7.S: Summary
- Page ID
- 32204
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Events in Earth history can be placed in sequence using the five principles of relative dating. The geologic time scale was completely worked out in the 19th Century using these principles without knowing any actual numeric ages for the events. The discovery of radioactivity in the late 1800s enabled absolute dating, the assignment of numerical ages to events in the Earth’s history, using decay of unstable radioactive isotopes. Accurately interpreting radiometric dating data depends on the type of rock tested and accurate assumptions about isotope baseline values. With a combination of relative and absolute dating, the history of geological events, age of Earth, and a geologic time scale have been determined with considerable accuracy. Stratigraphic correlation is an additional tool used for understanding how depositional environments change geographically. Geologic time is vast, providing plenty of time for the evolution of various lifeforms, and some of these have become preserved as fossils that can be used for biostratigraphic correlation. The geologic time scale is continuous, although the rock record may be broken because rocks representing certain time periods may be missing.