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 Chapter
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.
Divided into: (p. 591) Eons Eras Periods Epochs
21.2 Relative Age Dating Uniformitarianism –present is the key to the past –James Hutton, 1788 Beginning of MODERN GEOLOGY –Charles Lyell –Not immediately embraced by scientists of the day!
Relative Age Dating Six Principles –Principle#1: Superposition Oldest layer on bottom, youngest on top
Marine Regression sea regressing
Relative Age Dating Principle #2: Original Horizontality –Sediments originally deposited horizontally
Relative Age Dating Principle #3: Lateral Continuity –Layer continues equally in all directions; eventually pinches out
Relative Age Dating Principle #4: Cross-Cutting Relationships –Igneous intrusion or fault is younger than the rock it cuts across. fault cuts across Sedimentary layers
Igneous dike cuts across granite.
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
Unconformities Surfaces which represent geologic time not represented in the rock record Hiatus: Interval of time not represented
Types of Unconformities Disconformity: –Surface of erosion or non- deposition between beds which are parallel to one another
Types of Unconformities Angular Unconformity: Erosional surface on tilted or folded strata over which younger strata are deposited.
Types of Unconformities Nonconformity –Erosional surface cut into igneous or metamorphic rocks, overlain by sedimentary rocks. ?
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