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Earth Science
Earth Files - Age is Relative

Earth Science
Earth Science

Earth Files - Age is Relative
Reading Level
     edHelper's suggested reading level:   grades 9 to 10
     Flesch-Kincaid grade level:   9.46

Vocabulary
     challenging words:    bedding, conformity, Cross-beds, Disconformities, disconformity, nonconformity, redefined, unstratified, volcanism, angular, undeformed, metamorphic, horizontality, successive, unconformities, unconformity
     content words:    James Hutton, When Hutton, As Hutton, Grand Canyon

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Earth Files - Age is Relative
By Trista L. Pollard 		  
1     A farmer in Scotland made one of the most important discoveries in Earth science during the 18th century. This farmer observed the natural changes that took place on his land over the course of time. By observing these geologic changes, James Hutton developed the principle of uniformitarianism. Hutton, in addition to farming, was also a physician. He wrote on various topics including agriculture, weather, climate, and physics. Hutton concluded from his observations that his land was changed by the same geologic forces that changed our planet's surface thousands of years before. These geologic forces or processes, like volcanism and erosion, have helped to shape the Earth's crust since the earliest time. When Hutton spoke and wrote about uniformitarianism, geologists listened. In fact, his principle became one of the foundations of geology.
 
2     Over the years, scientists have redefined Hutton's principle to include the rates of geologic processes. Scientists still believe that the geologic processes we witness today are the same ones that occurred in the past. However, they have also added that the rates at which these processes occur can vary over time. So why is Hutton's principle so important? His principle has helped scientists today determine the age of our planet.
 
3     It has been estimated that our planet is about 4.6 billion years old. Prior to Hutton's observation, most scientists believed Earth was about 6,000 years old. They also believed that all of our geologic features formed at one time and not over a period of time. As Hutton observed his land, he realized that the changes occurred very slowly. He then concluded that the Earth's crust would need a tremendous amount of time to produce rock formations. In this case, he estimated that it would take about a million years. This would mean the Earth was much older than 6,000 years. Hutton's work encouraged other scientists to study the Earth's history.
 
4     When studying the history of our planet, scientists try to determine the Earth's relative age. They analyze the order in which rock formations and rock layers form. Every layer gives the scientists information about the sequence of events in Earth's geological history. Studying the strata of rock formations and applying some basic geological principles, scientists are able to estimate the relative age for each strata. The relative age of rock formations, especially rocks that have layers, indicates that each layer of the rock is older or younger than the layers that surround it. However, relative age does not give the rock formation's age in years.
 
5     Igneous, metamorphic, and sedimentary rocks form layers. When successive lava flows are positioned on top of each other, the igneous rock that is produced has layers. Metamorphic rocks like marble also have layers. Geologists, however, use sedimentary rock to estimate the relative age of rocks. Think about the Grand Canyon. The walls of the canyon are painted with the sedimentary rock layers that have developed over thousands of years. These strata provide the historical clues scientists need to understand the development of the canyon over time. When studying sedimentary rock layers, geologists use the law of superposition, the principle of original horizontality, and unconformities to help determine relative age.

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