geotime - This page last updated on 27-Feb-2011 EENS 1110...

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Unformatted text preview: This page last updated on 27-Feb-2011 EENS 1110 Physical Geology Tulane University Prof. Stephen A. Nelson Geologic Time From the beginning of this course, we have stated that the Earth is about 4.6 billion years old. How do we know this and how do we know the ages of other events in Earth history? Prior to the late 17th century, geologic time was thought to be the same as historical time. Archbishop James Ussher of Armagh, Ireland, 1654, added up generations from the Old Testament and determined that Earth formed on October 23, 4004 BCE. The goal of this lecture is come to come to a scientific understanding of geologic time and the age of the Earth. In order to do so we will have to understand the following: 1. The difference between relative age and numeric age. 2. The principles that allow us to determine relative age (the principles of stratigraphy). 3. How we can use fossils and rocks to understand Earth History. 4. How rock units are named and correlated from one locality to another. 5. How the Geologic Column was developed so that relative age could be systematically described. 6. How we can determine the numeric age of the Earth and events in Earth History. In order to understand how scientists deal with time we first need to understand the concepts of relative age and numeric age. z Relative age - Relative means that we can determine if something is younger than or older than something else. Relative age does not tell how old something is; all we know is the sequence of events. For example: The a volcano is younger than the rocks that occur underneath it. z Numeric age- Numeric age means that we can more precisely assign a number (in years, minutes, seconds, or some other units of time) to the amount of time that has passed. Thus we can say how old something is. For Example this metamorphic rock is 3.96 billion years old. To better understand these concepts, let's look at an archeological example: Imagine we are a group of archeologists studying two different trash pits recently discovered on the Tulane University campus and at the Audubon Zoo (where they all aksed for you). By carefully digging, we have found that each trash pit shows a sequence of layers. Although the types of trash in each pit is quite variable, each layer has a distinctive kind of trash that distinguishes it from other layers in the pits. Geologic Time 2/27/2011 Page 1 of 17 What can we say and learn from these excavations? z Relative age of trash layers - Because of the shape of the pits the oldest layers of trash occur below younger layers i.e. the inhabitants of the area likely deposited the trash by throwing it in from the top, eventually filling the pits. Thus the relative age of the trash layers is, in order from youngest to oldest.: { 5.25" Disk Layer - Youngest { Al Cans Layer { Tin Cans Layer { Ceramic Cups Layer { Stone Tools Layer - Oldest Notice that at this point we do not know exactly how old any layer really is. Thus we do not know the numeric age of any given layer. of any given layer....
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This note was uploaded on 02/01/2012 for the course EENS 1110 taught by Professor Staff during the Fall '10 term at Tulane.

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geotime - This page last updated on 27-Feb-2011 EENS 1110...

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