Presentation on theme: "Geologic Time Chapter 21.1-21.3. Geologic Time Two frames of reference –RELATIVE AGE DATING Places events in order according to their positions in the."— Presentation transcript:
Geologic Time Two frames of reference –RELATIVE AGE DATING Places events in order according to their positions in the rock record – ABSOLUTE AGE DATING Specific ages of rock units, etc. Given as Years Before Present.
Geologic Time Scale RELATIVE SCALE, with ABSOLUTE AGES added Modified as new information becomes available.
Relative Age Dating Principle #5: Inclusions –Rock fragments in a layer are older than the rock itself.
Relative Age Dating Principle #6: Fossil Succession –Fossil assemblages succeed one another in a regular and predictable order –William Smith, English surveyor, 1815 –Index fossils (guide fossils) Geographically widespread Existed for short geologic time Easily identified Abundant
Absolute Age Dating A rock layer or event can be assigned a number, years before present, with an error factor Radiometric dating –Radioactive elements spontaneously decay to a more stable element
Radiometric dating Unstable radioactive elements or isotopes (parents) change over time into stable products (daughters) – PARENT DAUGHTER Half-life: Time for ½ of parent to change to daughter Half-life is UNIQUE and CONSTANT Absolute Age Dating
Radiometric dating Types of Decay –Alpha – lose a He nucleus (2p + 2n) –Beta – lost an e- –Electron capture – gain an e- Absolute Age Dating