Time and Geology

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Chapter 3

• Time and Geology

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Geochronology

• The science that deals with determining the ages
  of rocks is called geochronology.

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Methods of Dating Rocks

• 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).
• 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).

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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.

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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.

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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)

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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.

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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)

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Periods

• Eras are divided into periods.

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Paleozoic Era

• Permian Period
• Carboniferous Period (Mississippian and
  Pennsylvanian Periods in North America)
• Devonian Period
• Silurian Period
• Ordovician Period
• Cambrian Period (oldest)

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Mesozoic Era

• Cretaceous Period
• Jurassic Period
• Triassic Period (oldest)

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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.

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Epochs

• Periods can be subdivided into epochs
• Epochs can be subdivided into ages

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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)

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Chronostratigraphic units

• Chronostratigraphic units are the actual rocks
  formed or deposited during a specific time
  interval.
• (They are sometimes called time-rock units.)

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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)

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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.

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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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Two models of atoms
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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.

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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.

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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.

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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.

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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.

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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
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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.

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Radioactive Decay of Uranium
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Subatomic Particles and Radiation Released by
Radioactive Decay

• Alpha particles - large, easily stopped by
  paper charge 2 mass 4
• Beta particles - penetrate hundreds of times
  farther than alpha particles, but easily stopped
  compared with neutrons and gamma rays. charge
  -1 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.

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Radioactive Decay

• Naturally-occurring radioactive materials break
  down into other materials at known rates. This is
  known as radioactive decay.

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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.

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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.

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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.

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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.

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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.

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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
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Decay Curve for Uranium-238
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Decay Curve for Potassium-40
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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.

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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)

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Minerals That Can Be Dated

• Uranium may be found in
• Zircon
• Urananite
• Monazite
• Apatite
• Sphene

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Dating Sedimentary Rocks
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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.

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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.

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Dating Sedimentary Rocks

• The ages of sedimentary rocks and fossils are
  determined using both relative and absolute
  dating.

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Dating Fossils

• The ages of fossils in a sequence of sedimentary
  rocks can be determined using both relative and
  absolute dating.

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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.

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Dating sedimentary rocks and fossils contd

1. 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.
2. Using this method, the age of the rocks in other
   areas is determined indirectly, from the ages of
   the fossils they contain.

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• 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.

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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.

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Carbon-14 dating

• 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).

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Carbon-14 dating

• 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 carbon-12 ratio
  is the basis for dating.

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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.

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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.

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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.

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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

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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.

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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

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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-year-old granite that
  must have been exposed at the time the sand
  grains were deposited.

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Other Old Earth Rocks

• Southwestern Greenland (4.0 b.y. granites)
• Minnesota (4.0 b.y. metamorphic rocks)
• Northwest Territories, Canada (3.96 b.y. Acasta
  gneiss)
• Beartooth Mountains, Montana (3.96 b.y. zircons
  in quartzite)
• China (3.8 b.y.)
• South Africa (3.7 b.y.)
• West Africa (3.6 b.y.)
• Still older rocks may remain to be found and dated

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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.
• Older rocks may have been deeply buried under
  sedimentary rocks, or beneath thrust sheets.
• Older rocks may have been heated, metamorphosed,
  or melted, and their dates "reset" to the time of
  heating, metamorphism, or melting.