Chapter 3
Time and Geology
Finding the age of rocks:
Relative versus Actual Dating
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. Actual (Absolute) dating - Quantifying the date of the
rock in years. This is done primarily by radiometric
dating (or detailed analysis of the breakdown of
radioactive elements within the rocks over time).
Geologic Time Scale
• The geologic time scale has been
determined over many years of research
through relative dating, correlation,
examination of fossils, and radiometric
dating.
• Boundaries on the time scale are placed
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.
The modern
geological
time scale
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)
Precambrian
The Archean and Proterozoic are together
referred to as the Precambrian, meaning
"before the Cambrian Period."
The Precambrian encompasses 87% of
geologic history.
Eras
There are three eras within the eon called Phanerozoic.
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
(split into Mississippian and Pennsylvanian
Periods in the United States)
• Devonian Period
• Silurian Period
• Ordovician Period
• Cambrian Period (oldest)
Mesozoic Era
• Cretaceous Period
• Jurassic Period
• Triassic Period (oldest)
Cenozoic Era
• Quaternary Period
• Neogene Period
• 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)
The former Tertiary Period is now split into two.
Epochs
Periods are subdivided into epochs.
and
Epochs are subdivided into ages.
Epochs of the Cenozoic Era
• Quaternary Period
– Holocene Epoch
– Pleistocene Epoch (oldest)
• Neogene Period
– Pliocene Epoch
– Miocene Epoch (oldest)
• Paleogene Period
– Oligocene Epoch
– Eocene Epoch
– Paleocene Epoch (oldest)
Chronostratigraphic units
Chronostratigraphic units represent the
actual rocks deposited or formed during a
specific time interval.
Chronostratigraphic units 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 a given epoch)
Stage (all rocks corresponding to a particular age)
Periods and Systems
Geochronologic units (time units) have the same
names as their chronostratigraphic units (time-rock
units).
For example, Cambrian Period is a time unit, and
Cambrian System is a time-rock unit.
The rocks of the Cambrian System were deposited
during the period called Cambrian.
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
Model of the atom
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. Uranium 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 nonradioactive 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 – atomic weight = 4; atomic number =
(The same as the nucleus of a helium atom. Has + charge of 2.)
•
Beta particles – an electron that is released when a neutron
splits into a proton and an electron.
(Like all electrons, the mass is negligible and there is a positive
charge.)
•
Gamma rays – electromagnetic waves much like x-rays, but
higher frequency
(Like all electromagnetic waves, including light, there is no charge
or mass associated this photon or "particle.")
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 decay rates of the various radioactive
isotopes are measured directly using a mass
spectrometer.
Decay Rates are Uniform
• Radioactive decay occurs at a constant or uniform
rate.
• The rate of decay is not affected by changes in
pressure, temperature, or other chemicals.
• As time passes, the number of parent atoms
decreases and the number of daughter atoms
increases at a known rate.
Half-Life
• Each radioactive isotope has its own unique
half-life.
• A half-life is the time it takes for one-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
704 million years
Uranium-238
Lead-206
4.47 billion years
Carbon-14
Nitrogen-14
5730 years
Rate of decay for Uranium-238
Rate of decay for Potassium-40
Rocks That Can Be Dated
Igneous rocks are best for age dating.
The dates from crystals in igneous rocks tell
us when the magma cooled.
When the magma cools and crystallizes, the
newly formed crystals usually contain some
radioactive elements, such as Potassium-40
or Uranium-238 that can be used for
radiometric dating.
Minerals That Can Be Dated
Potassium-40 decays and releases
Argon-40 gas, which is trapped in the crystal
lattice.
Potassium-40 is found in these minerals:
– Potassium feldspar (orthoclase, microcline)
– Muscovite
– Amphibole
– Glauconite (found in some sedimentary rocks)
Minerals That Can Be Dated
Uranium may be found in:
• Zircon
• Urananite
• Monazite
• Apatite
• Sphene
Dating Sedimentary Rocks
Radioactive mineral grains in sedimentary
rocks are derived from the weathering of
igneous rocks. The radiometric dates of
these grains give us the time of cooling of
the magma, which formed the original
igneous rock.
These dates do not tell us anything about
the age of the sedimentary rock.
Dating Sedimentary Rocks
If the sedimentary rock contains a mineral
that formed at the same time as the rock
formed, then it may be possible to use that
mineral to obtain a radiometric age date.
The sedimentary mineral glauconite contains
potassium, and can be used for radiometric
dating (employing the potassium-argon
technique).
Dating Sedimentary Rocks
The ages of
sedimentary rocks
and fossils are
determined using
both relative and
absolute dating.
Dating Sedimentary Rocks
For example, the
age of the shale
layers in both
instances is
between 110 and
180 million years.
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. Determine radiometric dates for the igneous rocks.
3. Use relative dating to determine the relative ages of
the sedimentary rocks. Bracket the age of the
sedimentary rocks using two igneous rocks with
known ages.
Dating sedimentary rocks and fossils
– cont'd
4. Correlate the sedimentary rocks with sedimentary
rocks in another area that 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 compiled by using this
method repeatedly at 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 including
their fossils.
Carbon-14 dating
1. Cosmic rays from the sun strike
Nitrogen-14 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, because radioactive carbon
dioxide is absorbed and used by plants.
The radioactive carbon dioxide gets into
the food chain and thus the carbon cycle.
All living things contain a constant ratio of
Carbon-14 to Carbon-12. (about 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 Carbon-14.
The change in the Carbon-14 to Carbon-12
ratio in fossil material is the basis for this
kind of radiometric 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.
Carbon-14
formed from
Nitrogen-14
and its fate in
the natural
environment.
Rubidium-Strontium Method
• When Rubidium-87 expels a beta particle, it
becomes Strontium-87.
• This method provides a useful check on the
potassium-argon method of dating.
Rubidium-Strontium Method
• Strontium-86 is not a radioactive isotope,
and it is employed in this method as well.
• Using a mass spectrometer, the ratio of
Rubidium-87 to Strontium-86 and the ratio of
Strontium-87 to Strontium-86 is determined
for several samples.
• This is plotted on a graph and the line thus
determined is called an isochron.
Rubidium-Strontium Method
• The slope of the line permits computation of
the age of the mineral crystals being studied.
In this instance,
the slope angle of
the isochron suggests
an age of 1.725 billion
years for a granite from
Sudbury, Ontario.
Fission Track Dating
• Charged particles from radioactive decay
pass through a mineral's crystal lattice and
leave trails of damage in the crystal called
fission tracks.
• These trails are due to the
spontaneous fission
(or radioactive decay) of
the uranium nucleus.
Fission Track Dating
Procedure:
– Enlarge tracks by etching in acid (to view with light
microscope) - or view them directly with electron
microscope
– Count the etched tracks (or measure the density of
such tracks in a given area of the crystal)
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)
• Other uranium-bearing minerals and natural
glasses
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 4.36
billion-year-old detrital zircon grains in a
sandstone in western Australia.
These grains probably came from the
weathering and erosion of 4.36 billion-yearold granite that must have been exposed at
the time the sand grains were deposited.
Other Old Earth Rocks
1. Southwestern Greenland (granite; 4.0 b.y.)
2. Minnesota (metamorphic rocks; 4.0 b.y.)
3. Northwest Territories, Canada (gneiss;
4.04 b.y.)
4. Hudson Bay, northern Quebec (zircons;
4.28 b.y.)
Still older rocks on Earth may remain to be found and
dated using radiometric methods.
Why are Earth Rocks Younger than
Meteorites and Moon Rocks?
The Earth is geologically active. The older rocks may have
been eroded away or destroyed by tectonic forces.
Older rocks may remain deeply buried under sedimentary
rocks, or under mountain ranges.
Older rocks may have been heated, metamorphosed, or
melted, and their isotopes "reset" to the time of the later
events of heating, metamorphism, or melting.