Dating methods and
the age of the Earth
Geology 101
Two types of dating
• Relative dating asks “Is a given event
older or younger than another event?”
• Numerical (absolute) dating asks “How
many years ago did an event take
place?”
What is an “event”?
• A discrete occurrence that can be
inferred from the rock or fossil record
• Examples: the deposition of a
sedimentary layer, or the death of an
organism
Relative dating
• Steno’s principles
(1669) are all very
common sense but do
order strata from oldest
to youngest
• Stratum = layer
• Strata = layers
• Stratigraphy = study of
the order of events in
sedimentary rocks
Principle of original horizontality
• Any distortion of
sedimentary strata
is due to
deformation after
the strata were
deposited
Principle of lateral continuity
• Strata are deposited
in a basin and thus
will be continuous
from end to end
• Any breaks are due
to erosion or
deformation after
deposition
Principle of superposition
• Younger strata
overlie older strata
• True of all gravitydriven deposits
Principle of cross-cutting
relationships
• The body that cuts
through is younger
than the body that
got cut through
• True for dikes,
faults, joints and any
non-concordant
stratum
Principle of inclusions
• Sometimes called
“xenoliths”
• The inclusion is
older than the rock
in which it is
included
Correlation
• Similar strata separated by gaps, were once
part of a continuous layer
• Led to the idea of “formations”: a mappable
unit of rocks sharing a common origin event
Faunal succession
• William Smith and
George Cuvier (around
1800): There is an order
to the fossils found in
strata
• Smith and Cuvier did
not think of evolution
but the principle led
Charles Darwin to think
of how the order might
arise
Evolution of geologic timescale
Modern geologic timescale
Points about the timescale
• Divided up by major changes in the fossils
found in the strata (Phanerozoic): mass
extinctions mark the boundaries
• Major divisions: eon, era, period, epoch
• Rocks don’t care about life: that is, one
period ≠ one formation
• Thus, lithostratigraphy is not biostratigraphy
Unconformities
• An unconformity is
a time gap in the
rock record
• Classified into
different types, but
all represent
significant missing
time
“The Great Unconformity”
Numerical dating
• Two types: radiometric and nonradiometric
• Radiometric dating always involves the
decay of a radioactive isotope
• Non-radiometric dating always involves
a change in the number or condition of
a material
Basic atomic structure
• Atoms are the basic unit
of matter with chemical
properties
• Made of sub-atomic
particles: electrons,
protons and neutrons
• All elements (atoms of
one type) have isotopes
that differ in mass
• Some isotopes are
unstable and will decay
(radio-isotopes)
Radiometric dating
• Typically need to know:
how much of the
radioactive isotope
(radio-isotope) was
present when the “geoclock” started (i.e., the
event occurred), how
much of the radioisotope is present
currently and/or the
half-life of the radioisotope
Radioactive decay series
• Not all decays are
as complicated as
this!
• Start with the
principal radioisotope and end with
the stable isotope
The “geo-clock”
The shape of the curve is called “exponential decay”,
because half of the radioisotope (“parent isotope”) is converted to the daughter isotope each successive half-life.
Many different systems
Non-radiometric dating
• Examples: tree ring
dating
(dendrochronology),
lichenometry, fission
track dating, electron
spin resonance (ESR)
dating
In the photo above, the path of an alpha particle ejected
from a radioisotope nucleus is called a “fission track”. By
enhancing these tiny tracks, counting them allows the
researcher to figure out how old numerically the sample is.
Two key questions about
numerical dating
• 1. Is the dating method appropriate for
the estimated age and composition of
the dated material?
• 2. What is the event being dated? In
other words, when does the geo-clock
start?
Test case
• You discover early
homonid (i.e., preHomo sapiens)
skeletons in a
sandstone deposit
• How do you obtain
the numerical age?
What method(s)
would you use?
So how old is the Earth?
• And how do we
know?
Count the days of recorded history
• James Ussher,
Archbishop of Armagh,
uses biblical history to
determine the first day
of Genesis.
• It is a complex
calculation.
• Calculates Sunday,
October 23, 4004 BCE
as the start date.
Estimate the Earth’s cooling
• William Thomsen, Lord
Kelvin (University of
Glasgow, Scotland), 1897.
• Reasoned that the Earth
was initially molten and had
cooled off over time,
enough to make a crust.
• Measured the rates of
cooling of iron spheres.
• Age: 20 to 40 million years.
Measure the ocean’s salinity
• John Joly (Trinity University,
Ireland), 1899.
• Reasoned that the oceans had
started off without any sodium
content, and thus, by measuring
the amount of sodium brought to
the ocean annually by all the
rivers in the world and measuring
the ocean’s salinity, one could
calculate the age of the ocean,
which is close to the age of the
Earth.
• Age: 80 to 100 million years
Biologists (and geologists) had a problem
with these ages
• As Charles Darwin,
originator of the theory
of evolution by means
of natural selection
pointed out, even 100
million years was not
long enough to give the
mechanism of evolution
time to generate the
diversity of Earth’s life.
Radioactivity provides the key
• As mentioned earlier, the
phenomenon of radioactivity
generates heat – something that
Kelvin could not have predicted.
• Henri Becquerel (Museum of
Natural History, Paris) found
radioisotopes emitting invisible
rays (1896).
• Ernest Rutherford (McGill
University, Montreal) suggested
uranium isotopes might be useful
in dating earth materials (1905).
Refining the technique
• Inspired by a talk by
Rutherford, Bertram
Boltwood (Yale, 1907)
reasons that by
measuring the amount of
lead, the stable end
product of uranium decay,
one could determine the
age of a rock sample.
• Ages ranged from 250 to
1300 million years.
...to the age of the Earth
• Arthur Holmes (Imperial
College, London, 1913) realizes
that the decay of thorium also
contributes to the final lead
amount and refines Boltwood’s
technique.
• By 1927, he publishes the age
of the Earth as about 1.6 to 3.0
billion years.
• In 1931, he co-authors a report
the establishes radiometric
dating as the only reliable
method to determine the Earth’s
age.
The current age
• Earth materials were not solid for a while after the Earth formed – big
problem, as radioisotopes are not “fixed” in a crystal until after
crystallization.
• Oldest rock: Acasta gneiss from the Slave craton, North America:
4.031 ± 0.003 Gyr
• Oldest mineral: Zircon from Jack Hills, Western Australia: 4.401 ±
0.008 Gyr
• Other solar system bodies cooled much more quickly after formation:
Canyon Diablo meteorite (shown in photo): 4.53 to 4.58 Gyr
Also useful: Lunar rocks, meteorites from Mars
Age of solar system can be independently
determined by helioseismology, which
uses the distribution of helium in the Sun’s
core to figure out how long the Sun has
been active.
Dating issues
• Often, non-geoscientists don’t ask
themselves the two basic questions
(see previous slide)
• In general, sedimentary rocks cannot be
radiometrically dated; fossils cannot be
radiometrically dated
• Reworking causes older material to be
incorporated into younger layers