Comment: 16:36 - 18:06 (01:30)
Source: Annenberg/CPB Resources - Earth Revealed - 10. Geologic Time
Keywords: "James Sadd", radioactivity, "Marie Curie", radioisotope, "Ernest Rutherford", "absolute age dating", "relative age dating", dinosaur, extinction
Our transcription: The relative age relationships that we see in rocks allow us to recognize geologic events in a historic sequence through time, but knowing only the order of events is a bit unsatisfying.
Without the ability to measure the absolute age of these rocks, it's impossible to fully understand Earth history.
And without a calibration of geologic time, we can't study the rates of geologic change that are used to make forecasts about future events.
Scientific discovery is often born of chance events coupled with keen perception.
In the late Nineteenth Century, Madame Marie Curie discovered that when certain types of rocks were stored in the same drawer overnight with undeveloped photographic film, the film was exposed.
This chance event led to the discovery of radioactivity. In this process certain types of atoms spontaneously disintegrate or decay.
The decay is random, but it proceeds at a constant rate through time for each particular type of atom or isotope.
Because many types of rocks contain radioactive isotopes, Ernest Rutherford reasoned in 1905 that the uniform rate of radioactive decay could be used to measure geologic time.
From this realization came the eventual development of techniques to measure the absolute age of geologic materials.
Now, for example, not only do we know that the dinosaurs disappeared in a global mass extinction, we know it happened 64 million years ago.
Geology School Keywords