The Rock Record

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Chapter 12
The Rock Record
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Warm-up 9/1/15

• List and briefly describe three different types
  of fossils.

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How old is the Earth thought to be?

• It is estimated that the earth is 4.6 billion
  years old, but how do we come to this
  conclusion???

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Rocks Record Earth History

• Rocks record events and changing life forms of
  the past.

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James Hutton

• 18th century physician and farmer who made
  valuable observations about the geologic changes
  occurring in his area

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Uniformitarianism

• His studies led to the Principle of
  Uniformitarianism (the forces and processes
  observed today have been happening for a long
  time)

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• The processes are the same, but they may occur at
  different rates

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Catastrophic Theory

• Before Huttons time, it was believed that the
  earth was 4,000 years old and formed from some
  sort of catastrophe

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Earth as a History Book

• Think of the rock layers of the earth as pages in
  a history book

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Relative Age vs. Absolute Age

• Relative age is the age of an object compared
  with the ages of other objects
• Stratalayers of rock

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Nicolaus Steno

• Developed three observations that are the basis
  for relative dating of Earths history.

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Types of Relative Age Dating

• Law of Superpositionan undeformed sedimentary
  rock layer is older than the layers above it and
  younger than the layers below it.

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Original Horizontality

• Layers of rock are deposited in a horizontal
  fashion.

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Crosscutting Relationships

• Law of Crosscutting Relationshipsprinciple that
  a fault or intrusion is always younger than the
  rock layers it cuts through

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Unconformities

• It is a break in the geologic record created when
  rock layers are removed by erosion

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Quiz

• 1. How old is the earth thought to be?
• 2. What is the Principle of Uniformitarianism?
• 3. What is the Law of Superposition?
• 4. What is the Law of Crosscutting Relationships?

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Absolute Age

• Absolute age?the actual age of the rock

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Types of Absolute Dating

• Rates of Erosion and Deposition? scientists
  measure the rate of erosion of a stream and back
  track how many years ago the stream must have
  been there to have eroded as much as it has
• Only practical for about 10,000 to 20,000 yrs ago

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Radioactive Decay

• The comparisons of the amounts of the parent
  material (the one you started with) and the
  daughter material (the one produced through
  nuclear decay)

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Radioactive Decay

• The amount of time it takes for the exactly one
  half of the parent material to decay to the
  daughter material is called the half-life.
  Scientist use this unchanging time to date a
  substance.

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The Fossil Record

• Paleontologists? scientists who study fossils to
  learn about the earths past
• Paleontology? the study of fossils

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Preservation of Organisms

• Mummification? preservation of dead organisms by
  drying

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• Amber? preservation of insects in hardened tree
  sap

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• Tar beds? preservation of animals in beds of
  thick petroleum that oozes to the earths surface
  (tar)

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• Freezing? preservation of animals or organisms in
  low temperature soils and ice

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• Petrifaction? process in which organic materials
  are replaced by minerals

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Traces of Organisms

• Trace Fossils tracks, footprints, borings, and
  burrows

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• Imprints, molds, and casts

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• Coprolites? the fossilized dung and waste
  materials of animals

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• Gastroliths? stones found in animals like
  dinosaurs that helps them to digest food

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Interpreting the Fossil Record

• Fossils are used to date rocks
• It can reveal changes in the environment

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Index Fossils

• Certain fossils found exclusively in rock layers
  of a particular geologic age
• For example, trilobite fossils from the Paleozoic
  Era

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Another index fossil

• Ammonite fossil from the Mesozoic Era