Dating the Earth’s Rocks
Quantitative Geologic Time
Early estimates
Calculation based on:
Old Testament – Earth is
6,015 years old
James Ussher (1581-1656)
1658 Earth formed on
Sunday, 23 October, 4004 BC
Early age estimates based on:
Thermodynamics I –
Cooling of the earth from
molten material based
on melting point of Pb
Georges Louis de Buffon (1707-1788)
1770s Earth cooled from molten ball
of magma - 75,000 years old
(Major contradiction to Ussher)
Early age estimates based on:
Evolutionary rates –
Organisms change
over time
Charles Lyell
Cenozoic Era began
80 million years ago
Charles Lyell
Early age estimates based on:
Sediment deposition rates –
1850s computation that Earth’s age ranges
from 1 million to 1 billion years
Early age estimates based on:
Salinity of sea water –
Knowing salinity of oceans
and how much salt is added
by rivers each year
John Joly (1857-1933)
1899 Earth’s seas
developed 90 million years ago
Early age estimates based on:
Thermodynamics II –
Melting temperatures and
cooling rates of rocks
William Thomson Kelvin (18241907)
1890s Earth formed 24-40
million years ago
Lord Kelvin
Finally, Radioactivity!
Radioactive Decay Provides a more
accurate method of
dating rocks
Henri Becquerel (1896)
Detected the phenomenon of
radioactive decay
Modern radioactive-isotope studies
Reviewing atoms:
•
Basic unit of matter of elements.
•
Each atom has a nucleus containing protons and neutrons.
Orbiting the nucleus are electrons.
•
The atomic number of an atom is the number of protons in the
nucleus.
•
The mass number is the number of protons plus the number of
neutrons.
•
Isotopes are variants of the same atom, but with a different
number of neutrons (and different mass number). Eg: uranium’s
nucleus always has 92 protons, but can have different number of
neutrons. Three common isotopes are U-234, U-235, and U-238.
Components of an Atom
Methods and concepts:
Radioactivity is natural, spontaneous
breakdown of the nuclear structure of atoms
Parent nuclide = daughter product + particle
expelled
Rate of nuclear decay is constant
Radioactive isotopes have a unique rate & mode
of decay. The rate is typically called a half life
representing the time it takes for ½ the parent
material to decay.
Half-life = span of time needed for one half of
original atom to decay to daughter product
• Unstable radioactive isotopes (parent) decay and give
rise to stable (daughter) atomic products.
All radioactive isotopes decay at
a fixed rate.
When a radioactive isotope, e.g.
U-238, is incorporated into a mineral
that crystallizes from magma, there is
no lead. The radiometric clock starts
at this point. Daughter (Pb) is trapped
within the crystal lattice.
How can we use this information to
tell how old a rock is?
• Crystallization of minerals locks in an original
quantity of radioactive atoms
• Radiometric dating of a crystal possible
because daughter products are retained
• Mass spectrometer is used to measure minute
amounts of isotopes
Incorporation of Radioactive Atoms (Igneous Rock)
Minerals in igneous rocks give the most reliable
age dates.
Principal Geologic Timekeepers:
Radioactive Parents & Stable Daughters
Uranium-lead: decay of U235 or U238 to lead
Potassium-40/Argon-40 method: trapped Argon
gas derived from decay of potassium
87Rb/86Sr method: decay of rubidium to
strontium
Carbon-14 method: limitation is the half-life of
5,730 years
Nuclear fission-track counting in crystals
Half-lives differ among elements
235 U
704 million years
dinosaur paleontology
14 C
5,700 years
ice-age paleontology, archaeology
238 U
4.5 billion years
determining age of earth, asteroids, etc.
Radiometric Dating I:
• Currently there are five isotopes that are extensively used
in radiometric dating ancient rocks: rubidium-87, thorium232, uranium-235, uranium-238, and potassium-40.
• The mineral must remain a closed system. Chemical or
physical processes can affect the parent/daughter ratio.
•Only fresh, unweathered rock is used, and several widely
spaced samples are taken for analysis.
Metamorphism “resets”
the atomic clock.
Metamorphic rocks will
appear younger due to
the loss of daughter
isotopes.
Can be used to
determine time of
metamorphism
Radiometric dating II:
establishes absolute ages of certain rock and
sedimentary layers (within statistical limits)
• Oldest known Earth material is 4.36 billion years
(based on zircon crystals from western Australia)
• Other very old rocks (over 4 billion years) come from
Canada and Greenland
• Age of meteorites: U/Pb and Rb/Sr dating yields 4.6
billion years
• Moon rocks: U/Pb and Rb/Sr dates range from 3.3 to
4.6 billion years
Radiometric Dating of Rocks
Bracketing Ages
We then use the radiometric dates of the
volcanic ash layers (bentonites) to provide upper
and lower limits of a particular bed.
Putting it all together
(Using radiometric
dating to find the
ages of index
fossils)
Fission Track Dating
• Fission-track dating measures
the number of microscopic
linear tracks left by the decay
of U-238. Useful for dating
samples from 1 million to 2
billion years old.
• The age of the sample is
determined from the number
of fission tracks present and
the amount of uranium the
sample contains: the older the
sample, the greater the
number of tracks.
• Naturally occurring glass and
minerals like garnet, zircon,
mica, epidote are datable.
The length of the track can provide
information on how fast the rock cooled
(long tracks = rapid cooling; short tracks =
slow cooling)
Fission Track Dating Can be used to determine:
• age of meteorites
• age of formation of obsidian
(volcanic glass)
• source of clastic sediments
• source of material for
archaeological artifacts
Ceremonial blades embedded in skull - Aztec
Chondritic meteorite
Carbon-14 Dating
• Used to date relatively recent events since half-life is 5,730
years (can be use to date up to 75,000 years).
• Can only be used to date material of biogenic origin (plants and
animals – things that were once alive).
• Carbon-14 is created in upper atmosphere when neutrons are
absorbed by nitrogen atoms (causing nitrogen to eject a proton).
• The isotope is incorporated into carbon dioxide and is absorbed
by living matter.
• As long as organism is alive carbon-14 is continually replaced –
ratio of carbon-14/carbon-12 is constant.
• When organism dies carbon-14 is converted to nitrogen-14 via
beta emission.
Radiometric dating vs. Historical records
Dendrochronology – “Tree-Ring” Analysis
Dendrochronology (dating using tree rings )
Correlation of Tree-Ring Sections