Chapter 8 Time and Geology

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Chapter 8 Time and Geology
GEOL 101 Introductory Geology
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Determining Geological Ages

• Relative age dates placing rocks and events in
  their proper sequence of formation
• Numerical (absolute age) dates specifying the
  actual number of years that have passed since an
  event occurred

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Principles of Relative Dating

• Law of superposition
• Undeformed section of sedimentary or layered
  igneous rocks
• Oldest rocks are on the bottom
• Principle of original horizontality
• Layers of sediment are generally deposited in a
  horizontal position
• Rock layers that are flat have not been disturbed
  (deformed)
• Principle of cross-cutting relationships
• Younger features cut across older features

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Superposition Strata in the Grand Canyon
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Horizontality
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Cross-cutting Relationship
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Cross-cutting Relationship
Which crater is youngest?
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Cross-cutting Relationships
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Principles of Relative Dating

• Inclusions
• A piece of rock that is enclosed within another
  rock
• Rock containing the inclusion is younger
• Unconformity
• Break in rock record produced by erosion and/or
  nondeposition of rock
• Represents period of geologic time

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Principles of Relative Dating

• Types of unconformities
• Angular unconformity tilted rocks (disturbed)
  are overlain by flat-lying rocks
• Disconformity strata on either side of the
  unconformity are parallel
• Nonconformity metamorphic or igneous rocks in
  contact with sedimentary strata

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Angular Unconformity
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Angular Unconformity
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Angular Unconformity
Animation
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Disconformity
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Disconformity
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Nonconformity
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Nonconformity
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Nonconformity
Inclusions
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Grand Canyon Unconformities
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Interpret Geologic History
Animation
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Correlation of Rock Layers

• Matching of rocks of similar ages in different
  regions is known as correlation
• Correlation often relies upon fossils
• similar sedimentary strata in widely separated
  areas
• identified and correlated by distinctive fossil
  content
• Principle of fossil succession
• fossil organisms succeed one another in a
  definite and determinable order
• time period recognized by its fossil content

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Correlation of Rock Layers
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Fossils Ages of Rocks
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Radiometric Dating

• Radioactivity spontaneous changes (decay) in the
  structure of atomic nuclei
• Basic atomic structure
• Types of radioactive decay
• Parent, daughter product, half-life
• Principle of radioactive dating
• Sources of error
• Carbon-14 (radiocarbon) dating
• Importance of radiometric dating

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Basic Atomic Structure

• Nucleus
• Protons positively charged particles with mass
• Neutrons neutral particles with mass
• Electrons negatively charged particles, orbit
  nucleus
• Atomic number
• An elements identifying number
• Number of protons in the atoms nucleus
• Mass number
• Sum of protons neutrons in atoms nucleus
• Isotope
• Variant of the same parent atom
• Differs in the number of neutrons
• Different mass number than the parent atom

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Radiometric Dating

• Types of radioactive decay
• Alpha emission
• Emission of 2 protons 2 neutrons (alpha
  particle)
• Mass number is reduced by 4 and the atomic number
  is lowered by 2
• Beta emission
• Electron (beta particle) is ejected from the
  nucleus
• Mass number remains unchanged and the atomic
  number increases by 1
• Electron capture
• Electron is captured by the nucleus
• Electron combines with a proton to form a neutron
• Mass number remains unchanged and the atomic
  number decreases by 1

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Radiometric Decay
Alpha Emission
Beta Emission
Electron Capture
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Radiometric Decay

• Parent an unstable radioactive isotope
• Daughter product the isotopes resulting from
  the decay of a parent
• Half-life the time required for one-half of the
  radioactive nuclei in a sample to decay

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Uranium Isotopes
Parent U238
Atomic - 92 Mass - 238
Atomic - 82 Mass - 206
Daughter PB206
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Radiometric Dating

• Principle of radioactive dating
• The percentage of radioactive toms that decay
  during one half-life is always the same (50
  percent)
• However, the actual number of atoms that decay
  continually decreases
• Comparing the ratio of parent to daughter yields
  the age of the sample

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Radioactive Decay Curve
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Radioactive Decay Curve
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Radiometric Dating
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Radiometric Dating

• Sources of error
• A closed system is required
• To avoid potential problems, only fresh,
  unweathered rock samples should be used
• Carbon-14 (radiocarbon) dating
• Half-life of only 5730 years
• Used to date very recent events
• C14 is produced in the upper atmosphere

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C14 Production and Decay
Neutron Capture
Beta Emission
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Radiometric Dating

• Importance of radiometric dating
• Radiometric dating is a complex procedure that
  requires precise measurement
• Rocks from several localities have been dated at
  more than 3 billion years
• Confirms the idea that geologic time is immense

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Other Dating Methods
Shell Growth Rings
Tree Rings
Rhythmic Layering in Glaciers
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Cross-Dating Principle in Dendrochonology
Youngest
Older
Oldest
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Geologic Time Scale

• Geologic time scale Earth history calendar
• Subdivides geologic history into units
• Originally created using relative dates
• Structure of the geologic time scale
• Eon the greatest expanse of time
• Phanerozoic visible life, most recent eon
• Proterozoic before life
• Archean ancient
• Hadean mythical subterraneon world of departed
  spirits, oldest eon
• Precambrian Proterozoic, Archean, Hadean

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Geologic Time Scale

• Era subdivision of an eon
• Precambrian no era subdivisions
• Eras of the Phanerozoic eon
• Cenozoic (recent life)
• Mesozoic (middle life)
• Paleozoic (ancient life)
• Eras are subdivided into periods
• Periods are subdivided into epochs

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Geologic Time Divisions
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Geologic Time Scale
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Geologic Time Scale

• Precambrian time
• Nearly 4 billion years prior to the Cambrian
  period
• Not divided into smaller time units because the
  events of Precambrian history are not know in
  great enough detail
• First abundant fossil evidence does not appear
  until the beginning of the Cambrian
• Visualizing vast period of geologic time

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Evolution vs Geologic Time
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Visualizing Geologic Time
Twice around the equator
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Geologic Time Scale Difficulties

• Not all rocks can be dated by radiometric methods
• Grains comprising detrital sedimentary rocks are
  not the same age as the rock in which they formed
• The age of a particular mineral in a metamorphic
  rock may not necessarily represent the time when
  the rock formed
• Datable materials (such as volcanic ash beds and
  igneous intrusions) are often used to bracket
  various episodes in Earth history and arrive at
  ages

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Sedimentary Ages using Radiometric Dating
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Stratigraphic Dating