Transcript Lecture21_dating2

I. Principles of Relative Dating
Review – Relative Dating
Principle of Original
Horizontality
Principle of Superposition
Principle of Cross-Cutting
Relationships
Inclusions
Principle of Faunal
Successions
I. Principles of Numerical Dating
Numerical Dating
Determining the age of a
rock or the Earth’s surface
in years
Based on some type of
natural clock (e.g., annual
tree or coral growth rings)
I. Principles of Numerical Dating
I. Isotope Dating—based
on the rate of decay of
radioactive isotopes within
rocks
A. Structure of atoms—very
small, one hundredmillionth of a centimeter
Atoms: smallest particles of
an element that retain all
of the element’s chemical
properties
Nucleus:
Protons, positive charge
Neutrons, neutral charge
Electrons orbit: if nucleus is
a basketball, electrons up
to 3 km away
I. Principles of Numerical Dating
I. Isotope Dating: based on
the rate of decay of
radioactive isotopes within
rocks
1. Charge:
Protons +
Neutrons (neutral)
Electrons Atoms always have the same
# of protons and electrons
so the charge is balanced.
2. Mass:
Protons
1 AMU or atomic mass unit =
1.67x10-24 gms
Neutrons = 1 AMU
Electrons = mass negligible
I. Principles of Numerical Dating
I. Isotope Dating: based on the
rate of decay of radioactive
isotopes within rocks
3. Atomic Number = number of
protons
Atomic number determines an
atoms identity
H=1, Mg=12, O=8, Fe=26
4. Atomic Mass = protons +
neutrons
Can change in different atoms
of the same element
For example Oxygen (8 protons):
8 neutrons 16O
9 neutrons 17O
10 neutrons 18O
5. Isotopes: atoms of the same
element (same atomic #), with
different # of neutrons
I. Principles of Numerical Dating
I. Isotope Dating: based on
the rate of decay of
radioactive isotopes within
rocks
B. What happens during
radioactive decay
1.Radioactive isotopes:
have nuclei that
spontaneous decay by
emitting or capturing a
variety of subatomic
particles (protons,
electrons, etc.)
Unstable isotopes  stable
isotopes
I. Principles of Numerical Dating
C. How it works (radioactive
decay)
1. Decay rates of radioactive
atoms are constant
2. Unaffected by temperature,
pressure, water, etc.
3. Half Life: time it takes for half
the atoms of the parent
isotope to decay, ranges from
tens of billions of years to
thousandths of a second.
Percentage of parent atoms that
decay in each half life is the
same (50%)
Increase in daughter =
decrease in parent
The actual number of atoms
that decay with each passing
half-life continually decreases
I. Principles of Numerical Dating
C. How it works (radioactive
decay)
4. When mineral crystallizes
(e.g., zircon mineral)
a. some amount of
radioactive parent may
get incorporated into the
mineral structure
b. but daughter is different
element and doesn’t “fit”
in the structure (or inert
and won’t combine), so at
the time of formation there
is 100% parent and 0%
daughter
I. Principles of Numerical Dating
C. How it works
(radioactive decay)
4. When mineral crystallizes
(e.g., zircon mineral)
c. With a given time, the
amount of parent slowly
declines and amount of
daughter increases within
the mineral.
d. So we can go back and
measure the ratio of
parent to daughter
(knowing the half life) and
determine when the
mineral crystallized.
I. Principles of Numerical Dating
Radioactive isotopes are
incorporated in minerals and
rocks in a variety of ways.
As minerals crystallize from
magma, radioactive isotopes
are included in mineral crystal
structure.
At the time of crystallization,
only parent isotopes are
included in the mineral.
Radioactive parent isotopes
then begin to decay producing
daughter isotopes.
I. Principles of Numerical Dating
ISOTOPE DATING uses this process
to measure the amount of time
elapsed since the mineral’s
formation.
With time, the amount of parent
isotope will decrease and the amount
of daughter isotope will increase.
The DECAY RATE is constant and
acts like a “clock”.
Decay rates are not affected by
temperature, pressure, or chemical
reaction with the parent isotope.
By measuring the ratio of parent to
daughter isotopes in the mineral and
comparing it with the rate of
radioactive decay, we can determine
the numerical age of a rock.
I. Principles of Numerical Dating
D. Dating Rocks
1.Igneous rocks – the best!
Dates when the minerals
formed
2. Metamorphic: during
metamorphism ions can
migrate, so dating tells us
when metamorphism
ended.
3. Sedimentary rocks: more
errors because it dates the
age of the individual pieces,
gives maximum age
I. Principles of Numerical Dating
C. How it works (radioactive decay)
I. Principles of Numerical Dating
C. How it works (radioactive decay)
I. Principles of Numerical Dating
C. How it works (radioactive decay)
I. Principles of Numerical Dating
C. How it works (radioactive decay)
I. Principles of Numerical Dating
E. Sources of Error
Groundwater can bring in new
ions and carry old ions away

Sample unfractured and
unweathered rock, makes
the age of the rock too
young)
Very young rocks: not enough
time to accumulate daughter
isotopes to be measured
accurately
So we date many different
samples and make sure the
ages agree
We date using different dating
methods to make sure they
agree
II. Types of Isotope Dating
We pick dating methods based on approximate age of the rock and the rock
composition
Old rocks – long half lives
Young rocks – short half lives
Composition: have to use a dating method corresponding to what is in the rock
I. Principles of Numerical Dating
II. Types of Isotope Dating
(page 252, table 8.1)
1. Uranium-thorium-lead
(granite)
2. Rubidium-Strontium
plagioclase feldspar (igneous
and metamorphic rocks)
3. Potassium-Argon
lots of minerals (plagioclase,
biotite, muscovite,
amphibole)
Argon is inert gas and will not
combine into any minerals
I. Principles of Numerical Dating
II. Types of Isotope Dating
4. Carbon 14 (radiocarbon
dating)
 14N
5730 year ½ life
14C
Useful between 100 and
about 50,000 years old
Can date things that contain
organic carbon (Used to
be living): bones, shells,
wood, charcoal, plants,
paper, cloth, pollen,
seeds)
I. Principles of Numerical Dating
II. Types of Isotope
Dating
4. Carbon 14 (radiocarbon
dating)
Anything living takes in
carbon in the form of CO2
(stable and unstable) by
eating and drinking and
photosynthesis
As long as the organism is
living, it is taking in more
14C but when it dies, it’s
14C starts declining (12C
remains constant)
Longer time since it died 
less 14C
I. Principles of Numerical Dating
III. Other Dating
Techniques:
Besides isotopic methods
A. Dendrochronology
(Tree-ring dating)
Trees grow rings for each
year
We can count rings to get
ages of trees
Pronounced changes in
climate (i.e. drought) causes
distinct patterns that can
then be correlated between
trees
Useful for dating:
landslides, avalanches, or
mudflows or wooden
artifacts
I. Principles of Numerical Dating
III. Other Dating
Techniques:
Besides isotopic methods
A. Dendrochronology
(Tree-ring dating)
Trees grow rings for each
year
We can count rings to get
ages of trees
Pronounced changes in
climate (i.e. drought)
causes distinct patterns that
can then be correlated
between trees
Useful for dating:
landslides, avalanches, or
mudflows or wooden
artifacts
I. Principles of Numerical Dating
III. Other Dating Techniques:
Besides isotopic methods
B. Varve chronology (lake
deposits)
Lakes produce annual layers of
sediment much like tree rings
Spring & summer  high
sediment input  thick, coarse,
light-colored layers
Winter little to no sediment,
especially when covered in ice—
very fine stuff deposited which
has been floating around for
months  dark, thin layers
Useful for dating: landslides into
the lake
Origin of Lake
Varves
(Summer)
Origin of
Lake
Varves
(Winter)
I. Principles of Relative Dating
III. Other Dating Techniques:
Besides isotopic methods
C. Lichenometry (dating lichen
colonies)
Lichen—simple plant-like colonies
the grow on exposed rock
For similar rocks and similar
climate: the larger the lichen
colony, the longer the time since
the growth surface was exposed
Develop a growth curve based on
measuring lichen of known age
(tombstones, buildings) then
extrapolate/interpolate to age of
unknown rock
Useful for dating: glacial deposits,
rockfalls, mudflows (expose new
rock to surface)
Review Principles of Absolute Dating
 Radioactive isotopes are
incorporated in minerals.
 As minerals crystallize from
magma, radioactive isotopes
are included in mineral crystal
structure.
 At the time of crystallization,
only parent isotopes are
included in the mineral.
Isotope Dating: based on
the rate of decay of
radioactive isotopes within
rocks
 Radioactive parent isotopes
then begin to decay producing
daughter isotopes.
Review Principles of Absolute Dating
Carbon 14
 14N
Short ½ life
14C
Useful between 100 and
about 50,000 years old
Isotope Dating: based on the
rate of decay of radioactive
isotopes within rocks
Can date things that contain
organic carbon (Used to
be living): bones, shells,
wood, charcoal, plants,
paper, cloth, pollen,
seeds)
I. Principles of Numerical Dating
III. Other Dating Techniques:
Besides isotopic methods
A. Dendrochronology (Treering dating)
Useful for dating: landslides,
avalanches, or mudflows or
wooden artifacts
B. Varve chronology (lake
deposits): Useful for dating:
landslides into the lake
C. Lichenometry (dating
lichen colonies): Useful for
dating: glacial deposits,
rockfalls, mudflows (expose
new rock to surface)