GEOLOGIC TIME Chapter 17
A.
Introduction
1. Concept
of Time/Space –theme underlying both archaeology and geology
a. Space=an exact location (point on a map)
b.
Time= history of that location as exacted from geologic evidence
2. Age
of the Earth p. 395
a. In 1625, Archbishop Ussher determined earth to be about 6000 years old
b. In 1860’s, Lord Kelvin estimated age to be 20-40 million years based on cooling rate of earth
c. In late 1800’s, Joly estimated age to be 90 million years based on the Na content of oceans
d. Today with radiometric dating techniques----
i. age of earth: about 4,500-4,600 million years old (4.5-4.6 billion years)
ii. oldest terrestrial rock : about 4 billion years old
3. James
Hutton---uniformitarianism: “the
present is the key to the past" p. 395
(catastrophism vs.
uniformitarianism---then & now)
4. Relative
vs. Absolute Time
a. Relative
Time: placing events in proper chronological
order
b. Absolute
Time: placing actual dates on events
1. Fundamental Relationships
a. Original Horizontality
b. Superposition: oldest (first) layer on bottom of sequence
c. Cross-cutting relationships-the features cutting are younger that the features being cut p.398, Fig 17.3
follow the sequence of events depicted in the block diagrams in Fig. 17.8-a, p.402
2. Fossils see pp. 189-190
a. Faunal
Succession
b. Index
Fossil---marine invertebrate that was short-lived and geographically widespread
3.
Geologic Mapping of
Sedimentary Rocks
a. Formation: a mappable rock unit
b. Geologic
section: Correlation (fossils) see Fig 8.18, p.191
c. Development
of the Geologic Column (and the Geologic Time Scale)
d. Unconformities—gap
in the geologic record due to missing sediment p.398
(for the test
you do NOT need to distinguish
different types; i.e., disconformities, angular unconformities, nonconformities)
1.
Radioactive Decay p. 405; see
Fig. 17.12, p.408
a.
Radioisotopes—example U-238 & U-235; same chemical
behavior, different nuclear behavior
b.
Nuclear processes:
alpha, beta, gamma radiation; electron capture see
p. 193, Fig. 8.20
c.
Decay Chains: parent/daughter; Chain for U-238 to Pb-206
see p. 195, Fig. 8.21 (NO need to memorize the chain for the test)
d.
Half-Life: see page 409; Fig 7.14
2.
Radiometric Dating with Parent/Daughter pairs
a.
Assumption is the “clock is started” with mineral
forms; closed system—no material enters or leaves
b.
Commonly used pairs:
Note: for the Test, the following is simplified from Table 17.1, p. 410
i.
K/Ar
Potassium/Argon
ii.
U/Pb Uranium/Lead----actually have two isotope pairs
iii.
Rb/Sr Rubidium/ Strontium
Note: for the Test
assume all use igneous rocks only; all are only good for events 10 my old or older
3. Carbon-14 see p. 410
a.
C-14 formed by cosmic radiation of N-14 in the
atmosphere
b.
C-14 decays back to N-14 with a half-life of 5730 years
c.
C-14/C-12 ratio used for dating is not a parent/daughter
ratio
d.
Good for events up to 50,000 years ago (some say up to
75,000-100,000)
e.
No good for events after the “dawn of the nuclear age”---1945
f.
Used extensively in archaeology but also in geology
1.
Explanation
a.
The rocks in the Geologic Column, B-3-c above, also
represent the time during which these rocks formed
b.
Unconformities, section B-3-d above, were used to divide
and subdivide major in the Geologic Column
c. Just as we divide years into months into
weeks into days, etc., so too is Geologic Time divided
d. Geologic time: Eons are divided into Eras; Eras area divided into Periods and
Periods are divided into Epochs
e.
Absolute time (from igneous rocks) delineates the
boundaries and is given as “mya” millions of years ago
or “mybp”, millions of years before present
2.
The Time Scale see Fig. 17.1,
p. 394
For
purposes of Test 2, know the following:
a.
Ignore the Eons, but know the Eras (Precambrian,
Paleozoic, Mesozoic, Cenozoic) in order
b.
Know the Periods in order for the Paleozoic, Mesozoic
and Cenozoic
c.
Know the Epochs in order for the Quaternary
(Pleistocene and Recent)
d.
Know the following approximate time boundaries:
10,000
year ago: end of the Pleistocene, start
of the Recent
2
mya: start of the Pleistocene (and
Quaternary)
65
mya: end of Mesozoic, start of Cenozoic
250
mya: end of Paleozoic, start of Mesozoic
550
mya: end of Precambrian, start of
Paleozoic
4500 mya: (= 4.5 billion): age of
earth (start of Precambrian)
3. Comprehending the vastness of Geologic Time
The vastness of geologic time
is far beyond our human frame of reference and is hard to fully appreciate
Example: consider a single step represents going back
in time 1000 years
1 step
(from the present): Normal invasion
of British Isles
2
steps (from the present): Time of
Julius Caesar
5
steps (from the present): Construction
of the Great Pyramids
10
steps (from the present): End of the
last Ice Age
32
miles (from present)
Cenozoic-Mesozoic
Boundary
115
miles (from present):
Mesozoic-Paleozoic
Boundary
300
miles (from present):
Paleozoic-Precambrian
Boundary
1900
miles (from present): Earliest know
terrestrial rock
2200
miles (from present): Age of the
earth