Dating Methods

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Dating Methods

• 10.11.07 / 10.12.07

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Essential Question

• How can scientists determine the ages of geologic
  material?

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What are strata?

• Layers of rock

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Strata
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Strata
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Folded strata
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What is an angular unconformity?

• When rock deposited in horizontal layers is
  folded or tilted, then eroded, and then covered
  by more horizontal layering

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Angular unconformity
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Angular unconformity
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What is the law of cross-cutting relationships?

• The fault or igneous intrusion is always younger
  than the rock layer it cuts through.

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Cross-cutting relationship
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What are nonconformities?

• Unstratified igneous or metamorphic rock is
  eroded and then covered by layers of sediment.

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Nonconformity
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Nonconformity
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What are disconformities?

• The boundary between younger sedimentary rock
  layers deposited on an eroded layer of older
  sedimentary rock

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disconformity
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What are faults?

• A break or crack in Earths crust along which
  rocks shift their positions

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Fault
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Earth history in 24 hours
scale 1 sec 52,000 years 1 min 3,125, 000
yrs 1 hour 187,500,000 yrs
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What is correlation?

• Matching geologic materials from different
  regions to ages

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Correlating tree rings
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Rates of erosion and deposition

• Measure the recent rates
• Estimate age of geologic feature
• Becomes less reliable as age increases
• Most reliable within last 10,000-20,000 years

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Niagara Falls
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Varve count

• Similar to counting tree rings
• Count annual layers of sediment
• Common in glacial lake beds
• Distinguished by layers of dark fine particals
  and light heavy particles

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Radiometric dating

• A method of determining the absolute age of an
  object by comparing the relative percentages of a
  radioactive (parent) isotope and a stable
  (daughter) isotope.
• Atoms of the same element with different numbers
  of neutrons are called isotopes.

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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

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The radioactive decay curve
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Dating sedimentary strata using radiometric
dating
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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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