1
Geologic time
2
John Wesley Powell’s expedition to the Grand Canyon in 1869
“Rocks conceal evidence for an ancient Earth”
3
Review: Origin of the solar system
4
Review: the historical view of geology
Aristotle’s view of geology was adhered to until the Middle Ages
Rocks created under the “influence” of stars
Earthquakes caused by heated air escaping explosively
Fishes lived in the Earth – motionless (fossils)
5
Review: Catastrophism
6
Determining geological ages
Relative age dates – placing rocks and events in their proper
sequence of formation
Numerical dates – specifying the actual number of years since
an event occurred
(known as absolute age dating)
7
Principles of relative dating
1)Law of superposition
Nicolaus Steno,1669
2)Principle of original horizontality
3)Principle of cross-cutting relationships
4)Inclusions
5)Unconformities
8
9
10
Principle of original horizontality
11
Rock containing the inclusions is younger
12
Grand Canyon, Fig. 8.2, p. 218
Principle of cross-cutting relationships
Fig. 8.4, p. 219.
Unconformity
Def: a break in the rock record produced by
erosion and/or nondeposition of rock units
13
Types of unconformities
Angular unconformity – tilted rocks are overlain by more flatlying rocks
Disconformity – strata on either side of the unconformity are
parallel
Nonconformity – metamorphic or igneous rocks in contact
1
with sedimentary strata
14
Angular unconformity
• James Hutton’s “Theory of the Earth,” 1785 (Scotland) (Uniformitarianism)
15
16
17
• Siccar Point, Scotland
Channel deposit showing a cross-cutting relationship
Cross-cutting relationship on the moon
Grand Canyon,
Fig. 8.6
18
Correlation of rock layers
Matching of rocks of similar ages in different
regions is known as correlation
19
Fig. 8.9 illustration of correlation
20
Correlation often relies upon fossils
Fig. 8.10
21
The geologic time scale
22
Isotopes and radioactive decay in dating
Number of neutrons can vary
Thus, an element can have more than one mass number = isotope
Ex: Oxygen has three isotopes (8,9, and 10 neutrons)
Fig. 2.4, p. 37
23
The Periodic Table of the Elements
24
Radioactivity
Spontaneous changes (decay) in the structure of atomic nuclei
25
Using radioactivity in dating
Parent – an unstable radioactive isotope
Daughter product – isotope resulting from the decay of a parent
Half-life – time required for one-half of the radioactive nuclei in a sample to decay
26
U-238 decays to Pb-206 through several steps.
Fig. 8.12, p. 228
27
Table 8.1, p. 231
28
Using radioactivity in dating
Fig. 8.13, p. 229
% of atoms that decay during one half-life is always the same (50 %)
Comparing the ratio of parent to daughter yields the age of the sample
29
Using radioactivity in dating
Radioactive decay can be expressed by the formula:
2
N = Noe -lt
Where:
N = number of parent atoms present today in the sample
No= original number of atoms (parent + daughter)
e = 2.71828... (this is a constant),
l= decay constant = 0.693/(half-life)
t = time (years)
30
31
Figure showing U-238 to Lead- 206 decay
radiocarbon dating
Half-life of only 5730 yrs
Used for recent events
C-14 produced in the upper atmosphere
incorporated in CO2, which is absorbed by living matter
Fig. 8.14, p. 233
32
33
Radiocarbon dating
34
Geologic time scale
35
Structure of the geologic time scale
Radiometric dating sources of error
A mineral must have remained within a closed system
Grains in detrital sedimentary rocks are not the same age as the rock
in which they formed
Particular minerals in a metamorphic rock may not represent the time
when the rock formed
Eon
Era
•Period
–Epoch
36
End of Chapter 8
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