Transcript ch03x - earthjay science

THE EARTH THROUGH TIME
TENTH EDITION
H A R O L D L. L E V I N
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1
CHAPTER 3
Time and
Geology
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FINDING THE AGE OF ROCKS:
RELATIVE VERSUS ACTUAL DATING
The science that deals with determining the ages of
rocks is called geochronology.
Dating rocks started some 400 years ago with
Nicholas Steno.
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METHODS OF DATING ROCKS
1.
Relative dating - Using fundamental principles of
geology (Steno's Laws, Fossil Succession, etc.) to
determine the which rocks are older and which are
younger. In other words, determine the sequences of
events without regard to a specific date.
2.
Actual (Absolute) dating - Quantifying the date of the
rock in years before the present. This is done
primarily by radiometric dating (or detailed analysis
of the breakdown of radioactive elements within the
rocks over time).
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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.
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GEOCHRONOLOGIC UNITS
The geologic time scale is divided into a number of
units of differing sizes. From the largest units to the
smaller units, they are:
•Eons > Eras > Periods > Epochs
These units are geochronologic units or time units.
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THE MODERN
GEOLOGICAL
TIME SCALE
Word modifiers:
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pro - before
phanero - evidence
paleo - ancient
meso – middle
neo – new
ceno – resent
eo – dawn
zoon - life
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FIGURE 3-1 Geologic Time Scale.
EONS
In order from oldest to youngest:
 Hadean Eon—This is an informal time unit
referring to the earliest Earth history where no rock
record has been preserved.
 Archean Eon—"ancient or archaic“—oldest rocks
on Earth (~4 billion to 2.5 billion years ago)
 Proterozoic Eon—"beginning life" (2.5 billion to
542 million years ago)
 Phanerozoic Eon—"visible life" (542 million years
ago to present)
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PRECAMBRIAN
The Archean and Proterozoic are together
referred to as the Precambrian, meaning
"before the Cambrian Period."
The Precambrian encompasses 87% of
geologic history.
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ERAS
Phanerozoic Eon is divided into three Eras
Youngest 

Oldest 
Cenozoic Era—"recent life" (such as mammals)
Mesozoic Era—"middle life" (such as dinosaurs)
Paleozoic Era—"ancient life" (such as trilobites)
PERIODS
Eras are divided into periods.
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PALEOZOIC ERA
Youngest
Permian Period
 Carboniferous Period— (split into Mississippian

& Pennsylvanian Periods in the United States)
Devonian Period
 Silurian Period
 Ordovician Period
 Cambrian Period

Oldest
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MESOZOIC ERA
Youngest
Oldest
Cretaceous Period
 Jurassic Period
 Triassic Period

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CENOZOIC ERA
Youngest
Oldest
Quaternary Period
 Neogene Period
 Paleogene Period

The former Tertiary Period is now split into two. 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)
Periods are subdivided into epochs.
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EPOCHS OF THE CENOZOIC ERA
Youngest

Quaternary Period


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Neogene Period
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Pliocene Epoch
Miocene Epoch
Paleogene Period


Oldest
Holocene Epoch
Pleistocene Epoch

Oligocene Epoch
Eocene Epoch
Paleocene Epoch
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CHRONOSTRATIGRAPHIC UNITS
Chronostratigraphic units represent the actual
rocks deposited or formed during a specific time
interval. Sometimes called "time-rock units."
Units include:
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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)
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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.
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ACTUAL GEOLOGIC TIME: CLOCKS IN THE
ROCKS
Early Attempts to determine the age of the Earth:
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Biblical – October 23, 4004 BCE
Evolution of Fossils – 80 million years
Sedimentation deposition rates – Ranged from a
million to over a billion years
Ocean Salinity – 90 million years
Cooling Rate – 24 to 40 million years
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PRINCIPLES OF RADIOMETRIC DATING
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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
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MODEL OF THE ATOM
FIGURE 3-5
IONS
Most atoms are electrically neutral, with the
number of protons equaling the number of
electrons in the nucleus.
If there is an unequal number of protons and
electrons, the atom has a charge (positive
or negative), and it is called an ion.
Negative (-) charged ion is an Anion
Positively (+) charged ion is a Cation
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ATOMIC NUMBER
Atomic number of an atom = number of
protons in the nucleus of that atom. Each
element such as Carbon ( 6 protons) or
Oxygen (8 protons) has its own unique
number of protons in the atom.
Example: The atomic number of uranium is 92. Uranium
has 92 protons.
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MASS NUMBER & ISOTOPES
Mass number is the sum of the number of protons
plus neutrons in the nucleus.
Example: Uranium-235 has 92 protons and 143 neutrons.
The mass number may vary for an element, because
of a differing number of neutrons. These are called
isotopes.
Example: Uranium-235 and Uranium-238
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ISOTOPES

Some isotopes are unstable. They undergo
radioactive decay, releasing particles and energy.

Some elements have both radioactive and nonradioactive isotopes.
Examples: carbon, potassium
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WHAT HAPPENS WHEN ATOMS DECAY?
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Radioactive decay occurs by releasing subatomic
particles and energy.
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The radioactive parent element is unstable and
undergoes radioactive decay to form a stable
daughter element.
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Example: Uranium, the parent element, undergoes
radioactive decay, releases subatomic particles and energy,
and through a series of steps decays to form the stable
daughter element, lead.
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RADIOACTIVE PARENT ISOTOPES &
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
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RADIOACTIVE DECAY OF URANIUM
FIGURE 3-6 Radioactive decay series.
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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
mass associated this photon or "particle.")
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or
RADIOACTIVE DECAY
Naturally-occurring radioactive materials break
down into other materials at known rates. This is
known as radioactive decay.

Many radioactive elements can be used as
geologic clocks. Each radioactive element decays
at its own constant rate.
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DECAY RATES ARE UNIFORM
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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.
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MASS SPECTROMETER & DECAY RATES
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.

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HALF-LIFE

A half-life is the time it takes for one-half of the
parent radioactive element to decay to a daughter
product.
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
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RATE OF DECAY FOR URANIUM-238
FIGURE 3-10 Rate of radioactive decay of uranium-238 to lead-206.
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RATE OF DECAY FOR POTASSIUM-40
FIGURE 3-12 Decay curve for potassium-40.
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ROCKS THAT CAN BE DATED
Igneous rocks are best for age dating.
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.
As mineral forms it preferentially incorporates the parent
and not the daughter. When the parent decays the
daughter is trapped in the crystalline structure
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MINERALS THAT CAN BE DATED
Potassium-40 is found in these minerals:
 Potassium
feldspar
 Muscovite
 Amphibole
Uranium may be found in:
 Zircon
 Urananite
 Monazite
 Apatite
 Sphene
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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).
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DATING SEDIMENTARY ROCKS
The ages of sedimentary rocks
and fossils are determined
using both relative and
absolute dating.
Illustration “A” shows a shale layer
sandwiched between two lava
flows of know ages. The ages of
the lava flows bracket the age of
the shale.
Illustration “B” shows a shale layer
underlain by a lava flow and
cross-cut by an igneous dike of
FIGURE 3-8 Igneous rocks that have provided absolute radiogenic ages can
known ages These ages bracket
often be used to date sedimentary layers.
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the age of the shale.
DATING FOSSILS
The ages of fossils
in a sequence of
sedimentary rocks
can be determined
using both relative
and absolute
dating.
FIGURE 3-9 The actual age of rocks that cannot be dated isotopically can
sometimes be ascertained by correlation.
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GEOLOGIC TIME SCALE
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.
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CARBON-14 DATING
1.
Cosmic rays from the sun
(mostly protons) strike atoms
in the upper atmosphere
resulting in the production of
high energy neutrons. When
one of these neutrons strikes
a Nitrogen-14 atom in the
atmosphere it causes a
proton to be ejected and
radioactive Carbon-14 forms.
Carbon-14 then combines
with oxygen to form
radioactive carbon dioxide.
FIGURE 3-14 Carbon-14 is formed from nitrogen in the atmosphere.
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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).
FIGURE 3-14 Carbon-14 is formed from nitrogen in the atmosphere.
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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.
FIGURE 3-14 Carbon-14 is formed from nitrogen in the atmosphere.
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CARBON-14 DATING

The half-life is so short (5,730 years) that
this method can only be used on materials
less than 50,000 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 50,000 years.
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RUBIDIUM-STRONTIUM METHOD
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Rubidium-87 expels a beta particle, it becomes Strontium-87.
Strontium-86 isotope is present in the rock, but is not
radioactive.
Using a mass spectrometer, the
ratio of 87Rb to 86Sr and the 87Sr to
86Sr ratios are determined for
several samples.
This is plotted on a graph and the
line thus determined is called an
isochron.
The slope of the line permits
computation of the age of the
mineral crystals being studied.
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FIGURE 3-13 Whole-rock rubidium-strontium isochron for a set
of samples of a Precambrian granite body exposed
near Sudbury, Ontario.
FISSION TRACK DATING
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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.
Useful in dating:
Micas (up to 50,000 tracks per cm2)
Other uranium-bearing minerals
and natural glasses
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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.

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THE OLDEST ROCKS—METEORITES
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.
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THE OLDEST ROCKS—MOON
Moon rocks have dates that range 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
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THE OLDEST ROCKS—EARTH
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.
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OTHER OLD EARTH ROCKS
1.
2.
3.
4.
Southwestern Greenland (granite; 4.0 b.y.)
Minnesota (metamorphic rocks; 4.0 b.y.)
Northwest Territories, Canada (gneiss; 4.04 b.y.)
Hudson Bay, northern Quebec (zircons;
4.28 b.y.)
Still older rocks on Earth may remain to be found and
dated using radiometric methods.
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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.
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IMAGE CREDITS
• FIGURE 3-1 Geologic Time Scale. Source: Harold Levin.
• FIGURE 3-5 Model of an Atom. Source: Harold Levin.
• FIGURE 3-6 Radioactive decay series. Source: Harold Levin.
• FIGURE 3-10 Rate of radioactive decay of uranium-238 to lead-206.
Source: Reprinted from Earth and Planetary Science Letters, by Steiger, R.
and Jäger, E. Subcommission on geochronology: Convention on the use of
decay constants in geo- and cosmochronology, 359-362, 1977, with
permission from Elsevier.
• FIGURE 3-12 Decay curve for potassium-40. Source: Harold Levin.
• FIGURE 3-8 Igneous rocks that have provided absolute radiogenic ages
can often be used to date sedimentary layers. Source: Harold Levin.
• FIGURE 3-9 The actual age of rocks that cannot be dated isotopically can
sometimes be ascertained by correlation. Source: Harold Levin.
• FIGURE 3-14 Carbon-14 is formed from nitrogen in the atmosphere.
Source: Harold Levin.
FIGURE 3-13 Whole-rock rubidium-strontium isochron for a set of samples
of a Precambrian granite body exposed near Sudbury, Ontario. Source:
Harold Levin.
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