Transcript No Slide Title

•Relative Dating
•Absolute Dating
William E. Ferguson
Kelvin Calculation of Earth Age
Unconformity Types
Figure 8.1 Lab
Folding Rock
Relative Dating Principles
Geologic Puzzle Time!
- superposition, horizontality, crosscutting relationships
• In assigned groups of 3, work out the age relationships
in Figs. 8.4 - 8.12 in your lab book
• Pay attention to contact metamorphism!
• Use pencil!!!!
• Put in squiggly lines between events for uncomformities
• Be ready as a group to transfer your answers to an
overhead & explain your relationships to the class.
Figure 8.4b
Figure 8.10b
Figure 8.9 Lab
F
B
K
N
A
J
D
M
H
C
L
G
E
Figure 8.10a Lab
Could
be
younger
G
H
I
A
B
C
D
L
F
E
M
J
K
Figure 8.11 Lab
S
J
K
D
X
E
A
H
M
R
B
G
P
F
Figure 8.12 Lab
R
N
H
P
D
A
T
F
L
G
J
Z
S
B
E
K
X
M
O
C
V
Radioactive Decay
What we need to know to apply radiometric dating
40K
has the simplest decay scheme, and is the only one
we’ll examine in detail.
• The half-life of a radioactive isotope = time required for half of it to
decay to a stable (non-radioactive) daughter isotope.
• 40K decays to 40Ar with half-life = 1300 million years.
• 40K and 40Ar can be measured in a mass spectrometer.
• 40K is decaying to 40Ar all the time, but 40Ar is a gas and can
escape if a mineral is above the “blocking temperature”. Below the
blocking temperature, 40Ar is trapped in the mineral.
• If a mineral is heated above the blocking temperature,
40Ar
• Different isotopes (of U, K, Rb, Sr, etc) used in dating have
different blocking temperatures.
is lost.
# of parent or daughter isotopes
# of Parent & Daughter
Atoms as a Function of Time
Daughter
Parent
Proportion of Parent Atoms Remaining
as a Function of Time
Fig. 10.14
% of parent isotope remaining
100
50
25
12.5
6.25
3.125
Percent of
Parent Atoms
Remaining as a
Function of
Time
% of parent isotope remaining
As more parent decays, the
decay rate slows down, but
the half-life doesn’t change
100
Slope =
rate of decay
Decay rate = lP
P = amt. Of parent
50
25
12.5
6.25
3.125
l= decay constant
= 0.693/half-life