1. Methods of Measurement
Geologic Time Scale
Methods of Measurement

2. Background So far we have discussed Earth’s formation
We have also discussed
how the Earth was
organized during its
formation.

3. What Next?
We know how the Earth was formed and how it changed during formation. But there are two lingering questions.
What has happened since then?
How do we know?

4. Dating Techniques (Review)
Dating Earth is HARD!
Nowhere on Earth will you find a complete rock record from day Earth’s formation up to today. IT DOESN’T EXIST.
We have only had the ability to accurately date rocks for about 60 years.
We are tiny.
Time units for humans are seconds, hours, days, years.
Time units for Earth is millions, hundreds of millions to billions of years.

5. Dating Techniques (Review)
Earth’s history is recorded in rocks of the crust…
Geology seeks to unravel this history!
Two basic time keeping methods:
Relative dating - Place events in their proper order.
Absolute dating - Determine, in years before the present, when event actually occurred.

6. Relative Age Dating
Most of the work was done in the 1800’s by geologists who…
Gathered information from numerous rock exposures.
Constructed a sequential chronology based on Earth’s life through time.

7. Relative Age Dating
Puts rock layers in age order from oldest to youngest.
Operates using the principle of Uniformitarianism- all present day processes have operated throughout geologic time.
Processes like…
Weathering and Erosion
Deposition
Volcanism
Plate Tectonics

8. Relative Age Dating
There are several principles of relative age dating that will ensure that rocks are always placed in chronological order.

9. Principles of Relative Age Dating
Superposition: In a sedimentary sequence that has not been overturned, the oldest rock is always at the bottom.

10. Principles of Relative Age Dating
Original Horizontality: Sedimentary layers are deposited flat. Any disruption must have come after layers were deposited.

11. Principles of Relative Age Dating
Cross-cutting relationships: If a rock unit cuts across another, it is always younger!

12. Principles of Relative Age Dating
Inclusions: if a unit contains inclusions (fragments) of another rock unit, the inclusion is older.

13. Relative Age Dating
Look at slide 8 and put the layers in order from oldest to youngest using relative age dating.
1___
2___
3___
4___
5___
6___
7___
8___
9___
10___
11___
12___

14. Correlation
When rocks are deposited it usually happens over large areas.
Many of the continents may share many of the same rock layers.
Correlation: the matching of rock layers from one area to another. This can be done by…
Using similarities in the rock
Examining key beds
Examining index fossils

15. correlated sequence 1 Kansas 2 Indiana 3 Ohio 4 Pennsylvania Mostly
nonmarine
limestone
correlated sequence

16. Where did the geologic time scale come from? Built up over time by
correlation and
relative dating
of rocks
from around
the world!

18. Absolute or Radioactive Dating
Relative age dating and correlation put the ENTIRE rock record together for us but did not tell what the real ages were.
Unstable or, radioactive, elements are everywhere.
If you know what the radioactive element will turn into, and how long it takes to decay you can calculate the age of anything.

19. Radioactive Dating
Half-life: The amount of time it takes for half of the parent (radioactive) element to decay into the daughter (stable) element.

20. Radioactive Dating Half-lives can be anywhere from billions of years
to fractions of a second.
If you had a sample that was 25% Thorium-232 and 75% Lead-208, how old is that sample?

21. Summary
Relative age dating has allowed us to put together all of Earth’s history into a story.
That story has chapters preserved all over the planet, we just had to put them in the right order.
The geologic time scale was put together using relative age dating.
We have had more exact dates for the rock layers for only about 60 years.