1. Finding the Earth’s Age
Besides looking at how rocks have layered, scientists also take measurements to find the actual, or absolute age, of rocks.

2. Finding the Earth’s Age
There are four primary measurements that scientists take to find a rock’s age: 1.) Measuring radioactive decay 2.) Measuring rates of erosion 3.) Measuring rates of deposition 4.) Measuring varve counts

3. Earth’s Age: Radioactive Decay
Atoms usually have an equal number of protons and neutrons in their nucleus. If an atom has more or less neutrons than protons, it is called an isotope. Because they are less stable than regular atoms, isotopes give off energy, often in the form of heat.

4. Earth’s Age: Radioactive Decay
An isotope of uranium, U-238, has a lot of this unstable (radioactive) energy. U-238 is so unstable, in fact, that over time it stops being uranium, and turns into other elements instead.

5. Earth’s Age: Radioactive Decay
As U-238 decays, it is called the parent element, and the elements it creates are called the daughter elements.

6. Earth’s Age: Radioactive Decay
It takes a very long time, however, for U-238 to decay into its stable, non-radioactive, daughter element, Pb-206 (an isotope of lead). How long?

7. Earth’s Age: Radioactive Decay
4.5 billion years! The age of the Earth itself!

8. Earth’s Age: Radioactive Decay
Here’s how it works: Say you have 10g of U-238. Over 4.5 billion years, half of that U-238 (5g) will have decayed into Pb-206. In another 4.5 billion years, half of that remaining 5g of U-238 will become Pb-206, and so on until eventually there is no U-238 left.

9. Earth’s Age: Radioactive Decay
The time it takes for half of a radioactive material (U-238) to turn into its daughter material (Pb-206) is called the radioactive material’s half-life.

10. Earth’s Age: Radioactive Decay
5.) So, if after
4.5 billion years
5g of the 10g of U-238
we started with has decayed into Pb-
206, and after another 4.5 billion years
another 2.5g has decayed, how much
of the 10g we started with will remain
in another 4.5 billion years?

11. U-238 > 10 million Earth’s Age: Radioactive Decay
By comparing the amount of U-238 and Pb-206
in a certain rock sample, scientists can figure out
the sample’s age. Since it’s half-life is so long (4.5
billion years), it is mostly helpful only for dating
samples that are over 10 million years old.
U-238 > 10 million

12. Earth’s Age: Radioactive Decay
There are radioactive isotopes besides U-238
that are helpful for dating rock samples:
Posassium-40 (K-40), has a half-life of 1.3 billion years, and is helpful for dating rocks 50,000 – 4.6 billion years old.
Rubidium-87 (Rb-87), has a half-life of 47 billion years, and is used to verify K-40 measurements.

13. Problems with Measuring Radioactive Decay
If too little time has passed or too much time has passed since a rock formed, it is difficult or even impossible to do any helpful measurements of radioactive decay. This is because there must be enough of both the parent element and the daughter element present for a good comparison to be made.

14. Earth’s Age: Radioactive Carbon Decay
Earth’s air has two isotopes
of Carbon in it:
C-14, which is unstable but not harmfully radioactive and eventually becomes its daughter element N-14
AND
C-12, which is stable and non-radioactive

15. Earth’s Age: Radioactive Carbon Decay
Since C-14 and C-12 are breathed by
most plants and animals, scientists can compare the amount of C-14 and C-12 in a fossil.
However, since C-14 has a half-life of only 5,730 years, the entire parent element can have decayed if scientists get a fossil over 100,000 years old.

16. Elements for Measuring
Radioactive Decay
U-238
K-40
Rb-87
C-14
Rocks, minerals, >10 million years old
Rocks, minerals, soils, 50,000 – 4.6 billion years old
Used to verify age of rocks millions or billions of years old
Organic material, <100,000 years old
6.) Suppose you have a shark’s tooth that you suspect is about 15,000 years old. Would you use U-238 or C-14 to date the tooth? Explain your choice and what would be wrong with the other option given.

17. Earth’s Age: Rates of Erosion
If scientists can measure the rate at which a stream or river is eroding (wearing away) its bed, they can approximate how old the stream or river is.

18. Earth’s Age: Erosion
For example, geologists have studied Naiagara Falls and found that the edge of the falls is eroding at an average rate of 1.3m (~5ft) per year. Using this, scientists dated the formation of the falls to 9,900 years ago.

19. Useful as finding and using the rate of erosion is, if a rock formation is older than 20,000 years (which most are), this method’s not very dependable.
Earth’s Age: Erosion

20. Earth’s Age: Erosion
The Grand Canyon, for instance, formed over a very long time and eroded at varying rates at different periods in its history.

21. Earth’s Age: Deposition
Using data collected over a long period, scientists have averaged the rates at which sediments, or small bits of rock, are laid down, or deposited. Turns out, every 1,000 years about 30cm, or 1ft, is deposited (on average).

22. Earth’s Age: Deposition
It’s hard to use deposition data in many cases because the rate of deposition often varies. For example, a flood can deposit many meters of sediment in just one day. Furthermore, rates of deposition have varied in different eras of Earth’s history

23. Earth’s Age: Varve Count
Just as people approximate a tree’s age by counting the number of rings in its trunk, scientists have noticed that many rock formations have annual layers. These layers, called varves, consist of thin dark bands surrounded by broader light bands.

24. Can be used to date the age of rock formations up to 20,000 years ago.
Measuring Earth’s Age
Radioactive Decay
Erosion
Deposition
Varve Count
Carbon, Potassium, Rubidium and Uranium can be used to date rocks and organic materials with general accuracy within thousands, millions and billions of years.
Can be used to date the age of rock formations up to 20,000 years ago.
Can be used to make very general, but not at all specific, age measurements.
Depending on how natural forces have shaped a rock structure, there may be varves that offer an annual record of a rock’s history.

25. Mini-Quiz!
Answer True or False A.) The Earth is believed by scientists to be about 4.6 billion years old. B.) One varve represents two years of deposition of sediments.

26. Mini-Quiz!
Choose the Answer that Best Fits C.) Sediment deposits that show definite annual layers are called _________. i.) varves ii.) dikes iii.) sills iv.) crossbeds