What is the sequence of events that shaped
this geologic setting?
Dike B
Dike A
Fault 2
Fault 1
TODAY:
Chapter 10 Part 2
Geologic Time
3) Fossils as time-pieces
4) Absolute dating w/ radioactivity
5) Geologic time scale
6)Regional correlation of rock layers
3. Fossils as timepieces
 Fossils
William Smith (late 1700s) noted that sedimentary strata in widely
separated area could be identified and correlated by their distinctive
fossil content
 fossil organisms succeed each other in
systematic fashion, always in order
e.g., age of trilobites, age of fishes, age of
reptiles,age of mammals
 any time period can be determined by
fossil content
“principle of fossil succession”
3. Fossils as timepieces
“Index fossils” = Fossils as time-indicators
geographically wide-spread
short span of time
Overlapping ranges
of fossils help date
rocks more exactly
(time-period is ‘skinnier’).
3. Fossils as timepieces
Using fossil assemblages, formations I and II of
Outcrop A and II and III of Outcrop B can be put in
relative sequence of each other (3).
4. Absolute dating w/ radioactivity
 Chemistry review
Atom
model
# protrons = atomic #,
defines the element
nucleus
protrons
neutrons
# neutrons can vary:
“isotopes”
See Fig. 10.13 for an example of radioactive decay.
4. Absolute dating w/ radioactivity
Half-life: The time required for half of the nuclei
to decay
Example: an isotope with a half life of 1 year
# of atoms
Parent
Now
1000
Next year
500
2 yrs from now 250
3 yrs from now 125
10 yrs from now 1
Daughter
0
500
750
875
999
4. Absolute dating w/ radioactivity
Fig. 10.14: Radioactive decay is exponential.
See Table 10.1 on radioactive elements used for radiometric dating.
What to remember…about radioactivity
 Some isotopes are unstable
 Parent nucleus breaks up (decays)
 By-product (daughter) will be different
element
 Rate of decay is constant & can be determined
 Half life = Time required for half of parent
atoms to decay to daughters
 Isotopes with long ½ lives (uranium) are used
to date old rocks
 Short ½ lives (carbon-14) are used for young
5. Geologic time scale - a calendar of Earth’s history
eons
Phanerozoic - last 570 million years
Cenozoic - recent life
Massive
extinction
events
0-66.4 m.y.a
(mammals, humans)
Mesozoic - age of middle life
66.4-245 m.y.a
(dinosaurs, 1st scrawny mammals)
Paleozoic - ancient life
245-540 m.y.a
(first fossils with shells,
fish, trilobites, clams, corals, ferns )
Precambrian - from birth of Earth up to before
complex life forms developed
(algae, bacteria, some fossils without shells like jellyfish, worms)
See Fig. 10.12, 10.17, 1.12
5. Geologic time scale
From Tarbuck and Lutgens, Earth, 7th ed.
6)Regional correlation of rock layers
Using relative dating principles and fossils, strata from different
localities can be pieced together to form one continuous record
of geologic history.
Box. 10.1., pp. 226-227
And now a few sample MC…
The principle of superposition states that __________.
A.
B.
C.
D.
a fault is younger than the rocks it cuts
sediments are deposited as essentially horizontal layers
the present is the key to the past
undisturbed sedimentary layers get progressively younger from bottom
And now a few sample MC…
What is an unconformity?
A.
B.
C.
D.
a gap in the geologic record
a period of deposition
a sedimentary layer of variable thickness
a sequence of deformed rocks
And now a few sample MC…
The study of faunal succession allows _________.
A.
B.
C.
D.
absolute dating of fossil-bearing strata
correlation of marine fossils with modern mammals
reconstruction of paleoclimates
rocks to be correlated from different outcrops
And now a few sample MC…
Which of the following radioactive isotopes is the most useful for
dating very young (<10,000 years old) wood and charcoal?
A.
B.
C.
D.
carbon-14
potassium-40
rubidium-87
uranium-238
And now a few sample MC…
Human beings (homo sapiens) evolved during which geologic era?
A.
B.
C.
D.
Cenozoic
Mesozoic
Paleozoic
Precambrian