Age of Earth

• Earth is 4.54 billion years old
• Age of objects can be determined through relative and radioactive dating
• Relative dating - compares age of two or more rocks then determines which is older/younger
• Radioactive dating - determines age by using the amount of radioactive decay
• Half life = time it takes for half of parent atoms to decay into daughter atoms
• Principles of Relative Dating

  • P. of Superposition
  • P. of Original Horizontality
  • P. of Cross-cutting relationship
  • P. of Inclusion
  • P. of Faunal Sucession
• Fossils - evidence of past organisms found within rock layers
• Law of Cross Cutting Relations - faults cut across sedimentary rock layers, so any fault that cuts across must have formed later than the rock layer
• Law of superposition - states that younger layers are deposited on top of older ones
• Law of original horizontality - states that when sediments are first deposited they lie flat or horizontal
• Principle of Inclusions


  Larger rock layers contain smaller fragments of older rocks.
• Principle of Faunal Successions


  Different fossil species in successive rock layers represent a chronological sequence of geological events.
• Cast Fossils


  when material seeps inside into spaces inside an organism and forms a fossil
• Ice/Amber - prevents decay