BAESI: Earth and Life Through Time

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BAESI Earth and Life Through Time

• Geologic Time Part 1, Relative Time
• Jonathan Hendricks
• SJSU Department of Geology
• jhendricks_at_sjsugeology.org

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Geologic Time

• Lecture Overview
• Perspectives on deep time.
• Relative age dating.
• The geologic time scale.
• Absolute age dating.

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Deep Time

• The earth is 4.55 billion years old.
• About 1 billion minutes ago was the year 106 AD.
• About 1 billion seconds ago was the year 1976 AD.

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Abbreviations

• Ga giga annum 1 billion years
• Ma mega annum 1 million years
• Ka kilo annum 1 thousand years

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Relative Age Dating

• Relative age dating consideration of the
  temporal ordering of geological events relative
  to one another.
• Much of the study of geology is concerned with
  this type of dating.
• Relative to Event A, did Event B take place
  beforehand or afterwards?
• You are younger than your mother.
• Trilobites evolved before dinosaurs.
• Better Trilobites evolved during the Cambrian
  period, dinosaurs during the Triassic period.

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Nicholas Steno (1638-1686)

• Danish anatomist.
• Observed that flooded streams deposit sediments
  evenly across flood-plains, burying previous
  layers.
• Developed 4 key principles of geology that are
  still fundamental today.

• Slide shows a painting of Nicholas Steno.

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5 Key Principles for Relative Age Dating

• Stenos Four Principles
• Superposition.
• Original horizontality.
• Original lateral continuity.
• Inclusions
• Huttons Principle of Cross-Cutting
  Relationships.

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Principle of Superposition

• in an undisturbed sequence of strata, the oldest
  strata lie at the bottom and successively higher
  strata are progressively younger. (Stanley,
  2005, p. 9)

Youngest

• Slide shows a picture of rocks exposed at a
  waterfall rocks at the bottom of the waterfall
  are older than those above.

Younger
Oldest
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Superposition in Hawaii

• Slide shows a picture of lava rock that has
  covered a highway in Hawaii.

Photograph by J. Hendricks
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Principle of Original Horizontality

• almost all strata are initially more nearly
  horizontal than vertical. (Stanley, 2005, p.
  9-10)

• Slide shows a photograph of horizontal strata at
  the Grand Canyon, Arizona.

Photograph by J. Hendricks
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Principle of Original Lateral Continuity

• sediment extends laterally in all directions
  until it thins and pinches out or terminates
  against the edge of the depositional basin.
  (Wicander and Monroe, 2006, p. 64)
• Thus, sedimentary rocks separated by a feature
  such as a valley can be assumed to have once been
  continuous.

• Slide shows a photograph of the Grand Canyon
  (Arizona) that illustrates the principle of
  lateral continuity.

Photograph by J. Hendricks
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Principle of Inclusions

• inclusions e.g., fossils, or fragments in a
  rock, are older than the rock itself (Wicander
  and Monroe, 2006, p. 79)

• Slide shows two pictures of fossil shells in a
  sandy layer of rocks.

Photographs by J. Hendricks
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Principle of Cross-Cutting Relationships

• an igneous intrusion or fault must be younger
  than the rock it intrudes or displaces (Wicander
  and Monroe, 2006, p. 64)

• Slide shows a photograph of igneous dikes that
  have intruded a larger body of rock (at Black
  Canyon of the Gunnison, Colorado).

Photograph by J. Hendricks
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Correlating Rock Units

• Temporal Correlation establishment of the degree
  of temporal equivalency between rock units or the
  events that they represent.

• Slide shows a cartoon of a simple series of rocks.

Figure by J. Hendricks
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Correlating Rock Units

• Slide shows a geologic map of the state of Kansas.

Map by Kansas Geological Survey
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Temporal Correlation

• Temporal correlation is most often done using
  guide fossils that are associated with particular
  intervals of geological time.

• Slide shows how guide fossils can be useful for
  correlating rock units across large distances.

Slide shows a cartoon of a simple series of rocks
and how guide fossils can be used to establish
temporal equivalency between them.
Figure by J. Hendricks
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Biostratigraphy

• Biostratigraphy science of using guide fossils
  to date rocks (relative age dating!).
• The best guide fossils have the following
  characteristics
• Abundant (easy to find).
• Easy to identify.
• Belong to species with short stratigraphic
  ranges.
• Are widespread.
• Biostratigraphy works because evolution and
  extinction have occurred.

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The Geological Time Scale
Today (Recent)

• Slide shows the geological time scale.

Younger
Hierarchy of Terms Eon Era
Period Epoch
65 Ma
251 Ma
542 Ma
Older
2.5 Ga
4.6 Ga
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What Determines the Boundaries?

• Answer mostly, but not exclusively,
  biostratigraphic data.
• For example, the division of the Permian and
  Triassic (Paleozoic-Mesozoic boundary) is based
  upon a mass extinction event.

• Slide shows the geological time scale and how the
  divisions correspond to mass extinction events on
  a plot of marine animal diversity over about the
  last 500 millions.

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The Geological Time Scale

• The geological time scale is one of the crowning
  achievements of science in general and geology in
  particular.
• Is a reference and communication system for
  comparing rocks and fossils throughout the world.
• Is the result of hundreds of years of
  investigation (origins date to the 1600s).
• Determination of when and where particular
  boundaries should be identified has led to many
  heated disputes.
• Continues to evolve as new information is
  gathered (it is a work in progress).
• This is particularly true of the age absolute
  ages assigned to boundaries between different
  time periods.

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BAESI Earth and Life Through Time

• Geologic Time Part 2, Absolute Time
• Jonathan Hendricks
• SJSU Department of Geology
• jhendricks_at_sjsugeology.org

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Absolute Age Dating

• Absolute age dating deals with assigning actual
  dates (in years before the present) to geological
  events.
• The science of absolute age dating is known as
  geochronology.
• Hypotheses of absolute ages are determined based
  upon known rates of natural radioactive decay of
  some isotopes of elements that occur in rocks.

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Early Estimates of Earths Age

• Before radioactivity was discovered, scholars
  used other methods to estimate the age of the
  Earth.
• They did this using a variety of methods
• Biblical genealogies
• Accumulation of sediment
• Earths temperature

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Accumulation of Sediment

• Early 1800s.
• Geologists studied rates of sediment accumulation
  in modern environments.
• Then, measured thickness of rock units and
  calculated how long it would have taken for those
  rocks to accumulate (uniformitarianism!).
• Typically calculated age of 100 million years.
• Problems gaps in record, erosion, not
  recognizing that some metamorphic rocks had once
  been sedimentary.

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Earths Temperature

• Lord Kelvin (1865) British physicist.
• Challenged uniformitarian view of an ancient
  Earth.
• Idea Earth very hot when formed, cooling ever
  since.
• Kelvin knew interior of Earth was still very hot.
• In order to maintain that heat, Kelvin calculated
  that Earth must only be 20-40 million years old.
• Problem Discovery of radioactivity!

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Breakthrough for Absolute Age Dating

• Discovery of Radioactivity

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Elements Isotopes

• Atomic number of an element the number of
  protons in the nucleus of a given atom
  (constant).
• Number of neutrons in nucleus may vary.
• Isotopes forms of an element with different
  numbers of neutrons.
• Isotopes are identified by their mass number.
• Mass number protons neutrons in atomic
  nucleus.
• Examples
• Carbon 12 6 protons, 6 neutrons
• Carbon 13 6 protons, 7 neutrons
• Carbon 14 6 protons, 8 neutrons

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Radioactive Decay

• Most isotopes are stable.
• Some are unstable and undergo radioactive decay.
• The atomic nucleus that undergoes radioactive
  decay parent.
• The resulting product is known as the daughter.
• Radioactive decay occurs at a constant rate.
• That rate varies between elements.

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Radioactive Decay Cont.

• During a given unit of time, the fraction of
  parent atoms that decay remains constant.
• The rate of radioactive decay for a given isotope
  (determined in the laboratory) is typically
  represented by its half-life.
• Half-life the amount of time needed for the
  number of parent atoms to be reduced by one-half.

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Radioactive Decay Cont.

• The number of daughter atoms grows in proportion
  to the decay of parent atoms.

• Slide shows a figure that illustrates how the
  growth of daughter atoms increases in proportion
  to the decay of parent atoms.

Parent or Daughter Atoms
Time Units
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Radiometric Dating

• Absolute ages can be calculated from a mineral
  sample by knowing
• The ratio of parent atoms currently in a sample
  relative to the number originally present.
• The half-life decay constant.
• Absolute ages pertain to time when system became
  closed.
• Prevention of loss of daughter isotopes to
  surrounding environment.
• Example time a mineral grain formed.

Magma (Hot!)
Magma (Cooling)
Igneous Rock
o
o
o


o


o
o

• Slide shows a figure that illustrates how
  radioactive parent atoms are incorporated into
  mineral structures, then leave behind daughter
  products as the parent atoms radioactively decay.

o



o

o
o


o

o

Parent Atom (Radioactive)
Daughter Atom (Stable)
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Equation for Radioactive Decay

• Np / No (1-?)y
• Np Number of parent atoms now
• No Number of original parent atoms when system
  became closed (Np Nd now)
• ? decay constant fraction of parent atoms that
  decay per unit time (for half-life, use 0.5)
• Solve for y, which here is the number of
  half-lives ago
• Multiply y times the half-life of the parent
  element

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Examples of Isotopes Useful for Radiometric Dating

• Uranium-Lead (238U ? 206Pb)
• Half-Life of Parent 4.5 billion years
• Effective dating range 10 million to 4.6 billion
  years.
• Samples zircon, uraninite
• Potassium-Argon/Calcium (40K ? 40Ar, 40Ca)
• Half-Life of Parent 1.3 billion years
• Effective dating range 50,000 to 4.6 billion
  years.
• Samples Muscovite, biotite, hornblende

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Examples of Isotopes Useful for Radiometric Dating

• Carbon-Nitrogen (14C ? 14N) radiocarbon dating
• Half-Life of Parent 5,73030 years
• Measure amount of 14C (daughter 14N leaks away)
• Effective dating range 100 to 70,000 years.
• Samples Plant matter, bone, tissue, shell, H2O,
  etc.

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Absolute Age Dating and Biostratigraphy
Rock Unit C (Youngest)
Dated Ash Layer
108.0 /- 0.8 Ma
Rock Unit B (Younger)
Guide Fossil X
Dated Ash Layer
112.0 /- 0.5 Ma
Rock Unit A (Oldest)

• If we find Guide Fossil X at a different
  locality preserved in a different type of rock,
  we can hypothesize that the age of that rock
  layer is also between about 108 and 112 million
  years old.