Geologic Time
Absolute Dating and
Geologic Time
Absolute Age
•It is the age of a rock unit, fossil or geologic
event expressed in units, such as years
• For example, your birthday . . .
– You were born on a specific
day, month and year
• Absolute age can be determined by Radiometric Dating
Radiometric Dating
• Uses the known decay rate (half-life) of
elements in rocks to determine the absolute
age of the rock
• U238Pb206 – useful in dating old igneous rocks
• K40Ar40 – useful for intermediate igneous rocks
Radiometric Dating
The Atom
• Fundamental unit of matter
• Made up of components called
subatomic particles
– Protons (positive charge)
– Neutron (no electrical charge)
– Electron (negative charge
Radioactive Decay
Stable Isotopes
• An atom is generally
stable if the number of
protons equals the
number of neutrons in
the nucleus
• Atomic Number
– An element’s identifying #
– Equals # of protons in atom’s
nucleus
• Mass Number
– Equals # of protons and
neutrons in an atom’s nucleus
Radioactive Decay
Isotopes
• Variant of the same parent atom
• Differ in the # of neutrons
• Result in different mass # than
parent
• For example:
– Carbon-14 (C-14)
– Types of carbon
• C-12 (stable)
• C-13 (unstable)
• C-14 (unstable)
Radioactive Decay

The nucleus of an atom (decays) changes
into a new element.

The proton number (atomic number) changes
14C
6
 147 N

Parent – unstable radioactive isotope

Daughter – isotopes resulting from decay of parent
How long does this take?
Radiocarbon Dating
Half-Life
• Half Life - time required for one-half of
radioactive nuclei in a sample to decay
 The
half life of C-14 is 5,730 years
The Carbon Cycle
When Does the Clock Start?
When a plant or animal
dies, the clock starts.
Organism dies
No more C-14 intake
C-14 begins to decay
How the Carbon Clock Works
 There
are two types of carbon used in dating
– C-12 (stable  does not decay)
– C-14 (radioactive  decays)
 When
organism is living, it has same ratio (C12:C-14)
found in the atmosphere
A living starfish has the same
ratio as the atmosphere
A fossilized starfish has a
different ratio
How the C-14 Ratio Works


The ratio of C-14:N-14 can tell us how many half-lives
Amount of
stable N-14
Amount of
unstable C-14
0
100
50
50
75
Ratio
Years dead
# of halflives
0
0
1:1
5,730
1
25
4:1
11,460
2
87.5
12.5
8:1
17,190
3
94
6
16:1
22,920
4
97
3
32:1
28,650
5
Carbon-14 can date elements up to approximately 100,000 years
– Used to date very recent events
– Important tool for anthropologists, archeologists and geologists
Radiometric Dating

Uses the known decay rate (half-life) of elements in
rocks to determine the absolute age of the rock


U238Pb206 – useful in dating old igneous rocks
K40Ar40 – useful for intermediate igneous rocks
C-14 Example
Turin Shroud
Believed to be burial cloth of crucified Christ
Example
Turin Shroud
Untouched
x-rayed
artistic impression
Example
Shroud of Turin
• Evidence:
– Blood Stains
– Wounds
– Weave of cloth
– Pollen grains
– Serum albumin
• Skeptics:
– Painting
– Blood - red
Carbon-14
Results
Carbon-14 dates the Turin Shroud to:
1356 A.D.
Is the Shroud authentic?
When was Jesus born?
More Examples
Dead Sea Scrolls
Qumran Caves
Aerial view
Caves
More Examples
Dead Sea Scrolls
Evidence:
• >500 manuscripts.
• 15,000 fragments describe
every book of Old Testament
• Some Papyrus samples give
exact dates w/in text 
coincide with C-14 results
Skeptics:
Dead Sea Scrolls
were found in jars
Fragment of Dead Sea Scroll
• Some of the scrolls appear
to be work of early
Christians.
Carbon-14
Results
Carbon-14
150 B.C – 5 B.C.
Is it Authentic?
Pre-dates Christianity
One More Example
Solomon’s Temple Stone Tablet
Evidence:
• Confirms existence of
Solomon’s Temple –
9th Century B.C.
• 10 lines of Phoenician
script describes life of
King Jehoash.
―Repair to temple.
―War, etc.
Carbon-14
Results
Carbon-14 dates stone tablet to:
9th Century B.C.
It’s authentic!!
Radiometric Dating
Inconsistencies
• In 1993, scientists found wood (trees) buried in
basalt flows (69 feet deep).
• Wood samples sent to two labss to be carbon-14
dated  44,000 years old.
• Basalt sent to two laboratories to be potassiumargon dated  45,000 million years.
69 feet deep
Wood
encased in
basalt
Known Carbon-14
Inconsistencies
• Mollusks that are living
dated at 2,300 years
• Seal skins (fresh)
dated 1,300 years
Okay . . .
Back to Geology!!
Earth’s History
 Based
on evidence preserved in rocks
 Immensity
of time span
difficult to appreciate
– Changes occur over
millions to billions of
years
– Human life span =
70-80 years
 Reconstruct
history
through relative dating
Geologic Time Scale
• Organized into
meaningful,
manageable units
• Oldest units at
bottom, most recent
at top
• Larger units (eons)
are at left, with
smaller units to right
(epochs)
• Each unit is identified
by rock type & fossils
Geologic Time Scale
 Scale
is divided by Eons, Eras, Periods and Epochs
– Eon is the largest division
 Pre-Cambrian Eon
• Before abundant life
• 7/8 of all time
 Phanerozoic Eon
• Visible life
• Represents 1/8 of time
Phanerozoic Eon
• Life explodes
• Divided into 3 eras
– Paleozoic Era (ancient)
 Age of Fishes
– Mesozoic Era (middle)
 Age of Dinosaurs
– Cenozoic Era (recent)
 Age of Mammals
• Boundaries defined by massive extinction
Paleozoic
Age of Fishes
Major Events
• Supercontinent – Pangaea
• Appearance of 1st life forms
• Invertebrate (contain shells)
• Life restricted to seas
Extinction  Climate change
Mesozoic
Age of Dinosaurs
Major Events
• Break up of Pangaea
• Seas invaded west N. America
• Gymnosperms (cone-bearing)
• Dinosaurs dominant
• Some mammals
Extinction  Large meteorite
Cenozoic
Age of Mammals
Major Events
• Angiosperms (flowering
plants)
• Marsupials (kangaroos,
opossums)
• Placentals (humans)
Extinctions  humans
Large mammals
Saber tooth tiger, etc.