# Unit 11 Notes - Absolute Time - Troy High School

Mechanics

Feb 21, 2014 (7 years and 8 months ago)

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Absolute Time

Historical Methods

Erosion and
Sedimentation

Scientists
estimate

the
amount of time it
would take for the
needed
erosion or
sedimentation

to occur

Only effective for
young

geologic
features

Tree Rings

One Ring

forms each year

Spring Wood

(growing
season)

Summer Wood

(dry
season)

Ring thickness depends on
temperature

and
rainfall

Ring Patterns

can be
correlated from tree to tree

Can be used to determine
the dates of construction
for different artifacts

Varves

A sediment that is
deposited on a
yearly

cycle

In any large body of water

Clearest in
glacial

lakes

Summer

and
Winter

lake
levels will differ

Glaciers

As ice
retreats (melts)

in
the summer and
accumulates

in the winter
it will leave
a varve

as well

Atoms with the same number of protons but
different numbers of neutrons are called
isotopes
.

Heavy isotopes (those with more neutrons) are
unstable and emit
.

The original atom is called the
parent

The new atom is called the
daughter

e.g.

Half
-
Life
: The amount of
time it takes for
half

the
sample to decay to a
stable

product

The proportions of
parent
isotopes

vs.
daughter
isotopes

in a sample can
be used to date that
sample

100%
50%
(1/2)
25%
(1/4)
12.5%
(1/8)
Formation
1 half life
2 half
lives
3 half
lives
Example:

Carbon
-
14 has a half
-
life of 5730 years.

If I extract all of the Carbon
-
14 and Nitrogen
-
14 out
of a leaf and I find that what I have is only ½ Carbon
-
14, how old is my leaf?

¼ Carbon
-
14, ¾ Nitrogen
-
14?

Parent
isotope

Daughter
isotope

Half
-
Life (yrs.)

Effective
Range (yrs.)

Material
to Date

Carbon
-
14

Nitrogen
-
14

5730

100
-
70,000

Once living
things

Uranium
-
238

-
206

4.5 billion

>10 million

Igneous

Oldest
rocks

Rubidium
-
87

Strontium
-
87

47 billion

>10 million

Igneous/
Metamorphic

Oldest
rocks

Potassium
-
40

Argon
-
40

1.3 billion

>50,000

Igneous
Rocks

Types of Decay

Alpha Decay:

When the parent isotope emits a Helium atom
along with the daughter isotope.

238
U
4
He +
206
Pb

Circle the alpha particle

Put a box around the parent isotope

Underline the daughter isotope

Beta Decay

When the parent isotope emits an electron
along with the daughter isotope.

14
C e
-

+
14
N

Circle the beta particle

Put a box around the parent isotope

Underline the daughter isotope

important?

Because it can be verified by different
methods it has proven to be very accurate

This helps scientists to put absolute dates
on events that have happened in the past

What was a historical method of
absolute dating?

1
2
3
4
5
0%
0%
0%
0%
0%
1.
Erosion

2.
Sedimentation

3.
Tree Rings

4.
Varve

5.
All Of The Above

1
2
3
0%
0%
0%
1.
It allows us to accurately
measure how much
glacier melt there is

2.
It allows us to measure
the ages of things
accurately even if they
are really old

3.
It allows is to see how
many electrons there are
in atoms

Radioactive element X has a half
-
life of 1000 years. How old is
something if only 25% of it is left?

1
2
3
4
5
0%
0%
0%
0%
0%
1.
1000 years

2.
2000 years

3.
3000 years

4.
4000 years

5.
8000 years

What is alpha decay?

1
2
3
4
0%
0%
0%
0%
1.
When the parent isotope
emits an electron along with
the daughter isotope.

2.
When an atom fuses
together

3.
When an atom gains an
electron

4.
When the parent isotope
emits a Helium atom along
with the daughter isotope.

What is beta decay?

1
2
3
4
0%
0%
0%
0%
1.
When the parent isotope
emits an electron along with
the daughter isotope.

2.
When an atom fuses
together

3.
When an atom gains an
electron

4.
When the parent isotope
emits a Helium atom along
with the daughter isotope.

What is electron capture?

1
2
3
4
0%
0%
0%
0%
1.
When the parent isotope
emits an electron along with
the daughter isotope.

2.
When an atom fuses
together

3.
When an atom gains an
electron

4.
When the parent isotope
emits a Helium atom along
with the daughter isotope.