Transcript 02a-dating

WHEN ?
65 million
years ago
WHAT ?
85% of all species
including all of the
dinosaurs, many fish,
plankton and many
plants either died out
completely or
suffered heavy
losses
WHY ?
a giant meteorite crashing into the
earth, severely disrupting the earth's
ecosystem
or volcanic activity, climate
change, environmental pollution
WHEN ?
Next?
WHAT ?
50% of species
including all plants
and animals
WHY ?
Human induced climate change
III. DATING FOSSILS
1. few fossils can be dated directly
2. the system depends on the rule that older things
are buried deeper
3. this works by sediments being deposited over
time by erosion
a. creature dies near site of erosion
and parts do not decompose
b. Dead creature is buried &
other creatures die in same area
c. over time, remains of newer
dead creatures are buried
above older remains
What can be dated?
Dirt – no
Fossils – seldom
Volcanic ash – yes!
Using K / Ar
Example of dating fossils
SOIL SURFACE
A
VOLCANIC
ASH # 1
C
B
VOLCANIC
ASH OR LAVA
#2
E
D
1. we know that A is younger than
volcanic ash # 1
A
C
B
2. we know that B, C and E are older than
volcanic ash # 1 and younger than
volcanic ash # 2
3. we know that D is older than
volcanic ash # 2
E
D
4. we assume that C is younger than B
and E because it is shallower
5. we assume that B and E are the same
age by distance above
volcanic ash # 2
C. RADIOMETRIC DATING
1. Radio-isotopes --atoms that decay with a set half life
2. half life --- time required for ½ of the parental
isotope to decay to a daughter isotope
87.5 % PARENTAL ISOTOPE
12.5 % DAUGHTER ISOPTOPE
%
P
A
R
E
N
T
A
L
I
S
O
T
O
P
E
75 % PARENTAL ISOTOPE
25 % DAUGHTER ISOTOPE
75
50 % PARENTAL ISOTOPE
50 % DAUGHTER ISOTOPE
50
25 % PARENTAL ISOTOPE
75 % DAUGHTER ISOTOPE
12.5 % PARENTAL ISOTOPE
87.5 % DAUGHTER ISOTOPE
6.25 % PARENTAL ISOTOPE
93.75 % DAUGHTER ISOTOPE
25
12.5
¼
½
1
2
3
half lives of the isotope
4
3. given the half life and the relative amounts of the
elements, the age of the item can be determined
using an equation
4. however, an approximation can be done with no more
math than the ability to divide by 2
and subtract from 100
Parental
isotope
remaining
Half lives
Parental
isotope
remaining
Half lives
50 %
1
50 %
1
25 %
12.5%
6.25 %
3.125 %
2
3
4
5
75 %
87.5%
1/2
1/4
93.75 %
1/8
96.875 %
1/16
4. Examples of commonly used isotopes
parental
isotope
daughter
isotope
14C
14N
40K
40Ar
235U
207Pb
half life
in years
range (yrs)
best for
100 –
100,000
biotic
1.3 billion
100,000 –
4.6 billion
ash
713 million
10 million –
4.6 billion
rock
5730
5. What age are the following?
a. 0.78125 % of the original 14C remains.
b. 99.60938 % of the original 40K remains.
a. 0.78125 % of the original 14C remains.
50%  25%  12.5%  6.25%  3.125% 
1
2
3
4
5
1.5625 %  0.78125 %
6
7 half lives
7 half lives [from the pattern on the chart]
7 half lives x 5730 years/ half life = 40,110 years
b. 99.60938 % of the original 40K remains.
50%  75%  87.5%  93.75%  96.875% 
1
1/2
1/4
1/8
1/16
98.4375 %  99.21875 %  99.60938 %
1/32
1/64
1/128 half lives
1/128 half life x 1300 million years/ half life
= 10.15615 million years
Work on assignment sheet