutron breaks down into a proton and an electron: the electron is given off as a radioactive particle
The number of neutrons & protons are changed, so a new element is formed
Uranium-238 is the parent
Lead-206 is the daughter

16. Absolute Dating
Each parent has a specific rate of decay – this is the half-life of the element
The time it takes for half of the atoms to decay
Carbon-14 has a half-life of 5730 years

17. Absolute Dating
Radiometric dating – measuring the amounts of parent and daughter materials in a rock and knowing the half-life, a geologist calculates the age of a rock
Geologists must decide what isotope to use
Must have an appropriate half-life

18. Absolute Dating
Radiocarbon dating – using carbon-14 to date fossils, bones and wood up to 50,000 years old
Must be a formerly living thing to have Carbon