Chapter 3
Time and Geology
Geochronology
The science that deals with determining
the ages of rocks is called geochronology.
Methods of Dating Rocks
1. Relative dating - Using fundamental principles
of geology (Steno's Laws, Fossil Succession,
etc.) to determine the relative ages of rocks
(which rocks are older and which are younger).
2. Absolute dating - Quantifying the date of the
rock in years. This is done primarily by
radiometric dating (or analysis of the
breakdown of radioactive elements in the rocks
over time).
Geologic Time Scale
• The geologic time scale has been
determined bit-by-bit over the years
through relative dating, correlation,
examination of fossils, and radiometric
dating.
• Boundaries on the time scale are drawn
where important changes occur in the
fossil record, such as extinction events.
Geochronologic Units
The geologic time scale is divided into a number
of types of units of differing size. From the
largest units to the smaller units, they are:
• Eons
• Eras
• Periods
• Epochs
These units are geochronologic units.
Geochronologic units are time units.
Eons
Eons are the largest division of geologic
time. In order from oldest to youngest, the
three eons are:
• Archean Eon - "ancient or archaic" - oldest
rocks on Earth
• Proterozoic Eon - "beginning life" (2.5 billion
to 542 million years ago)
• Phanerozoic Eon - "visible life" (542 million
years ago to present)
The Precambrian
The Archean and Proterozoic are together
referred to as the Precambrian, meaning
“before the Cambrian Period”.
The Precambrian covers 87% of geologic
history.
Eras
There are three eras in the Phanerozoic
Eon. Eras are divided into geologic
periods. In order from oldest to youngest,
the three eras are:
• Paleozoic Era - "ancient life" (such as
trilobites)
• Mesozoic Era - "middle life" (such as
dinosaurs)
• Cenozoic Era - "recent life" (such as
mammals)
Periods
Eras are divided into periods.
Paleozoic Era
• Permian Period
• Carboniferous Period (Mississippian and
Pennsylvanian Periods in North America)
• Devonian Period
• Silurian Period
• Ordovician Period
• Cambrian Period (oldest)
Mesozoic Era
• Cretaceous Period
• Jurassic Period
• Triassic Period (oldest)
Cenozoic Era
• Neogene Period (youngest – today)
• Paleogene Period (oldest)
On maps and in publications prior to 2003, you will
see the two periods of the Cenozoic Era listed as:
• Quaternary Period
• Tertiary Period (oldest)
These are not the same as the current two periods.
Epochs
Periods can be subdivided into epochs
Epochs can be subdivided into ages
Epochs of the Cenozoic Era
• Neogene Period
– Holocene Epoch (youngest - today)
– Pleistocene Epoch
– Pliocene Epoch
– Miocene Epoch
• Paleogene Period
– Oligocene Epoch
– Eocene Epoch
– Paleocene Epoch (oldest)
Chronostratigraphic units
Chronostratigraphic units are the actual
rocks formed or deposited during a
specific time interval.
(They are sometimes called time-rock units.)
Chronostratigraphic units
Chronostratigraphic units include:
•
•
•
•
•
Eonothem (all rocks corresponding to a given eon)
Erathem (all rocks corresponding to a given era)
System (all rocks corresponding to a given period)
Series (all rocks corresponding to an epoch)
Stage (all rocks corresponding to a particular age)
Periods and Systems
Geochronologic units (time units) have the
same names as the chronostratigraphic
units (time rock units) that they represent.
For example, the Cambrian System is a
rock unit, and the Cambrian Period is a
time unit.
The rocks of the Cambrian System were
deposited during the Cambrian Period.
Principles of Radiometric Dating
Review of Atoms
Atom = smallest particle of matter that can
exist as a chemical element.
The structure of the atom consists of:
• Nucleus composed of protons (positive
charge) and neutrons (neutral)
• Electrons (negative charge) orbit the
nucleus
• Various subatomic particles
Two models of atoms
Ions
Most atoms are neutral overall, with the
number of protons equaling the number of
electrons.
If there is an unequal number of protons
and electrons, the atom has a charge
(positive or negative), and it is called an
ion.
Atomic Number
Atomic number of an atom = number of
protons in the nucleus of that atom.
Example: The atomic number of uranium
is 92. It has 92 protons.
Mass number
Mass number is the sum of the number of
protons plus neutrons.
Example: Uranium-235 has 92 protons
and 143 neutrons.
The mass number may vary for an
element, because of a differing number of
neutrons.
Isotopes
• Elements with various numbers of neutrons
are called isotopes of that element.
Example: uranium-235 and uranium-238
• Some isotopes are unstable. They undergo
radioactive decay, releasing particles and
energy.
• Some elements have both radioactive and
non-radioactive isotopes.
Examples: carbon, potassium.
What happens when atoms decay?
• Radioactive decay occurs by releasing
subatomic particles and energy.
• The radioactive parent element is unstable
and undergoes radioactive decay to form a
stable daughter element.
• Example: Uranium, the parent element,
undergoes radioactive decay, releases
subatomic particles and energy, and
ultimately decays to form the stable
daughter element, lead.
Radioactive Parent Isotopes and
Their Stable Daughter Products
Radioactive Parent Isotope
Stable Daughter Isotope
Potassium-40
Argon-40
Rubidium-87
Strontium-87
Thorium-232
Lead-208
Uranium-235
Lead-207
Uranium-238
Lead-206
Carbon-14
Nitrogen-14
Radioactive Decay of Uranium
As uranium-238 decays to lead, there are
13 intermediate radioactive daughter
products formed (including radon,
polonium, and other isotopes of uranium),
along with and 8 alpha particles and 6
beta particles released.
Radioactive Decay of Uranium
Subatomic Particles and Radiation
Released by Radioactive Decay
•
Alpha particles - large, easily stopped by paper
charge = +2
•
Beta particles - penetrate hundreds of times
farther than alpha particles, but easily stopped
compared with neutrons and gamma rays.
charge = -1
•
•
mass = 4
mass = negligible
Neutrons - highly penetrating; no charge mass = 1
Gamma rays (high energy x-rays) - Highly
penetrating electromagnetic radiation; can
penetrate concrete. Lead shield can be used.
Photons (light). No charge or mass.
Radioactive Decay
Naturally-occurring radioactive materials
break down into other materials at known
rates. This is known as radioactive decay.
Radioactive Decay Rate
• Many radioactive elements can be used as
geologic clocks. Each radioactive element
decays at its own constant rate.
• Once this rate is measured, geologists can
estimate the length of time over which
decay has been occurring by measuring
the amount of radioactive parent element
and the amount of stable daughter
elements.
Mass Spectrometer
• The quantities and masses of atoms and
isotopes are measured using an
instrument called a mass spectrometer.
• The mass spectrometer came into use
after WWI (1918). This led to the discovery
of more than 200 isotopes.
Measuring Decay Rates
The decay rates of the various radioactive
isotopes are measured directly using a
mass spectrometer.
Basically, the mass of a quantity of a
radioactive element is measured. Then
after a particular period of time, it is
analyzed again. The change in the number
of atoms over time gives the decay rate.
Decay Rates are Uniform
• Radioactive decay occurs at a constant
exponential or geometric rate.
• The rate of decay is not affected by
changes in pressure, temperature, or other
chemicals.
• The rate of decay is proportional to the
number of parent atoms present.
Half-Life
• Each radioactive isotope has its own
unique half-life.
• A half-life is the time it takes for half of the
parent radioactive element to decay to a
daughter product.
Half Lives for Radioactive Elements
Radioactive Parent
Stable Daughter
Half life
Potassium-40
Argon-40
1.25 billion yrs
Rubidium-87
Strontium-87
48.8 billion yrs
Thorium-232
Lead-208
14 billion years
Uranium-235
Lead-207
Uranium-238
Lead-206
704 million
years
4.47 billion
years
Carbon-14
Nitrogen-14
5730 years
Decay Curve for Uranium-238
Decay Curve for Potassium-40
Rocks That Can Be Dated
Igneous rocks are best for age dating.
The dates they give tell when the magma
cooled.
When the magma cools and crystallized,
the newly formed crystals may contain
some radioactive elements, such as
potassium-40 or uranium that can be
dated.
Minerals That Can Be Dated
Potassium-40 decays and releases argon
gas, which is trapped in the crystal lattice.
Potassium-40 is found in these minerals:
– Potassium feldspar (orthoclase, microcline)
– Muscovite
– Amphibole
– Glauconite (greensand; found in some
sedimentary rocks)
Minerals That Can Be Dated
Uranium may be found in:
– Zircon
– Urananite
– Monazite
– Apatite
– Sphene
Dating Sedimentary Rocks
Dating Sedimentary Rocks
Radioactive mineral grains in sedimentary
rocks are derived from the weathering of
igneous rocks. Their dates are the time of
cooling of the magma that formed the
igneous rock.
The date does not tell anything about
when the sedimentary rock was deposited.
Dating Sedimentary Rocks
If the sedimentary rock has a mineral that
formed on the seafloor as the rock was
cemented, then it may be possible to age
date it.
The greensand mineral, glauconite,
contains potassium, and can be dated
using the potassium-argon technique.
Dating Sedimentary Rocks
The ages of
sedimentary rocks
and fossils are
determined using
both relative and
absolute dating.
Dating Fossils
The ages of fossils
in a sequence of
sedimentary rocks
can be determined
using both relative
and absolute
dating.
Dating sedimentary rocks and
fossils
1. Locate a sequence of sedimentary rocks that
contains some igneous rocks (such as a lava
flow, volcanic ash bed, intrusion, or underlying
igneous rock).
2. Get a radiometric date for the igneous rocks.
3. Use relative dating to determine the relative
ages of the sedimentary rocks. Bracket the
sedimentary rocks between two igneous rocks
of known age.
Dating sedimentary rocks and
fossils – cont’d
4. Correlate the sedimentary rocks with
sedimentary rocks in another area which
contain the same fossils. They are correlated
(or "matched up") on the basis of the fossils
they contain. They must contain the same
species of fossils.
5. Using this method, the age of the rocks in
other areas is determined indirectly, from the
ages of the fossils they contain.
The geologic time scale was established by
doing this repeatedly for many locations around
the world.
The geologic time scale is a composite vertical
sequence representing all known rock units and
their fossils, worldwide, in sequential order.
Absolute ages of rocks have been determined
through radiometric dating where possible.
The geologic time scale provides a calibrated
scale for determining the ages of rocks
worldwide by examining their fossils.
Carbon-14 dating
1. Cosmic rays from the sun strike nitrogen14 atoms in the atmosphere and cause
them to turn into radioactive carbon-14,
which combines with oxygen to form
radioactive carbon dioxide.
Carbon-14 dating
2. Living things are in equilibrium with the
atmosphere, and the radioactive carbon
dioxide is absorbed and used by plants.
The radioactive carbon dioxide gets into
the food chain and the carbon cycle.
All living things contain a constant ratio
of carbon-14 to carbon-12. (1 in a
trillion).
Carbon-14 dating
3. At death, carbon-14 exchange ceases
and any carbon-14 in the tissues of the
organism begins to decay to nitrogen-14,
and is not replenished by new C-14.
The change in the carbon-14 to carbon12 ratio is the basis for dating.
Carbon-14 dating
• The half-life is so short (5730 years) that
this method can only be used on materials
less than 70,000 years old.
• Assumes that the rate of carbon-14
production (and hence the amount of
cosmic rays striking the Earth) has been
constant over the past 70,000 years.
Fission Track Dating
• Charged particles from radioactive decay
pass through mineral's crystal lattice and
leave trails of damage called fission
tracks.
• These trails are due to the spontaneous
fission (or radioactive decay and
breakdown) of uranium.
Fission Track Dating
Procedure to study:
– Enlarge tracks by etching in acid (to view with
light microscope)
– Or view with electron microscope
– Count the etched tracks (or measure track
density in an area)
The number of tracks per unit area is a
function of age and uranium concentration.
Fission Track Dating
Useful in dating:
• Micas (up to 50,000 tracks per cm2)
• Tektites (glassy rocks produced when
meteorite impact melts bedrock forming
molten droplets which cool quickly as they
are thrown into the atmosphere.)
• Natural and synthetic (manmade) glass
The Oldest Rocks
The oldest rocks that have been dated are
meteorites. They date from the time of the
origin of the solar system and the Earth,
about 4.6 billion years old.
The Oldest Rocks
Moon rocks have similar dates, ranging in
age from 3.3 to about 4.6 billion years.
The oldest Moon rocks are from the lunar
highlands (lighter-colored areas on the
Moon), and may represent the original
lunar crust
The Oldest Rocks
The oldest dates of Earth rocks are 4.2
billion-year-old detrital zircon grains in a
sandstone in western Australia.
These grains probably came from the
weathering and erosion of 4.2 billion-yearold granite that must have been exposed
at the time the sand grains were
deposited.
Other Old Earth Rocks:
1. Southwestern Greenland (4.0 b.y. granites)
2. Minnesota (4.0 b.y. metamorphic rocks)
3. Northwest Territories, Canada (3.96 b.y. Acasta
gneiss)
4. Beartooth Mountains, Montana (3.96 b.y.
zircons in quartzite)
5. China (3.8 b.y.)
6. South Africa (3.7 b.y.)
7. West Africa (3.6 b.y.)
Still older rocks may remain to be found and dated
Why are Earth Rocks Younger than
Meteorites and Moon Rocks?
1. The Earth is geologically active. The older
rocks may have been eroded away.
2. Older rocks may have been deeply buried
under sedimentary rocks, or beneath thrust
sheets.
3. Older rocks may have been heated,
metamorphosed, or melted, and their dates
"reset" to the time of heating, metamorphism,
or melting.