GEOLOGIC TIME SCALE PRESENTED BY: SUBHASHREE S
Geologic Time By examining layers of sedimentary rock, geologists developed a time scale for dividing up earth history. Earlier in the 20 th century, radiometric-dating techniques allowed scientists to put absolute dates on divisions in the geologic time scale. In this segment, we will learn how geologists: • determine the relative ages of rock units, • determine the divisions of the geologic time scale, and • how radiometric techniques can be used to date some rocks. How do geologists determine how old rocks are? 1. Relative dating -- determine whether the rock is older or younger than other rocks 2 . Absolute dating -- use radiometric dating techniques to determine how long ago the rock formed in the exact number of years *Not all rocks can be dated absolutely, so combinations of techniques are used.
Example of Relative Age Dating and Correlation
Relative Age Dating
Principles associated with Relative Dating The comparing of rock units to decipher their age relative to one another • Principle of Superposition Rock layer above is younger than the ones below it. (Oldest on bottom, youngest on top) May not apply to rocks that have been folded (can get turned upside-down). • Principle of Original Horizontality Sedimentary layers are deposited in approximately horizontal sheets. If layers are folded, episode of deformation must have occurred after rocks formed. Age of folding is younger than youngest deformed rock unit. • Principle of Crosscutting Relationships Any feature (e.g. fault or intrusion) that cuts across rocks is younger than the youngest rock that is cut. • Principle of Faunal Succession Organisms have evolved and gone extinct through time Fossil content of rock changes in a systematic way, reflecting evolutionary changes Fossil content can be used to help determine age of rock and correlate rocks. Paraphrased as "Organisms within rock units change with time". Other parameters used for relative dating are unconformities and index fossils
Absolute Dating: Absolute dating is the process of determining an age on a specified chronology in archaeology and geology. Some scientists prefer the terms chronometric or calendar dating, as use of the word "absolute" implies an unwarranted certainty of accuracy. Thus, by using the appropriate radioactive isotope (knowing its half- life time), and measuring the quantity of the isotope present in the rock, one can deduce how long it has taken to decay down to the present amount in the rock. Radiometric Dating Techniques use naturally-occurring radioactive isotopes Isotope -- form of an element that has additional neutrons Radioisotope -- isotope that spontaneously decays, giving off radiation .
Decay products • Radioisotopes may decay to form a different isotope or a stable isotope.
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