Clues to Earth's Past

  Fossil

  Remains, imprints, or traces of prehistoric organisms.

  Permineralized remains

  Fossils in which the spaces inside are filled with minerals from groundwater.



  Carbon film

  A thin film of carbon residue left, that forms a silhouette of the original organisms.

  Mold

   A type of body fossil that forms in rock when an organism is buried, decays or dissolves and leaves a cavity in rock.

  Cast

   Type of fossil that forms when sediments or water enter the cavity, form new rock, and produce a copy of the original object.

  Index fossil

   Remains of species that existed on Earth for relatively short periods of time, were abundant and were widespread geographically.

 Principle of superposition

  States that in undisturbed rock layers, the oldest rocks are on the bottom and the rock become progressively younger toward the top.



  Relative age

   The age of something compared with other things.

  Unconformity

  Gaps in rock sequences in the rock layer due to erosion or periods without any deposition.

  Absolute age

  The age, in years, of a rock or other object.





  Radioactive decay

  Process in which some isotopes break down into other isotopes and particles.

  Half-life

      Time it takes for half of the atoms in the isotope to decay.



  Radiometric dating

     Process used to calculate the absolute age of rock by measuring the ratio of parent isotope to daughter product in a mineral and knowing the half life of the parent.





  Uniformitarianism

   Principle stating that Earth processes occuring today are similar to those that occured in the past.