People study rock formations to determine geologic timelines.
When calculating geologic timelines, it is often helpful to think in relative and absolute terms. For example, conceptualize the age of the Earth using an analogy: If you were to write a history of the Earth, allowing just one page per year, it would be 4.6 billion pages long, or 1.45 miles. By studying the order of geologic events, you can place them in a timeline.
Instructions
1. Study the rock formations to determine the relative geologic time frame. Each of the layers of rock represents a different period of time in the Earth's history. The older rock layers are always at the bottom of a rock formation and the youngest layers are always at the top. This is called the law of superposition.
2. Refer often to the Geologic Time Scale and become familiar with the geologic time period and ages. The Geologic Time Scale lists the era, period and absolute age of the Earth in millions of years. You can see distinct changes in the flora and fauna for each time period.
3. Study the fossils contained within the rock formation to determine the geologic era. For examples, trilobites are three-lobed arthropods formed 540 million years ago in the Cambrian period and proliferated in the Paleozoic period. Brachiopods are marine animals that resemble clams and are still common today, but the most diverse forms occurred about 400 million years ago.
4. Study radiometric dating to determine the exact date, or absolute age of the fossil. For example, brachiopods are younger than trilobites, but in order to figure out the exact age of a brachiopod fossil, analyze the layer of rock encasing the fossil. If your fossil is contained within two layers of volcanic ash, analyze the two ash layers to determine the age. According to radiometric dating, if the age of the upper layer of ash is 468 million years and the lower layer is 470 million years, using to the law of superposition, the age of the fossil is between 468 and 470 million years.
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