Principles of Geology

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Principles of Geology

• Chapter 9
• Geologic Time

Mian Liu 2007
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the canyons of this region would be a Book of
Revelations in rock-leaved Bible of geology.
J. W. Powell
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Dating rocks

• Relative dating
• Using a set of principles to put rocks in their
  proper sequences of formation
• Absolute dating
• Using radioactive decay to determine the exact
  age of rocks

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Principles of relative dating

• Law of superposition
• Developed by Nicolaus Steno in 1669
• In an undeformed sequence of sedimentary rocks
  (or layered igneous rocks), the oldest rocks are
  on the bottom

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Principles of relative dating

• Principle of original horizontality
• Layers of sediment are generally deposited in a
  horizontal position
• Rock layers that are flat have not been disturbed

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Principles of relative dating

• Principle of cross-cutting relationships
• Younger features cut across older feature

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Principle of cross-cutting relationships
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Principles of relative dating

• Inclusions
• An inclusion is a piece of rock that is enclosed
  within another rock
• Rock containing the inclusion is younger

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Principles of relative dating

• Unconformity
• An unconformity is a break in the rock record
  produced by erosion and/or nondeposition of rock
  units

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Principles of relative dating

• Types of unconformities
• Angular unconformity tilted rocks 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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Correlation of rock layers

• Correlation by physical criteria
• For a limited area
• Correlation using index fossils
• Principle of fossil succession

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Overlapping ranges of fossils help date rocks
more exactly than a single fossil
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Dating rocks

• Relative dating
• Using a set of principles to put rocks in their
  proper sequences of formation
• Absolute dating
• Using radioactive decay to determine the exact
  age of rocks

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Absolute Dating based on Radioactive Decay
Parent
daughter
Why does it work? 1. The decay rate is CONSTANT,
independent of external conditions in the
earth. 2. The daughter/Parent ratio can be
precisely measured.
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Dating with radioactivity

• Parent an unstable radioactive isotope
• Daughter product the isotopes resulting from
  the decay of a parent

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Dating with radioactivity

• Radioactivity
• Spontaneous changes (decay) in the structure of
  atomic nuclei

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Half-life the time required for one-half of the
radioactive nuclei in a sample to decay
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Dating with carbon-14 (radiocarbon dating)

• Half-life of only 5730 years
• Used to date very recent events
• Carbon-14 is produced in the upper atmosphere
• Useful tool for anthropologists, archeologists,
  and geologists who study very recent Earth history

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Tree ring dating (dendrochronology)

• Growth ring is added each year on the outer rim
• Characteristics of each ring (size, density, etc.
  ) varies with environment, especially climate
• A regional pattern may be established, allowing
  comparison between live trees and trees killed by
  geological events in the past, thus providing age
  constraints to those events (dendrochronology).

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The geologic time scale

• The geologic time scale a calendar of Earth
  history
• Subdivides geologic history into units
• Originally created using relative dates
• Structure of the geologic time scale
• Eon the greatest expanse of time

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The geologic time scale

• Structure of the geologic time scale
• Era subdivision of an eon
• 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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End of Chapter 9