Dating rocks main content.

Dating rocks

Dating rocks

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The immensity of time
In order to understand geological processes, we need to envision time periods far greater than the hourly, daily, and yearly progression that dominates our everyday lives. Major changes in the continents, oceans, atmosphere, and biosphere occur over millions of years, and usually cannot be detected from one year to the next. But the Earth’s rocks provide clues that allow scientists to piece together the planet’s 4.5-billion-year history. 

Fossils record relative time
Prior to the 20th century, geologists could only determine the relative ages of rocks — whether one was younger or older than another. Working in the Bath district of western England, William Smith (1769–1834), an engineer and surveyor, saw that rock layers could be dated.