2Geologic Time Two frames of reference RELATIVE AGE DATING Places events in order according to their positions in the rock recordABSOLUTE AGE DATINGSpecific ages of rock units, etc.Given as Years Before Present.
3Geologic Time Scale RELATIVE SCALE, with ABSOLUTE AGES added Modified as new information becomes available.
14Order of Relative AgesOldest YoungestD B C E
15Relative Age Dating Principle #5: Inclusions Rock fragments in a layer are older than the rock itself.
16Relative Age Dating Principle #6: Fossil Succession Fossil assemblages succeed one another in a regular and predictable orderWilliam Smith, English surveyor, 1815Index fossils (guide fossils)Geographically widespreadExisted for short geologic timeEasily identifiedAbundant
25Absolute Age DatingA rock layer or event can be assigned a number, years before present, with an error factorRadiometric datingRadioactive elements spontaneously decay to a more stable element
26Absolute Age Dating Radiometric dating Unstable radioactive elements or isotopes(“parents”) change over time into stableproducts (“daughters”)– PARENT → DAUGHTERHalf-life: Time for ½ of parent to change todaughterHalf-life is UNIQUE and CONSTANT
27Absolute Age Dating Radiometric dating Types of Decay Alpha – lose a He nucleus (2p + 2n)Beta – lost an e-Electron capture – gain an e-