1. 9. GEOLOGIC TIME
1. Relative age dating - Order of events are known, but not dates
2. Radiometric (Absolute) age dating - Dates are known
DATING
Means determining when geologic events happened
GEOLOGIC EVENT
(1) The formation of rocks (2) Faulting (3) Tilting
(4) Folding (5) Metamorphism (6) Erosion etc… 1

2. What came first... chicken or the egg?
1. RELATIVE AGE DATING – determined by spatial relationships between adjacent rock bodies
What came first...
chicken or the egg?
WHEN DID THE KOREAN WAR HAPPEN? 2

3. “Principle of Original horizontality”
WHEN DID THE ROCKS IN THIS AREA FORM?
IN WHAT SEQUENCE? HOW CAN WE KNOW?
“Principle of Original horizontality”
“Principle of Cross-cutting relationships”
“Principle of Inclusions”
“Principle of Superposition”
“Principle of Unconformities” 3

4. PRINCIPLE OF ORIGINAL HORIZONTALITY
Sediments are always deposited horizontally
If layers are inclined, then some tectonic forces must have tilted them - they didn’t form that way
Page 303 4

5. PRINCIPLE OF SUPERPOSITION
If one layer is on top of another, then the top layer came later (it’s younger)
Youngest
Oldest

6. PRINCIPLE OF CROSS CUTTING RELATIONSHIPS
If a rock unit is cut or disrupted by a fault or a dike, then the rock must have been there first. That is, the rock is older, the fault or the dike are younger!
dike
Sandstone
shale
dike
F A U L T 6

7. PRINCIPLE OF INCLUSIONS
If a rock unit contains pieces of another rock, it must be younger than the pieces of rocks.
UNCONFORMITY  7

8. PRINCIPLE OF UNCONFORMITIES
Unconformities represent gaps in the geologic record that formed because layers were not deposited for a time or else layers were removed by erosion
Unconformity: is like the place where pages are missing from a book

9. EXAMPLE -1
What type of unconformity is F?
What type of Fault is D? 9

10. EXAMPLE - 2
Folding ?
What type of unconformity is E?

11. EXAMPLE - 3

12. EXAMPLE - 4 12

13. FOSSILS AND THE RELATIVE
TIME SCALE
Dinosaur
Ammonite
Trilobite 13

14. What is the possible age of a rock that contains fossils of both:
Shark and Mucrospirifer?
Shark and Dinosaurs?
Estimate the age of the fossiliferous limestone 14

15. RADIOMETRIC (Numerical)
AGE DATING
Provides actual # of years of rocks (minerals)
Places rocks and events in Geologic Time Scale
Makes use of Radiometric techniques such as K-Ar, C-14, U-Pb etc.

16. HOW OLD IS THE EARTH? Archbishop James Ussher (1654)
the world was created in 4004 (B.C.E.)
Dr. John Lightfoot (biblical scholar)
Was more specific: the Earth was created on October 26, 4004 at 9:00 a.m. (B.C.E)
Age of the earth: = 6,021 years

17. RADIOMETRIC DATING: BASICS
Radioactivity: Spontaneous nuclear transformations that change the number of protons & neutrons in the nucleus of an isotope.
Forces that bind p & n may be unusually weak, thus the nucleus becomes unstable!
Isotope: One of two or more species of the same chemical element having the same number of protons but differing from one another by having a different number of neutrons.
U has always 92 protons, but varying # of neutrons; U234, U235, U238

18. HALF LIFE (λ)
Time it takes for half of the parent isotope to decay into daughter isotope
Page 322
Figure 9.22
C-N years

19. INSTRUMENT FOR RADIOMETRIC DATING
Mass Spectrometer

20. % of Parent Isotope (element) remaining
Half-life
% of Parent Isotope (element) remaining 1
Starting amount of parent isotope (at time=zero): 100 % 2 3 4
EXAMPLE
Q1. Suppose the half-life value of a radioactive element is 50 m.y. How long will it take for 50% of the parent isotope to change (decay) to the daughter product?
Q2. If the % of parent isotope found in a sample is 6.25%, how old is the sample?

21. WHAT CAN BE DATED? Metamorphic Rock Organic Material (fossils)
Dating Method
Minerals/materials that can be dated
U-Pb
Zircon, apatite
K-Ar
Muscovite, biotite, hornblende
Rb-Sr
Muscovite, biotite, K-feldspar
C-N
Wood, coal, peat, bone, shell, water
Metamorphic Rock
Organic Material (fossils)
Igneous Rock

22. C-14 METHOD
Constant generation of C-14 in the upper atmosphere by cosmic particle bombardment of N (nitrogen).
Nitrogen (N-15) emits a proton and becomes C-14. This is radioactive element with a λ of about 5,730 yrs.
Plants and animals ingest this radioactive C-14 while they are alive. When they die, the ingestion stops, and the radioactive C-14 clock begins to count down.
N atomic # = 7
C atomic # = 6

23. Oldest Rocks on Earth (Acasta Gneiss, N Canada)
Ca b.y. old
AGE OF THE EARTH
4.56 b.y.
Oldest Mineral on Earth
(Zircon, Jack Hills Conglomerate, W Australia) : ~ 4.4 b.y. old

24. THE SOLAR SYSTEM
It is believed that all planets of our solar system, including the asteroid belt, originated at about the same time. Therefore, the age of one is the age for all!
Age determined for Earth from rocks and minerals that originate on Earth does not give the best estimate for the age of the Earth because most of these rocks have been recycled during subduction or by metamorphic processes.