Name__________________________________
GS 106 Geologic Dating
1. Relative Dating
R
A
P
E
L
F
H
D
G
S
Using the geologic principles discussed in chapter 18 of Lutgens and Tarbuck deduce the sequence of
geologic events recorded in the cross section above. List the events from youngest to oldest along with the
geological principle(s) used to place the event in sequence. Rock units R, P, D, L, and E are sedimentary,
while units A and F are igneous extrusive and units S and G are igneous intrusive. The line H is a fault.
GEOLOGIC EVENT
GEOLOGIC PRINCIPLE(S) UTILIZED
_________________________
__________________________________
_________________________
_________________________________
_________________________
_________________________________
_________________________
_________________________________
_________________________
_________________________________
_________________________
_________________________________
_________________________
_________________________________
_________________________
_________________________________
_________________________
_________________________________
_________________________
_________________________________
_________________________
_________________________________
_________________________
_________________________________
_________________________
_________________________________
_________________________
_________________________________
_________________________
_________________________________
2. Grand Canyon Geology
The visually magnificent Grand Canyon is a classic geologic locality where the deep
down cutting of the Colorado River has exposed layer after later of sedimentary rock.
You will examine a geologic map of the Grand Canyon to learn about geologic time
periods and unconformities.
Examine the map
Different colors represent different sedimentary rock units. A key is provided which
gives the names and ages of the sedimentary rock units.
What is the name of the pale blue map unit which covers most of the map?
Find the Colorado River, what is the color of the map unit exposed at the bottom of the
canyon? What is the name of this unit?
Notice how the map units create stripes parallel to the river. This pattern is controlled by
the orientation of the rock layers. Are the rock layers horizontal or vertical?
What would the map look like if the rock layers were oriented in the direction which you
didn’t choose above?
Examine Cross Section B
What is the total thickness of the horizontal sedimentary units? __________________
What is the top most horizontal sedimentary unit? ________________________
During which geologic time period did the top most unit form? ____________________
What is the bottom most horizontal sedimentary unit? ________________________
During which geologic time period did the bottom most unit form? _________________
What is the name of the unit labeled Pc?
When did it form?
What is the name of the unit labeled Ph?
When did it form?
Are any time periods missing between the deposition of units Pc and Ph? yes
If yes, what periods?
no
What type of depositional contact occurs between units Pc and Ph?
conformity angular unconformity
disconformity
nonconformity
What is the name of the unit labeled Dtb?
When did it form?
What is the name of the unit labeled Cm?
When did it form?
Are any time periods missing between the deposition of units Dtb and Cm? yes
If yes, what periods?
no
What type of depositional contact occurs between units Dtb and Cm?
conformity angular unconformity
disconformity
nonconformity
What type of depositional contact occurs between the Tapeats Sandstone and the Vishnu
Schist? conformity angular unconformity disconformity
nonconformity
Examine Cross Section D
What type of depositional contact occurs between the Tapeats Sandstone and the preCambrian sediments?
conformity
angular unconformity
disconformity
nonconformity
Examine Figure 18.6 In Your Text
Which is older the Vishnu Schist or the Zoroaster Granite? How can you tell?
Is the metamorphism of the Vishnu Schist the oldest event recorded in the Grand
Canyon? If there is an older event what would it be?
Examine Figure 18.6 in your text. How many periods of uplift and erosion are recorded
in this cross section of the Grand Canyon? Identify each!
Name__________________________________
GS 106 Absolute Dating
1. Radioactive Decay
1) Complete the data table below using the coin flip data we generated in class.
2) Plot the number of parents, the number of daughters, and the number of parents
plus daughters against the number of half-lives using the attached graph paper.
DO NOT CONNECT THE DOTS! Instead draw a smooth best fit curve through
each set of data points. A smooth best fit curve will come as close as possible to
the data points while staying as smooth as possible. Make sure to label the axes
and provide a key identifying the data points.
# of half lives
0
1
2
3
4
5
6
7
8
# of parent
(p)
# of daughter
(d)
p+d
% p remaining
0
1
2
3
4
5
6
7
2. Absolute Dating
1. On the attached graph draw a curve showing how the percentage of parent remaining
decreases as the number of half lives passed increases.
2. The mica muscovite (KAl3Si3O10(OH)2) is often present in granite. What process forms
the muscovite grains in granite?
3. Some of the potassium in muscovite is the isotope
40
K which is radioactive. What is the half life of 40K? ___________________________
4. When geologists analyze muscovite in granites they find minute quantities of 40Ar
even though no 40Ar was present in the grain when it formed (see formula). What
process forms this 40Ar?
5. A geologist analyzes a tiny flake of muscovite from a granite from Northern Ontario
Canada and finds it to contain 2,000,000,000 atoms of 40K and 8,000,000,000 atoms of
40
Ar.
a) How many atoms of 40K were in the muscovite when it formed? __________________
b) What percentage of the initial 40K atoms still remain? _________________________
c) How many half lives have passed? ________________
d) How old is the muscovite grain? __________________________
e) How old is the Canadian granite? __________________________________
6. A geologist analyzes a tiny zircon grain from a granite pebble in a conglomerate from
the San Bernardino Mountains in California and finds it to contain 7,000,000,000 atoms
of 235U and 3,000,000,000 atoms of 207Pb.
a) How many atoms of 235U were in the zircon when it formed? ______________
b) What percentage of the initial 235U atoms still remain? ______________
c) How many half lives have passed? ______________________
d) What is the half life of 235U? ___________________________
e) How old is the zircon grain? __________________________
f) How old is the conglomerate? __________________________________
Radioactive Decay Plot
100
90
Percentage of Parent Remaining
80
70
60
50
40
30
20
10
0
0
1
2
3
4
Number of Half Lives
5
6
7
8