Earth’s Clock
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
Tree rings
Growth lines
Annual layers
Nature’s Clock
• Radioactivity > age of rocks
!Constant rate
!Radioactive elements in rocks
• Clocks in rocks
• Different:
! Elements ! Rocks
! Times
Review of Atoms
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
Radioactive Decay (cont.)
• Example:
!
238U
=>
206Pb
! 238U decays to lead
! Loses 10 protons, 22 neutrons
Rate of Radioactive Decay
• Half-life: time it takes for 50% of parent
atoms to decay to daughter atoms
! Measurable
o Amount of energy released
! Unchanged by:
o Composition
o Temperature
o Pressure
A Representative Calculation
• New
mineral:
! Contains a radioactive isotope
! Contains no daughter products
! Half-life = 30 yrs
• Note: mineral could form by crystallization,
metamorphism, or precipitation
Halflife
Time,
years
# Parent
Atoms
# Daughter
Atoms
0
0
100%1024
1
30
50% 512
50% 512
2
60
25% 256
75% 768
3
90
12.5% 128
87.5% 896
4
120
6.25% 64
91.75% 960
5
150
3.12% 32
94.86% 992
6
180
1.56% 16
95.42%1008
0%
0
Graphing Radioactive Decay
100
Growth
of
daughter
% of atoms
75
50
Decay
of
parent
25
0
0
1
2
3
4
Half-lives
5
6
Very Long Half-Lives
Parent => Daughter Half-life
Minerals
238U
=> 206Pb
4.5 b.y.
zircon
87Rb
=> 87Sr
48.8 b.y.
feldspar
=> 143Nd
106 b.y.
mica
147Sm
Shorter Half-Lives
Parent => Daughter Half-life
40K
238U
14C
Minerals
=> 40Ar
1.3 b.y.
feldspar
=> 206Pb
700 m.y.
zircon
=> 14N
5,730 y.
organic
material
Which Rocks Give Useful Ages?
• Date mineral grains
• Good ages 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,
etc.
! Chemical rocks
o No radioactive elements
Dating Rocks (cont.)
• Metamorphic rocks
! Sometimes
• Not all rocks datable
! Igneous rocks are best.
Combinations
• Absolute ages of igneous rocks
• Relative ages of others
• Estimate age of undatable rocks
Volcanic ash,
25 m.y.
Lava flow,
40 m.y.
Sandstone
between
25 & 40 m.y.
The
Geologic
Time
Scale
Age of
Earth =
4.6 b.y.
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
Precambrian
(88%)
Earliest life
Oldest rocks
m.y.
3.0 b.y.
4.4 b.y.