Transcript File - Mr. LaFranca`s Earth Science Class

Notepack 19
AIM: How can we tell the age of rocks?
Do Now: Regents Question:
Put the layers of rock in order from oldest
to youngest.
Geological Time
• Geological Time refers to time as it relates to
the Earth.
• There are 2 types of time
– Relative age – Time based on comparison
• This rock is older than that rock
– Absolute age – Time based with a number
• This rock is 4 million years old
HOW CAN WE KNOW THE AGE
OF ROCKS?
Relative age
• Comparing ages
• Superposition
•
*rock layers
• index fossils – fossils
of a known age found
in a layer of rock
Absolute age
• Finding a number/date
• radioactive decay data
How do we find absolute age of rocks?
• Scientist use radioactive dating to find an
approximant age of rocks and fossils
• Radioactive isotopes are atoms that have too many
neutrons in their nucleus and decay over time!
• Isotopes are very unstable!
• Since everything is made up of atoms, scientist can
use these isotope to measure the age of anything
including rocks
• The oldest known rock 4.3 billion years old
How do we find absolute age? cont.
• Radioactive dating involves understanding
how radioactive isotopes decay over time!
• As isotopes decay (breakdown), the original
atom changes into a different atom.
• This decaying process occurs at a specific rate
which scientist can measure; this is known as
a half life.
Half -life
• A half life refers to
the time in which
half of the amount
of isotopes atoms
(parent material)
decays (changes)
into a more stable
atom (daughter
material).
Here is 100 atoms of Carbon - 14.
100
carbon 14
After 5700 years (1 half life)
50
carbon 14
50
nitrogen
After another 5700 years – 11400 years total
(2 half lives)
25
carbon 14
75
nitrogen
After another 5700 years – 17100 years total
(3 half lives)
12.5
carbon 14
87.5
nitrogen
Each half-life, the amount
of atoms gets cut in half.
100%
One half-life.
50%
Two half-lives.
25%
Three half-lives.
12.5%
Four half-lives.
6.25%
Which Isotope should you use to date something?
• Each isotope has its own unique
half-life.
• Carbon-14 has the shortest halflife on the chart. (57,000 yrs)
• Because fossils contain carbon
and has been around in a
relatively short time, scientist
use Carbon-14 to determine the
age.
• Potassium-40 and Uranium-238
are used to date the Earth.
• Rubidium-87 is used to date the
universe. (49,000,000,000 years)
How to answer questions on the Regents?
• If you are asked how many half-lives did
something go through, just start with the original
amount and divide by 2 until you get to the
number that is needed. Each time you divide by
2 equals 1 half-life.
• How many half-lives did carbon-14 go through if
you started with 84 grams and ended with 21
grams?
• 84/2= 42 (1 half-life), 42/2=21 (2nd half-life)
How to answer questions on the Regents?
• If you are asked what the original amount of a
substance was if it went through a number of halflives, just start with the amount you are given and
multiple it by 2 for each half-life that you are given.
Each time you multiple by 2 equals 1 half-life.
• What was the original amount of carbon-14 if it
went through 4 half-lives and the you currently
have 2 grams?
• 2 x2= 4 (1 half-life), 4x2=8 (2nd half-life), 8x2=16
(3rd half-life), 16x2=32 (4th half-life)
How to answer questions on the Regents?
• If you are given the amount of an isotope and its
stable atom and were asked to calculate how many
half-live did the sample go through, just add the
amount of the isotope and the stable atom to get
the total amount. Then divide this number by 2 until
you get to the amount given for the isotope. Each
time you divide by 2 equal one half-life. .
• If you have 10 grams of Carbon-14 and 30 grams of
nitrogen, how many half-lives occurred?
• 10+30=40
• 40/2= 20 (1 half-life), 20/2=10 (2nd half-life)
1. If a sample contains 50g of Carbon14 and 50g
of Nitrogen14, how many half-lives has it
undergone?
1. If a sample contains 50g of Carbon14 and 50g
of Nitrogen14, how many half-lives has it
undergone?
One half-life. After one have life 100g of C14
becomes 50 grams of C14
2. If a sample contains 25g of Carbon14 and 75g
of Nitrogen14, how many half-lives has it
undergone?
2. If a sample contains 25g of Carbon14 and 75g
of Nitrogen14, how many half-lives has it
undergone?
•
Two half-lives. After one half-life there is 50
g C14. after a second half-life there is 25 g
C14
3. If a sample contains 25g of Carbon14 and
175g of Nitrogen14, how many half-lives has
it undergone?
3. If a sample contains 25g of Carbon14 and
175g of Nitrogen14, how many half-lives has
it undergone?
3!
Total = 200 g C14.
1st half life – 100g
2nd half life – 50g
3rd half life – 25 g
4. How old is a bone in which the Carbon14 in it
has undergone 3 half-lives?
4. How old is a bone in which the Carbon14 in it
has undergone 3 half-lives?
5,700 + 5,700+ 5,700 = 17,100 years
5. What happens to the amount of Nitrogen14
as the Carbon14 decays?
5. What happens to the amount of Nitrogen14
as the Carbon14 decays?
It increases
6. If a 20g of Carbon14 has a half-life of 5,700
years, what would be the half-life of a 40g
sample?
6. If a 20g of Carbon14 has a half-life of 5,700
years, what would be the half-life of a 40g
sample?
5,700 years
Trick question – the half-life of C14 is always
5,700 years.