1. Relative vs. Absolute Dating

2. Important Facts

3. Uniformitarianism The Present is the KEY to the PAST.
Laws of nature are the same now as they have always been.
Past events explained and estimated by modern processes.
Charles Lyell credited with this concept. Explained in his book “The Principles of Geology.”

4. Other Important Words…
Lithification: the consolidation of a loosely deposited sediment into a hard sedimentary rock.
Strata: rock layers

5. Relative vs. Absolute Age of Rock
Relative Dating: determination of the age of a rock or geologic event by comparing with other rocks/geologic events
Can you infer who is older?
She is.

6. 4 ways to determine relative age
4 ways to determine relative age. (Also called the Principles of Geology)

7. Original Horizontality
When sedimentary rock layers form, they are “created” in a horizontal fashion.

8. Law of Superposition Top Layer = Youngest
In undisturbed layers of sedimentary rock or lava flows…
Bottom Layer = Oldest

10. Cross-Cutting Relations
Any feature that cuts across a rock or layer of sediment is younger than the rock or layer it cuts
Youngest Layer

11. Fossil Correlation
The same group of fossils are of the same age, and therefore, strata containing them are also of similar age.

12. There some speed bumps on the road to Relative Dating

13. Uncomformity
A gap or missing layers of rock due to erosion.

14. Angular Unconformity
Older rock under the line of erosion are tilted at an angle, while the younger strata above are horizontal.

15. Disconformity
Strata above AND below the line of erosion are horizontal.

16. Nonconformity
Unconformity that separates igneous or metamorphic rock from a layer of sedimentary rock.

17. Relative vs. Absolute Age of Rock
Absolute: A form of geologic dating that gives an actual date/age in history that the item was formed or died.
6 months
82 years

18. Phenomenon of Radioactivity
Some elements, such as uranium (U) and thorium (Th) are unstable.
They decay spontaneously.

19. The Atom

20. Radioactive Decay
238 U spontaneously emits a particle from its nucleus called an alpha particle (2 protons + 2 neutrons).   

21. Radioactive Decay

22. Uranium-Thorium Decay

23. Beta Particle Emission
But, Th is also unstable, and it emits a beta particle …

24. Thorium-Protactinium Decay

25. U – Pb Series
This process is called radioactive decay, and eventually uranium (parent) decays to lead (daugher product)

26. U – Pb Series
The rate at which this process occurs is measured in terms of a “half life”.

27. Half Life= Number of years for ½ of the original number of atoms to decay from U to Pb.

29. Radiometric Dating
Assume: Rate of decay (half life) of long-lived elements has not varied since the Earth was formed.
This is an example of uniformitarianism.

30. Half-Lives of Radioactive Isotopes

31. Examples of very old rocks found on Earth
3.4 billion year old granites – South Africa
3.7 billion year old granites – Greenland
Oldest terrestrial rocks ~ 4 billion years
Central Canada and Central Australia
These are deep in the interiors of the continents
Core drilling aids in excavation of these rocks.