18GEOLOGIC TIME

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18GEOLOGIC TIME
Grand Canyon The strata exposed contain clues to
millions of years of Earth history.
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Vastness of Geologic Time Deep Time
12-15 by
Mar 21 820 am
3.6 by
Jan 1 1200 am
4.6 by
An appreciation for the immensity of geologic
time is essential for understanding the history
of our planet
Nov 27 907 pm
430 my
Nov 16 631 pm
570 my
245 my
Dec 12 126 pm
Dec 26 813 pm
65 my
100, 000 yrs
55 my
Dec 27 315 pm
Dec 31 1148 pm
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Relative versus Absolute Ages
Relative age places events in sequential or
chronologic order older than, younger than Gulf
War, WWII Absolute age specific dates (numerical
ages) for rocks or geologic events numerical
age 1991 1940
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Relative Dating Principles
1. Superposition - in a sequence of undisturbed
strata, older units are on the bottom and younger
units are on the top
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Relative Dating Principles
2. Original horizontality - sediments are
initially deposited as flat layers 3. Lateral
continuity - strata extend laterally in all
directions until it thins, pinches out, or
reaches the edge of the depositional basin
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Relative Dating Principles
4. Cross-cutting relationships - a feature that
cuts across another is younger than the feature
that is cut (dikes, faults, plutons are younger
than the rocks they cut across)
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Relative Dating Principles
4. Cross-cutting relationships - a feature that
cuts across another is younger than the feature
that is cut (dikes, faults, plutons are younger
than the rocks they cut across)
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Relative Dating Principles
5. Inclusions - inclusions are always older than
the rock that they are included within
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Relative Dating Principles
5. Inclusions - inclusions are always older than
the rock that they are included within
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UNCONFORMITIES
Surfaces of erosion representing missing time
(hiatus) a buried land surface Angular
Unconformity Disconformity Nonconformity
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Angular Unconformity

• ANGULAR UNCONFORMITY
• an erosional surface on top of tilted strata
  over which younger rocks have been deposited

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Disconformity

• DISCONFORMITY
• an erosional surface between parallel strata

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Nonconformity

• NONCONFORMITY
• an erosional surface on top of igneous or
  metamorphic rocks

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Summary of the Principles of Relative Dating
Superposition Original Horizontality Lateral
Continuity Cross-cutting relationships Inclusions
Unconformities Angular unconformity Disconformity
Nonconformity Fossil Succession
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Applying the Principles
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CORRELATION
the demonstration of age equivalency of rock
units in different areas lateral continuity,
position, rock type key beds (e.g., coal seams,
ash layers) index (guide) fossils
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FOSSILS
Fossils remains or traces of prehistoric life
Most organisms are uniquely adapted to their
habitat By comparing fossil organisms to their
living relatives, scientists are able to
determine what the ancient environment was like
during the time the fossil organism was alive
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FOSSILS
remains or traces or prehistoric organisms
preserved in rocks Body fossils remains of
organisms, hard parts or soft parts Tissue
(amber, ice, tar, bogs, desiccation) Bones,
shells, other hard parts (replacement,
mineralization, carbonization, molds casts,
impressions) Trace fossils tracks, trails,
nests, burrows, coprolites, gastroliths Condition
s that favor fossilization Hard parts Rapid
burial Escape physical, chemical, biological
destruction water, waves, scavengers,
atmosphere, bacterial decay (decomposition)
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FOSSILS
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FOSSILS
Impressions of fish show fine details
Insect in amber
Dinosaur footprint in limestone near Tuba City AZ
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FOSSIL SUCCESSION
fossil organisms succeed one another through
time in a regular and predictable order, so rock
layers can be dated by their fossil content
Fishes first appear in the Cambrian Mammals first
appear in the Triassic Flowering plants first
appear in the Cretaceous
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FOSSIL SUCCESSION
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FOSSIL SUCCESSION
Mammals
Ammonites
Early ferns
Graptolites
Trilobites
Fossil algae
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CORRELATIONand FOSSILS
demonstration of age equivalency of rock units in
different areas lateral continuity, position in
sequence, rock type key beds (e.g., coal seams,
ash layers)
index (guide) fossils widespread
easy to ID short lived abundant
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CORRELATION and FOSSILS
A group of fossils can be used to date rocks more
precisely than could be accomplished using only
one of the fossils.
Overlapping fossil ranges help to narrow down the
age of a rock layer.
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CORRELATION and FOSSILS
Overlapping fossil ranges help to narrow down the
age of a rock layer.
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ABSOLUTE DATING
Atom nucleus protons () and neutrons (no
charge) electrons (-) in shells surrounding the
nucleus protons and neutrons have mass (1 amu
each) and contribute to the mass of the
atom electrons are essentially mass-less and do
not contribute significantly to the mass of the
atom
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ATOMIC NUMBER and ATOMIC MASS
The number of protons in the nucleus determines
the identity of an atom and its atomic
number. hydrogen H p1 oxygen O p8
carbon C p6 The atomic mass of an
atom is based on the number of protons and
neutrons in the nucleus.
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ISOTOPES
Isotopes are atoms with the same atomic number
but with different atomic masses (different
numbers of neutrons but the same number of
protons). Some isotopes are radioactive and can
be used to date rocks.
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Radiometric Dating
Radioactive isotopes (parent) spontaneously decay
to form other isotopes (daughter), releasing
energy in the process Number of parent and
daughter atoms are measured in a mass
spectrometer The constant rate of decay gave
geologists a clock to accurately date rocks.
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RADIOACTIVE DECAY

• Alpha decay
• nucleus emits 2 p and 2 n
• Beta decay
• neutron emits an electron
• transforms to a proton
• Electron capture
• proton captures a passing electron
• transforms to a neutron

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RADIOACTIVE DECAY
Some radioactive elements undergo only one decay
step, others require multiple decay steps
Uranium-lead decay series U238 -gt
Th234 (alpha decay) U238 has 92
protons, 146 neutrons Th234 has 90
protons, 144 neutrons Th234 -gt Pa234 (beta
decay) Th234 has 90 protons, 144
neutrons Pa234 has 91 protons, 143
neutrons
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RADIOACTIVE DECAY
Rubidium-Strontium Rb87 -gt Sr87 (beta
decay) Rb87 has 37 protons, 50 neutrons Sr87 has
38 protons, 49 neutrons Potassium-Argon K40 -gt
Ar40 (electron capture) K40 has 19 protons, 21
neutrons Ar40 has 18 protons, 22
neutrons Carbon-14 C14 -gt N14 (beta decay) C14
has 6 protons, 8 neutrons N14 has 7 protons, 7
neutrons
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Carbon 14
N14 is converted to C14 in the atmosphere by
neutron capture, in which the nucleus captures a
neutron and emits a proton (N has 7 p C has 6
p) C14 is absorbed along with the other isotopes
(C12, C13) of carbon into the tissues of living
organisms Once the organism dies, C14 is not
replenished so the ratio of C14 to C12 in the
organism decreases as C14 decays to N14 by beta
decay (neutron emits electron, transforms to
proton) It takes 5,730 /- 30 years for one-half
of the original amount of C14 in the organism to
decay
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HALF LIFE
A half-life is the time it takes for one-half of
the unstable parent element to decay to atoms of
a daughter element. To determine the age of a
rock, scientists determine how many half-lives
have elapsed since the rock formed, and then
multiply by the value of one half-life.
As time progresses, the amount of radioactive
parent decreases and the amount of daughter
increases.
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Radioactive decay curve
100 parent, 0 daughter PD 10 Time rock
forms (age 0)
50 parent, 50 daughter PD 11 After 1
half-life
25 parent, 75 daughter PD 13 After 2
half-lives
12.5 parent, 87.5 daughter PD 17
6.25 parent, 93.75 daughter PD 115 After
4 half-lives
3.125 parent, 96.875 daughter PD 131
After 5 half-lives
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Radiometric Dates

• How old is a sample that contains 25 of its
  original K-40?
• How many half-lives have elapsed?
• What is the value of one half-life of K-40?
• Multiply the value of one half-life by the number
  of half-lives that have elapsed.

2
1.3 b.y.
1.3 b.y. x 2 2.6 b.y.
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Radiometric Dates

• How old is a sample that contains 65 daughter
  Pb-207?
• How much parent remains?
• How many half-lives have elapsed?
• What is the value of one half-life of U-235?
• Multiply the value of one half-life by the number
  of half-lives that have elapsed.

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1.5
713 m.y.
713 m.y. x 1.5 1069.5 m.y. or 1.0695 b.y.
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Radiometric Dates

• What fraction of the original C-14 remains in a
  sample after 11,460 years?
• What is the value of one half-life of C-14?
• How many half-lives have elapsed?

5730 years
11,460 / 5730 2

• Create a table
• Fraction C-14 half-lives Age

1/1 0 0 years
1/2 1 5730 yrs.
1/4 2 11,460 yrs.
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Radiometric Dates

• How many half-lives have elapsed to yield a
  sample with 125 atoms of C-14 and 375 atoms of
  N-14?
• How many atoms of C-14 were there to begin with?

500
b. Create a table atoms C-14 atoms N-14
half-lives
500 0 0
250 250 1
125 375 2
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Radiometric Dates

• How old is a sample that contains a U-235 to
  Pb-207 ratio of 11?

a. Create a table fraction U235 frction Pb207
half-lives
10
1/1 0/1 0
11
1/2 1/2 1

• What is the value of one half-life of U235?
• Multiply the value of one half-life by the number
  of half-lives elapsed.

713 m.y.
713 m.y. x 1 713 m.y.
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Bracketing the Absolute Ages of Sedimentary Rocks
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Geologic Time Scale

• Eons
• Eras
• Periods
• Epochs
• Phanerozoic Eon 0 - 540 my (lasted 540 my)
• Mesozoic Era 65 - 248 my (lasted 183 my)
• Jurassic Period 144 - 206 my (lasted 62 my)
• Miocene Epoch 5.3 - 23.8 my (lasted 18.5 my)
• PRECAMBRIAN time lasted nearly 4 billion years!
  88 of Earths history

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The Record of Life in the Rocks

• PHANEROZOIC EON
• Cenozoic Era Age of Mammals
• diversification of flowering plants, insects,
  birds, mammals
• appearance of humans
• Ice Age
• Mesozoic Era Age of Reptiles
• origin of flowering plants, dinosaurs, birds,
  and mammals
• Paleozoic Era Ancient Life
• invertebrates
• first land plants, first vertebrates (back-boned
  animals, including fishes, amphibians, and
  reptiles
• PRECAMBRIAN Origin of Life
• Proterozoic Eon
• Earliest shelled animals
• Archean Eon
• Earliest record of life
• Hadean Eon
• No record of life