Important Principles in Geology

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Important Principles in Geology
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James Hutton

• Scottish geologist during the 18th century
• Considered the father of modern geology
• Although at the time people thought the Earth was
  a few thousand years old, he proposed that the
  Earth was many millions of years old
• He tried to calculate soil accumulating in the
  ocean to figure out how much time had passed
  since the formation of the Earth

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Uniformitarianism

• Hutton used the term to compare the present day
  rock cycle with the past rock cycle
• The idea is that the present is the key to the
  past
• From these comparisons he could interpret how
  rock layers (strata) were formed but not the
  length of time
• He could determine which layer was younger or
  older, just by the position of the strata
• This is now known as stratigraphy

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Stratigraphy

• Geologists can make predictions by looking at
  sedimentary processes going on today
• They realize that you can age certain layers by
  comparing the materials and fossils that it
  contains
• This has become an important tool to unravel the
  history of the Earth

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Relative Age Dating

• Before absolute dating of rocks was developed in
  the 20th century, geologists had to rely on
  relative age dating
• Relative dating places rock in order from oldest
  to youngest
• Geologists use specific principles to determine
  the age of rocks

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Principle of Superposition

• In a sequence of unformed sedimentary rock, the
  oldest beds are on the bottom and the youngest on
  the top

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Principle of Original Horizontality

• Sediment layers are nearly horizontal when
  originally deposited.
• Strata that are not horizontal have been deformed
  by movements of the Earths crust.
• Oldest layer is still the bottom.

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Principle of Crosscutting Relations

• Geological features, such as faults and fractures
  are younger than the rocks that they cut.

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Principle of Inclusion

• If one rock body contains fragments of another
  rock body it must be younger than the fragments
  of rock it contains.
• The inclusions are older than the rocks which
  contain them

B is older than C
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Principle of Faunal Succession

• Groups of fossil plants and animals occur in the
  geological record in a definite order.
• A period of geological time can be recognized by
  its specific fossils.