1. How do we know that the earth is 4.6 billion years old?
How Old is it Really?
How do we know that the earth is 4.6 billion years old?

2. How do Scientists Determine the Age of Rocks and Fossils?
2 methods: Relative Dating and Absolute Dating
- Relative dating estimates the time during which an organism lived.
It compares the placement of fossils in layers of rock.
Scientists infer the order in which species existed based on the Principle of Superposition
- In undisturbed layers of rocks, the older rocks will be found beneath the younger rocks
Relative dating doesn’t give an
actual age, just indicates how
old rocks and fossils are when
compared to each other
Youngest Rock
Oldest Rock

3. Absolute Dating- Radiometric Dating
Allows scientists to assign an actual age to rocks and fossils.
- relies on the decay of unstable radioactive isotopes
- as you will recall:
(Yes believe it or not I do expect to remember)
Isotopes are atoms of an element with different
numbers of neutrons
neutrons
protrons

4. Radioactive isotopes
Radioactive isotopes are unstable. They break down and become an atom of a stable element
- they can release protons and neutrons (alpha emission) or
- a proton can break down and turn into a neutron (beta emission)
Alpha emission 2p and 2n are released beta emission a proton breaks down releasing an electron the proton becomes a neutron

5. How Does Radiometric Dating Work?
Based on the half-life of the radioactive element
- the half-life is the amount of time required for half of the radioactive isotopes to decay
- comparing the amount of parent and daughter elements in a sample allows age to be determined

6. Elements and their Half-Lives
There are many radioactive elements
only some are commonly used for absolute dating
Parent Daughter Half-Life
- Carbon Nitrogen yrs
- Potassium Argon billion yrs
- Uranium Lead billion yrs
- Uranium Lead million yrs
C and K are beta emitters – U is an alpha emitter Ar 40 is a relatively inert gas.

7. C14 is very accurate but can. only be used to date things
C14 is very accurate but can only be used to date things ,000 years old
- is used to date things that were once alive

8. How was the age of the earth determined
Potassium 40-Argon dating is good for dating rocks 100,000 – 4.6 billion yrs old
The oldest rocks on earth are 3.9 billion years old
- meteors and rocks brought from the moon are billion years old
- scientists believe that this is a good indicator of when the solar system and its planets formed