Geologic Time

1
Geologic Time

• Time is an important component of geology
  separates geology from all other sciences
• Mans ancient history comprises tens to hundreds
  of thousands of years events in geologic time
  may have occurred millions or even billions of
  years ago!
• 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

2
History of the concept of geologic time

• Prior to the 18th century Earth age considered to
  be about 6000 yrs, 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

3
History of the concept of geologic time

• 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)
• In the early 20th century Pierre and Marie Curie
  discovered radioactive decay, which later led to
  the methods of measuring radioactive isotope
  ratios to determine decay rates that provided
  geologists with methods of absolute dating

4
Geologic principles and relative dating

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

5
Geologic principles and 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 (allows relative dating of strata and fossils)

6
Geologic principles and relative dating

• 2) The principle of original horizontality
  sediments deposited from water will be
  essentially horizontal (thus any tilting must
  have occurred after lithification)
• 3) The principle of lateral continuity sediments
  extend laterally in all directions until
  terminating against the edge of a basin or
  thinning and pinching out (as in a facies change)
• 4) The principle of cross-cutting relationships
  intrusions or faults are younger than the rocks
  that are intruded or displaced (intrusions cause
  contact metamorphism

7
Geologic principles and relative dating

• 5) The principle of inclusions inclusions, or
  fragments of rock, contained within a rock layer
  are older than the rock layer itself
• 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, and (c) the relative ages of
  fossil assemblages can be determined

8
Unconformities gaps in the rock record

• Conformable beds represent sedimentary strata
  with no breaks in deposition
• 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
• Any such non-represented interval in geologic
  time is a hiatus

9
Unconformities gaps in the rock record

• 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 (may be difficult
  to recognize gaps in fossil record can be a
  clue)
• 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 eroded metamorphic or igneous
  rocks are covered by sedimentary rocks (if
  intrusion, sed rocks older, if non conformity,
  ig/met rocks older)

10
Geologic principles and relative dating

• By applying the principles of relative dating we
  can interpret the geologic history of any
  exposure, i.e., fig. 17.11
• Interpretation includes the correlation of rock
  units this can be done via lithostratigraphy
  (marker beds), biostratigraphy (index fossils),
  and chronostratigraphy (absolute dating)
• Characteristics that make effective index fossils
  include distinctive morphology, rapid evolution,
  widespread distribution and abundance
• Often biostratigraphic correlation is aided by
  the use of fossil assemblage zones

11
Absolute dating

• Absolute dating is based upon the fact that
  radioactive elements decay to form more stable
  isotopes
• Important data atoms are the smallest particles
  of elements atoms made up of a nucleus composed
  of 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 the total number of protons
  and neutrons in a nucleus is the atomic mass

12
Absolute dating

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

13
Absolute dating

• There are three types of radioactive decay, and a
  number of elements that undergo radioactive decay
  with varying half-lives (Table 17.1)
• Radioactive elements useful in absolute dating
  include Uranium 238 (Lead 206), Rubidium 87
  (Strontium 87), and Potassium 40 (Argon 40)
• 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

14
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 (closed system)
  volcanic ash can also be used
• Other methods of absolute dating include fission
  track dating (measurement of microscopic damage
  done by by emission of atomic particles during
  spontaneous decay of uranium), carbon-14 dating
  (useful for up to only 70,000 yrs), and
  dendrochronology

15
The Geologic Time Scale

• Developed by piecing together the work of
  numerous researchers at widespread localities
  over a long period of time applying the
  techniques of relative dating
• The development of absolute dating allowed the
  early relative time scale to be tied to absolute
  dates, based upon dates from thousands of rock
  exposures
• In this manner, geologists have been able to
  determine the absolute ages of various geologic
  periods and determine their duration

16
Stratigraphic terminology

• Rock units formation (sub units member, bed
  supra units group, supergroup)
• Rock time designations eonothem, erathem,
  system, series, stage
• Time units eon, era, period, epoch , age