1. How Old is the Earth Anyway?
Unlocking the ancient Earth

2. Relative Time Scale
Is based on the rock and fossil evidence found in the crust of the Earth
The relative age of the rock is due to position. Tells you what happened first, second, third, etc.
Sedimentary rocks are especially useful because:
They are most likely to contain fossils
They are the most common and widespread rock type

3. Rules for the Relative Dating of Sedimentary Rocks – The present is the key to the past (uniformitarianism)

4. Principle of Original Horizontality
Sediments (that make up Sedimentary rocks) will settle in horizontal layers, unless they have undergone deformation

5. Law of Superposition
The oldest layers are found below the younger layers

6. Principle of Crosscutting Relationships
Any rock that cuts existing rock is younger than the rock that it cuts.

7. Fossil Succession
Fossils represent the remains of once living organisms
Most fossils are the remains of extinct organisms
The kinds of fossils found in rocks of different ages differ because life on Earth has changed over time

8. Other Tools: Index Fossils
Attributes of an Index Fossil:
Fossils of life that existed during limited periods of geologic time.
Widespread distribution
Any fossil found in a rock is the same age as the rock it was found in.
They are used as guides to date the age of the rock in which they are preserved.

9. Other tools: Unconformity
A surface between two rock layers that represents a gap in the rock record.

10. Types of unconformities
Angular Unconformity
Disconformities

11. Angular unconformity:

12. Disconformities:

13. Crosscutting Relationships:

14. Absolute Age- Radiometric Dating
Is used to determine the absolute age of earth materials
Determines how old a rock is in years.
It is based on the natural radioactive decay of radioactive elements found in igneous rocks.

15. A little background:
An atom is made up of protons, neutrons and electrons
An element is an atom with a specific number protons.
The number of neutrons and electrons in an element can change, but if you change the number of protons in an element, you change the element.

16. Diagram: The protons and neutrons are found in the nucleus of the atom
The electrons are in orbit around the nucleus.

17. Radioactive decay
Most atoms are stable; that is, the number of protons and neutrons never changes.
In some atoms, the number of protons and neutrons makes the atom unstable.
The unstable atom emits its excess energy and may eject one or more subatomic particles or it will split into two atoms of smaller elements in order to become more stable.
This is radioactive decay.

18. Half-Life
As the element loses particles from the nucleus it changes or splits from the radioactive PARENT element to a stable DAUGHTER element.
This rate of decay is measured in Half-lives
Half-life is the amount of time it takes ½ (50%) of the parent element to decay to the daughter element
Radiometric dating is based on the ratio of the Parent element to the Daughter element

19. Rate of Decay: Half-life Parent Daughter When formed 100% (1) 0 (0)
In % and (fraction)
Daughter
When formed
100% (1)
0 (0)
1 half-life
50% (1/2)
2 half-lives
25% (1/4)
75% (3/4)
3 half-lives
12.5% (1/8)
87.5% (7/8)
4 half-lives
6.25% (1/16)
93.75% (15/16)
5 half-lives
3.125% (1/32)
96.875% (31/32)

20. Different elements have different half-lives: (in your ESRT add U235)
Parent
Isotope
Daughter
Half-life of parent (years)
Useful range (years)
Carbon 14
Nitrogen 14
5,730
100-30,000
Potassium 40
Argon 40
1.3 billion
100,000 – 4.5 billion
Rubidium 87
Strontium 87
47 billion
10 million – 4.5 billion
Uranium 238
Lead 206
4.5 billion
10 – 4.6 billion
Uranium 235
Lead 207
710 million

21. To solve half-life problems:
Find the number of years for that element’s half-life
Find the number of half-lives that have occurred (from table)
Multiply the element’s half-life by the number of half-lives that have passed