Earth s
Clock
Lecture #17
Absolute Age
• Absolute / numerical / chronological age =
age measured in years
•  How to measure time?
  Constant process
o  Ex: Earth s rotation
  A record of the process
o  Ex: marking days on wall
Natural Clocks
Growth rings
Tree rings
Annual layers
Nature s Clock
•  Radioactive decay --> age of rocks
  Constant rate
  Radioactive elements found in rocks
•  Clocks in rocks
•  Different elements
  Found in variety of rocks
Review
Proton (+)
<- Nucleus ->
Neutron (no charge)
Electron
(-)
Protium
<- 2 isotopes of hydrogen ->
Deuterium
Isotopes: atoms with = # of protons,
≠ # of neutrons
Isotopes
•  Uranium
  238U: 92 protons + 146 neutrons
  235U: 92 protons + 143 neutrons
•  Both are unstable:
  Will break apart
•  Stable isotopes don’t change
Radioactive Decay
• Def: spontaneous change in nucleus of
unstable isotope
•  General reaction:
  Unstable isotope => Stable isotope + energy
  Parent isotope => Daughter isotope + energy
A Radioactive System
•  Example:
  238U
Parent
=>
206Pb
Daughter   238U decays through several unstable
daughters, to lead
  238U loses 10 protons, 22 neutrons
Half-Life
• Def: time it takes for 50% of parent atoms
to change to daughter atoms
  Measurable
o  Amount of energy released
  Constant with respect to:
o  Composition
o  T
o  P
Tutorial: Half-Lives
•  New
mineral:
  Contains a radioactive isotope
  Contains no daughter atoms
  Half-life = 30 million yrs
• Note: mineral could form by crystallization,
metamorphism, or precipitation
#1. Half-Life and Years
Halflife
Time,
Ma # Parent
Atoms
# Daughter
Atoms
0
1
0
30
100%
1024
50%
512
0%
0
50%
512
2
60
75%
768
3
90
25%
256
12.5%
128
4
120
5
6
87.5%
896
91.75%
960
150
6.25%
64
3.12%
32
180
1.56%
16
95.42%
1008
94.86%
992
Tutorial: #2. Graph Isotopes
100
Growth
of
daughter
% of atoms
75
50
Decay
of
parent
25
0
0
1
2
3
Half-lives
4
5
6
Isotopic Systems
P => D
238U
Half-life
Minerals
Used
for: => 206Pb
4.5 b.y.
zircon
40K
=> 40Ar
1.3 b.y.
feldspar
old rocks
14C
=> 14N
5,730 y.
organic
material
very old
rocks
<50,000
yrs old
Which Rocks Give Useful Ages?
•  Method dates mineral grains
• Good age if minerals & rock formed at
the same time
  Mineral must start with:
o  Only parent
o  No daughter
Dating Rocks
•  Igneous rocks
  Very good
•  Sedimentary rocks
- No good!
  Clastic rocks
o  Dates grain, not when it was deposited
  Chemical rocks
o  No radioactive elements
Dating Rocks (cont.)
•  Metamorphic rocks
  Sometimes
•  Not all rocks datable
  Igneous rocks are best.
Tutorial: #3-#7. Combining Methods
•  Use relative ages of all rocks
•  Plus absolute ages of igneous rocks
•  Estimate age of undatable rocks
D = P, so 1 half-life has
passed, or 30 Ma
30-90 Ma
D = 7P, so 3 half-lives
have passed, or 90 Ma
The
Geologic
Time
Scale
Age of
Earth =
4.6 b.y.
Precambrian (88%)
Evolution of Life
Humans
2 m.y.
Age of
Mammals
EXTINCTION
First birds
65 m.y.
Age of
Dinosaurs
EXTINCTION
245
m.y.
Large swamps
Amphibians
Land plants
Fish
Trilobites
FIRST SHELLS
545
m.y.
3.0 b.y.
Oldest rocks 4.4 b.y.
Earliest life