1. LESSON 10 – Earth’s History: Estimating Geologic Time
Modified from
How do you reconstruct geologic history with rocks?
History

2. Determining geological ages
Geologists use rocks to tell time in two ways
In the field, they can look at a landscape and decipher the order of events that produced it
To actually know “when” an event occurred, or to know when a rock formed, requires laboratory analysis

3. This field sketch shows observations of a landscape
This field sketch shows observations of a landscape. How do we ascertain the order in which the rocks were placed there? By first determining the rocks’ relative ages.

4. How do you determine the order of events?
Relative age – the ordering of objects or features from oldest to youngest. Things that happened first, then next, and last
Absolute age – establishing the date of an event (in years before the present time).

5. How are geologic events placed in relative order?
By the application of a set of geological “rules”
First is the Principle of Superposition – This states that when rocks were placed, newer ones were placed or formed on top of the older ones
New Rocks
So here at the Grand Canyon – the old rocks are on the bottom of the pile, and the new ones are on top.
Old Rocks

6. How are geologic events placed in relative order?
Principle of Original Horizontality – sedimentary layers are (more or less) horizontal when they form.
(These Flat layers that are no longer horizontal)

7. Principle of Original Horizontality – sedimentary layers are (more or less) horizontal when they form.
Older rock layers
Newer rock layers
(These Flat layers that are no longer horizontal)

8. Figure 18.3

9. How are geologic events placed in relative order?
Principle of Cross-Cutting Relationships – geologic features that cut across rock must be younger than the rock they cut through.
Older Rock
Newer Rock

10. How are geologic events placed in relative order?
Principle of Inclusions – This states that objects enclosed in rock must be older than the time of rock formation

11. Review: Principles of Relative Dating
Law of Superposition
In an undeformed sequence of surface-deposited rocks, the oldest rocks are on the bottom.
Includes sedimentary rocks, lava flows, ash deposits and pyroclastic strata.
Does not include intrusive rocks, which intrude from below.

12. Review: Principles of Relative Dating
Principle of original horizontality
Layers of sediment are generally deposited in a horizontal position
Rock layers that are flat have not been disturbed
Principle of cross-cutting relationships
Younger features cut across older features (faults, intrusions etc)
Principle of Inclusion
Objects enclosed in rock must be older than the time of rock formation

13. Fossil Record
Evidence of organisms preserved in the sedimentary rock record.
Typically hard shelled organisms or bones of other organisms are the only evidence preserved as fossils.
Organisms must be buried quickly after they die to be preserved.

14. Principle of Fossil Succession:
Fossil organisms succeed one another in a definite and determinable order, so any time period can be recognized by its fossil content
Drawn from theory of evolution
Fossil organisms become more similar to modern organisms with geologic time
Extinct fossils organisms never reappear in the fossil record

15. How was the geologic time scale constructed?

16. Figure 18.11

17. How have scientists determined the age of Earth?
Radiometric dating (radioactive decay) provided a measure for the absolute ages of rocks, and the 4.5-billion-year age of Earth.
(Image from

18. How have scientists determined the age of Earth?
Radioactivity
Isotopes – atoms of the same element with the same number of protons, but a different number of neutrons
Radioactive decay – a change in the number of protons, neutrons, or both that transforms an unstable isotope towards a stable one
Fig 7.16

19. How have scientists determined the age of Earth?
Radioactive decay of specific elements can be used to determine the Absolute Age (the actual age of a rock or event)
Some elements decay (change) into other elements over a specific amount of time (half-life).

20. How is the absolute age of a rock determined?
Use Half-Life to measure the isotopic abundance
An unstable “parent” isotope decays towards a stable “daughter” isotope.
Assuming each daughter comes from a parent isotope, and that we know the average time it takes to convert, we can use the ratio of the two to calculate the age of the rock they are in.
Half-life – the amount of time it takes for ½ of the parent to turn into the daughter isotope.
Fig 7.17

21. 1st Half Life
Figure 18.14
After one half-life, 50% of the parent isotope will have become daughter isotope, regardless of the sample size.
2nd Half Life
After two half-lives 50% of the remaining parent isotope will have become daughter isotope. This means 75% of the original parent isotope will have changed.

22. Half-Life Example
A sample with an original amount of 40g of carbon-14 is found and now contains 2.5g of carbon-14, how many years old is this sample?
40g 20g 10g 5g 2.5g
This sample has undergone 4 half-lives, and the half-life of carbon-14 is 5700 years:
5700 yrs. x 4 = 22,800 years!

23. Validating the method