Rock Record and Geologic Time

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Rock Record and Geologic Time
Grand Canyon in Arizona (stratification, bedding)
stratification, bedding, stratum (strata)
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Timescale (Relative)
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Timescale (Absolute)
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How do we tell geologic time?

• Relative time
• Principles of stratigraphy - 17th 18th century
• Fossils - 18th century
• Paleomagnetism 20th century
• Absolute time
• Radiometric dating late 19th-20th century

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Relative dating

• Placing rocks and events in sequence
• Law of superposition oldest rocks are on the
  bottom
• Principle of original horizontality sediments
  are deposited in flat, horizontal layers

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Glen Canyon Dam, Lake Powell
Rio Colorado Red River Red sediment now
trapped in Lake Powell so the river is no longer
red.
Grand Canyon in Arizona
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Superposition is well illustrated by the strata
in the Grand Canyon
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Original Horizontality
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Relative dating

• Other geologic principles
• Cross-cutting relationships - fractures, faults
  and intrusions must be YOUNGER than rocks they
  cut
• Inclusions one rock contained within another
  (rock containing the inclusions is younger)

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Cross-cutting Relationships

• fractures, faults and intrusions must be YOUNGER
  than rocks they cut

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Inclusions
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(No Transcript)
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(8) Erosion of folded unit and of intrusion B (7)
Intrusion of B (or before 6) (6) Folding of all
previously deposited layers (5) Deposition of
layer E (4) Deposition of layer I (3) Deposition
of layer F (2) Deposition of layer H (1) Oldest
Event Deposition of layer D
(12) Youngest Event Deposition of
unit C (11) Erosion of unit G and intrusion
A (10) Intrusion of A (9) Deposition of unit G
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Telling Time with Fossils

• use first occurrence and last occurrence
• rapidly evolving (short-lived) organisms divide
  time into the finest divisions
• best index fossils have a wide geographic range
  (planktonic ocean organisms)

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Index (Zone) Fossils

• trilobites Cambrian
• ammonoids Devonian to Cretaceous
• bivalves Devonian to Cretaceous
• foraminifera - Cenozoic

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Radioactivity and radiometric dating

• Radioactivity
• Spontaneous breaking apart (decay) of atomic
  nuclei
• Radioactive decay
• Parent an unstable isotope
• Daughter products isotopes formed from the
  decay of a parent

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Radioactivity and radiometric dating

• Radiometric dating
• Half-life the time for one-half of the
  radioactive nuclei to decay
• Requires a closed system
• Cross-checks are used for accuracy
• Complex procedure
• Yields numerical dates
• http//www.youtube.com/watch?v1920gi3swe4

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The radioactive decay curve
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Proportion of Parent Atoms Remaining as a
Function of Time
Half-lives and remaining parent isotope
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Radiometric/Isotopic dating

• Radioactive elements (parents) decay to
  nonradioactive (stable) elements (daughters).
• The rate at which this decay occurs is constant
  and knowable (measurable).
• Therefore, if we know the rate of decay and the
  amount present of parent and daughter, we can
  calculate how long this reaction has been
  proceeding.

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Radioactivity and radiometric dating

• Carbon-14 dating
• Half-life of only 5730 years
• Used to date very recent events
• Carbon-14 produced in upper atmosphere
• Incorporated into carbon dioxide
• Absorbed by living matter
• Useful tool for anthropologists, archeologists,
  historians, and geologists who study very recent
  Earth history

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Parent Isotope Stable Daughter Product Currently Accepted Half-Life Values
Uranium-238 Lead-206 4.5 billion years
Uranium-235 Lead-207 704 million years
Thorium-232 Lead-208 14.0 billion years
Rubidium-87 Strontium-87 48.8 billion years
Potassium-40 Argon-40 1.25 billion years
Samarium-147 Neodymium-143 106 billion years