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EESA06H3 (234)
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Chapter 19

Chapter 19 Study Notes

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Environmental Science
Nick Eyles

Chapter 19: Time and Geology Background Information Geoscientists (that work in fields or with maps and illustrations in a laboratory) are usually concerned with relative time (geologic processes are long), and they try to figure out the sequence in which geologic events have occurred in the past. o A geoscientist may be able to characterize how old a rock is relative to another set of rocks (i.e: rocks near the surface (rock A) is younger than the rocks underlying it (rock B), which is younger than the rock underlying that (rock C) ). o But this doesnt tell us the EXACT age of the rock, which can be determined using radioactive isotope dating. o Isotopic dating has shown the North American craton to be between 4 billion years and 1 billion years old. Main Focus of chapter 19 How can we apply some basic principles to figure out the sequence in which geologic events have taken place? It is not only important to know how old rocks are relative to one another, but to figure out the actual age of the rock. This helps us compare rocks that are geologically separated. By doing so, we can understand the history of a region, the continent and the whole earth. The plate tectonics theory depends on intercontinental correlation of rock units and geologic events. Fossil use has helped us identify certain strata that recurred in the same order. (Note: Strata is defined as a thin layer of rock) o This helped geologists identify stratigraphic successions and eventually allows the creation of the standard geologic time scale. Uniformitarianism Prior to the 1800s the earth was thought to have created in merely 6,000 years. It was thought that the features we observe in rocks and landscapes were created supernaturally and the formation of the earth was placed in biblical chronology. Some marine fossils that we find on mountains were believed to have gotten there through a large flood that affected the entire world, and reached to the top of mountains (Noahs flood). However, James Hutton (father of modern geology) argued that the geological features in the past can be explained using present-day processes. Thus, he concluded that the www.notesolution.com earth actually took a long time to build. He explained that mountains are not permanent, and are subject to erosions and other such physical events. (Quote by Hutton: We find no sign of a beginning no prospect of an end). His law is based on the principle that physical law is independent of time and location. Charles Lyell ( wrote a landmark book : Principles of Geology) agreed with Huttons idea and said that the geologic processes operating in the present are the same processes that operated in the past and this is known as the principle of Uniformitarianism. (Easier said as: The present is the key to the past) Uniformitarianism doesnt mean that the earth was created at a uniform rate. Sudden events can also shape the earths geology. So some scientists prefer to use the term actualism. Numerical age ( aka absolute age) : age given in years Relative time: sequence of events relative to one another. How can the sequence of past geologic events be determined? Relative Time Geologic sequences can be determined by breaking down the problem into several little problems o Example: the geology of the Grand Canyon can be broken down into Horizontal layers of rock Inclined layers of rock Rock underlying the inclined layers ( plutonic and metamorphic rock) And the canyon itself Principles Used to Determine Relative Age Individual parts of the larger problem can be solved using simple principles The geologic sequences can be put together using: o Contacts: surfaces separating two different types of rocks or rocks of different ages o Formations: bodies of rocks with considerable thickness with characteristics that will distinguish them from other bodies of rocks. Formations are typically named after the local towns or landmarks.w www.notesolution.com
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