Transcript Absolute Dating

Absolute Dating
J
F
M
Earth
Forms
A
First
Life
Earth
Cools
M
J
Abundant
Oxygen
J
A
S
O
Multicellular
Organisms
Plants
and
Animals
N
D
J
Dinos Humans
15-25 11:20 pm
1 second
1/ 31,556,925.974 of the year 1900
9,192,631,770 flips
of the magnetic field of a cesium atom
What is Absolute/Numerical Dating?
•The age of an earth material or
event in years
Relative age
3rd
2nd
1st
youngest
Absolute
12 kya
95 mya
1.7 bya
8 kya
Estimating Earth's Age Early (failed) Attempt
Bishop Usher – Biblical Interpretation
4004 BC – before the birth of Christ
October 21
9:00 in the morning
Estimating Earth's Age Early (failed) Attempt
Sedimentation rates - 3 my – 500 my
Halley/Joly - Ocean Salinity – 100 my
Lord Kelvin – Rate of Cooling – 30 my
Radioactive Revolution
around 1900
• Radioactive decay - spontaneous
transformation of an element to
another isotope of the same element
or another element.
Pieces of an Element
• Protons - positively charged
• Neutrons - no charge
• Electrons - negatively charged
e
P
N
P
e
Isotope - element with different
number of neutrons in the nucleus.
Hydrogen - stable
Tritium - unstable
e
e
P
P
N
N
Radioactive Decay
Tritium
Helium 3
(daughter)
(parent)
e
P
N
N
unstable
e
P
nuclear
decay
N
P
stable
e
Radioactive Decay
Alpha Decay – loss of a positively
charged He ion
Beta Decay – neutron splits into
proton and electron
Alpha Decay
234
4 He
238
Th +
U 
90
2
92
( 2protons + 2 Neutrons)
Beta Decay
234
234
Th 
Pa + e90
91
Neutron splits into a
proton and an electron
Half-life
The fixed period of time during which
half the parent atoms present in a
closed system decay to form daughter
atoms
1
Half-Life
2
3
4
5
Half-life
Half-lifes
0
Potassium 40 / Argon 40
12 grams / 0 grams
1
6
/
6
2
3
/
9
3
1.5 /
10.5
Half-Life Exercise
Sample: 1024 grams
Isotope A
(10 k) half-life
Isotope B
Daughter
Parent
960 grams
64 grams
10,000
20,000
30,000
40,000
1st
2nd
3rd
4th
512 / 512
256 / 768
128 / 896
64 / 960
Parent/Daughter
Half Life (yrs)
Range (yrs)
4.5 billion
10 million to
4.5 billion
14C
5730
100 to 70,000
3H/3He
12.3
1 to 50
238U/206Pb
Parent/Daughter
238U/206Pb
14C
3H/3He
Material Dated
Igneous and
Metamorphic rocks
Organic material
Water
Uranium (U) - Thorium (Th) - Lead (Pb)
238U decays to 206Pb (4.5 billion)
235U decays to 207Pb (713 million)
232Th decays to 208Pb (14.1 billion)
>Rocks containing Uranium provide three
possible techniques.
>Because all three occur together, it allows a
method to cross-check the dates.
>Uses zircons, uraninite and uranium ores
Potassium (K) - Argon (Ar) Dating
>Potassium (K) is an extremely common element.
>Half-life (t1/2) is 1.3 billion years.
>Range is 100,000 to 4.6 billion years.
>Useful for relatively young and very old rocks.
>Found in muscovite, biotite, orthoclase and
glauconite.
>Used to date volcanic rocks.
Carbon 14 / Carbon 12
>Cosmic rays hit Nitrogen 14 changing it to
Carbon 14.
>Carbon 14 is taken in by organisms.
>When organism dies, amount of C-14
decreases.
Magnetic Polarity Dating
Dating techniques
that can be used
on this geologic
column:
• absolute dating
• paleomagnetism
• superposition
• fossil correlation
What is the Parent / Daughter ratio of a sample of radiogenic
material if the
original sample was 200g.
and 3 half-lifes have passed.
If the half-life is 20,000 years, how old is the sample?
What is the half-life of an isotopic pair if
the sample is 2 million years old
and contains 625g of parent and 9375g of daughter isotope?