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Earth Files - Absolute Age

Vocabulary
 challenging words: half-lives, nonradioactive, radiocarbon, varve, half-life, analysis, beta, isotope, varves, deposition, sandstone, geologic, sedimentary, shale, absolute, tremendous content words: Grand Canyon, United States, Niagara Falls

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 Earth Files - Absolute Age By Trista L. Pollard

1     When geologists determine the relative age, they are comparing the age of rocks in relation to other rocks. In other words, rock formations are either older or younger than other rock formations. This is why scientists use absolute dating methods to find out the actual age of rocks.

2     Absolute age is the numeric age of rocks, events, or objects stated as years before present time. To estimate absolute age, geologists may study rates of erosion, rates of deposition, or deposit varve count. When scientists want to estimate the absolute age of a stream, they analyze the rate of erosion of its bed. However, this method is only effective when the stream is 10,000 to 20,000 years old. Think about the Grand Canyon in the United States and Niagara Falls in the U.S. and Canada. Niagara Falls was formed within the last 20,000 years, so scientists can use erosion rates to determine its absolute age. The Grand Canyon, however, is much older at over a million years. The erosion rates for this geologic structure have varied over the time it has existed on Earth.

3     Estimating the absolute age of sedimentary rocks like limestone, sandstone, and shale require scientists to study rates of deposition. Geologists collect information about the rock deposition over long periods of time. Using this information, they are able to estimate the average rate of deposition for these and other sedimentary rocks. Did you know that over a period of 1,000 years, only 30 centimeters of sedimentary rock is deposited? Like all occurrences in nature, some sedimentary rocks may not be deposited at an average rate. There are areas around the world that experience frequent flooding. Flooding increases the amount of sediment deposition in an area. Similar to rates of erosion, rates of deposition only give geologists an estimate for absolute age.

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