Geologic Time

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Geologic Time

• Time is an important component of geology this
  separates geology from most other sciences
• Mans most ancient history extends back just
  thousands of years in time events in geologic
  time have occurred over millions or even billions
  of years!

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Geologic Time

• There are two methods of geologic dating
  relative and absolute
• Relative dating involves sequencing of geologic
  events
• Absolute dating provides specific dates for rock
  units or geologic events

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How Old is the Earth?

• Prior to the 18th century the age of the Earth
  was considered to be about 6000 yrs, and was
  based upon biblical genealogies
• In the 18th century scientists began to search
  for ways to determine the age of the Earth that
  reflected scientific evidence
• Georges Louis de Buffon estimated age of earth
  at 75,000 yrs by calculating cooling rates of
  metal balls
• Others attempted to calculate the time needed for
  deposition of all the Earths sedimentary rocks
  (problems varying rates, erosion), 1 Ma to 2 Ba

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How Old is the Earth?

• 19th century John Joly calculated time for ocean
  to reach current levels of salinity 90 Ma
  (problems unknown amount of salt in seafloor
  deposits, didnt know about subduction that
  recycled salts)
• Lord Kelvin calculated cooling rates based upon
  evidence from European mines 20-400 Ma (didnt
  account for the unknown phenomena of radioactive
  decay)

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How Old is the Earth?

• In the early 20th century Pierre and Marie Curie
  discovered radioactive decay, which led to
  methods of measuring the decay of radioactive
  isotope ratios and provided a way to establish
  absolute dates
• Using radiometric absolute dating geologists have
  dated Earths oldest rocks, Moon rocks, and
  meteorites. These dates provide an estimated age
  of our planet of 4.6 billion years

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Geologic Principles and Relative Dating

• Geology developed as a science in late 18th
  century
• Scottish geologist James Hutton is considered the
  father of geology he developed the principle
  of uniformitarianism, which says that geologic
  processes have operated uniformly during Earth
  history
• Charles Lyell in 1830 published the Principles
  of Geology, which documented Huttons work and
  established the concept of geologic time

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Relative Dating

• Principles of relative dating allow events to be
  placed sequentially, which provides
  interpretation of geologic history and
  development of a relative time scale
• Six basic principles developed from the work of
  Hutton William Smith, an English canal builder
  and Nicolas Steno of Denmark
• 1) The principle of superposition in an
  undisturbed succession of sedimentary rocks, the
  oldest are on the bottom and the youngest are on
  top

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Superposition
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Relative Dating

• 2) The principle of original horizontality
  sediments deposited from water will be
  essentially horizontal
• 3) The principle of lateral continuity sediments
  extend laterally in all directions until
  terminating against the edge of a basin

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Original Horizontality/Lateral Continuity
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Relative Dating

• 4) The principle of cross-cutting relationships
  intrusions or faults are younger than the rocks
  that are intruded or displaced (igneous
  intrusions cause contact metamorphism)

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Relative Dating

• 5) The principle of inclusions inclusions, or
  fragments of rock, contained within a rock layer
  are older than the rock layer itself

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Relative Dating

• 6) The principle of fossil succession fossil
  assemblages succeed one another throughout time
  in a regular and predictable order based upon
  these points
• (a) life has varied through time
• (b) fossil assemblages are recognizably different
  from one another
• (c) the relative ages of fossil assemblages can
  be determined

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Fossil Succession
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What is the sequence of events?
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Unconformities gaps in the rock record

• Sedimentary strata may have distinct bedding
  planes that represent gaps in deposition these
  are gaps in the rock record, intervals where
  geological time is not represented and are called
  unconformities

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Unconformities
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Unconformities

• There are three types of unconformities
• 1) disconformities a surface of erosion or
  non-deposition between younger and older beds
  that are parallel to each other
• 2) angular unconformities an unconformity in
  which the beds are at an angle to each other (one
  or both may be tilted, but they are not parallel
  to each other)
• 3) nonconformity when eroded metamorphic or
  igneous rocks are covered by sedimentary rocks

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Nonconformity
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Correlation

• Interpretation of rock exposures includes
  correlation of rock units this can be done via
• Lithostratigraphy (marker beds)
• Biostratigraphy (index fossils)
• Magnetostratigraphy (paleomagnetism)
• Chronostratigraphy (absolute dating)

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Lithostratigraphy
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Index Fossils

• Characteristics that make effective index fossils
  include
• distinctive morphology
• rapid evolution
• widespread distribution
• abundance
• Often biostratigraphic correlation is aided by
  the use of fossil assemblage zones

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Biostratigraphy
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Magnetostratigraphy
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Absolute dating

• Absolute dating is based upon the fact that atoms
  of radioactive elements decay to form stable
  isotopes
• Important data about atoms
• Atoms are the smallest particles of elements
• Atoms are composed of a nucleus containing
  protons and neutrons and an outer shell that
  contains electrons
• The number of protons determines the type of
  element and is known as the atomic number

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Absolute dating

• Not all atoms of the same element have the same
  number of neutrons in their nuclei these
  variable forms are called isotopes

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Radioactive Decay

• Some isotopes are unstable, or radioactive, and
  decay to a more stable form this decay rate is
  constant and measurable geologists measure this
  rate to determine the absolute ages of rocks

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Radioactive Decay

• Decay of the original isotope, or parent
  elements, into its product, the daughter element,
  is measured in half-lives the time it takes for
  half of the original number of parent atoms to
  decay into the daughter product atoms
• Absolute dating is accomplished by measuring the
  ratio of daughter product atoms to parent atoms,
  and comparing this ratio to the known quantity of
  a non radioactive element measurements are done
  with a mass spectrometer

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Absolute Dating

• There are three types of radioactive decay, and a
  number of elements that undergo radioactive decay
  with varying half-lives
• Radioactive elements useful in absolute dating
  include
• Uranium 235 (Lead 207) Half-life of 713 Million
  Years
• Potassium 40 (Argon 40) Half-life of 1.3 Billion
  Years
• Uranium 238 (Lead 206) Half-life of 4.5 Billion
  Years
• Rubidium 87 (Strontium 87) Half-life of 47
  Billion Years

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Absolute dating

• Radioactive dating can only be accomplished on
  rocks that contain radioactive elements, this
  usually is limited to igneous rocks that have not
  been secondarily altered this includes volcanic
  ash

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The Geologic Time Scale

• The Geologic Time Scale is a graphical
  representation of the history of the Earth,
  divided into units related to geologic events as
  evidenced by the fossil record
• It was first developed as a relative time scale
  that was pieced together by numerous researchers
  at widespread localities over a long period of
  time, using the techniques of relative dating
• The development of absolute dating allowed the
  relative time scale to be tied to absolute dates,
  based upon dates from thousands of rock
  exposures, which provided todays Geologic Time
  Scale

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