Chapter 7 -- Geologic Time and History of the Earth

Chapter 7 -- Geologic Time and History of the Earth - GEO...

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GEO 1013 E. R. SWANSON – spring 2009 A SHORT GEOLOGIC HISTORY OF THE EARTH The Geologic Time Scale The concept of relative dating and the discovery that fossils could be used to determine the age of rocks quickly led to the construction of the geologic time scale. This time scale, based on fossils, was divided into eras according to the life forms (fossils) found in rocks, and these time scale names make more sense if you understand that the zoic refers to life and that paleo = old, meso = middle and ceno = recent. The major divisions of the geologic time scale are the Paleozoic, Mesozoic and Cenozoic eras. These eras are divided further into time periods with names typically derived from the regions were rocks of the various time periods were first described. The Paleozoic Era, for example, is divided into the Cambrian, Ordovician, Silurian and Devonian periods, all defined in England. Devonian rocks were first described in Devonshire. Younger Paleozoic periods include the Pennsylvanian and Mississippian rocks from the U.S.A., and Permian rocks from the Perm province of Russia. There are no hard and fast rules however. Most of the world refers to our Pennsylvanian and Mississippian rocks as Carboniferous for the abundant coal beds in these rocks. Likewise, Cretaceous (from the Latin creta meaning chalk), the last period in the Mesozoic, is so named because limestone (including chalk) is abundant in rocks of this age. All of these periods are based on major changes in the fossil record, although not as great as the radical fossil changes (mass extinctions) found between rocks of the Paleozoic, Mesozoic and Cenozoic Eras. And what about rocks that are older than Paleozoic? What are they called, how are they divided, and are these rocks older than life? Well, these rocks are all older than the first Paleozoic period, the Cambrian. For this reason, Pre-Paleozoic rocks are said to belong to Precambrian time . Some evidence for life can be found in Precambrian rocks, but fossils in Precambrian rocks are extremely rare and primitive. Also, most Precambrian rocks are igneous, which would not have had fossils, or they are sedimentary rocks that have been metamorphosed, a process that would have destroyed any existing fossils. Fossils, therefore, are not particularly useful in dating Precambrian rocks, but dating based on the decay of radioactive elements is particularly helpful here. Just as we know less about the distant reaches of the Universe than about our own solar system, we know very much less about Precambrian earth history than we do about the history of the Cenozoic Era, a time closer to our own. This represents a fairly significant lack of knowledge because the Precambrian Era represents 88% of Earth’s history, compared to one 1.4% of the geologic time belonging to the Cenozoic. Things ancient, therefore, tend to get lumped together, and we play fast and lose in the Precambrian talking about 4.3 or 4.4 billion as if 100 million years was next to nothing when, in fact,
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This note was uploaded on 09/09/2009 for the course GEO 301 taught by Professor Long during the Spring '07 term at University of Texas.

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Chapter 7 -- Geologic Time and History of the Earth - GEO...

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