112lect7 - GY 112 Lecture Notes D. Haywick (2006) 1 GY 112...

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GY 112 Lecture Notes D. Haywick (2006) 1 GY 112 Lecture Notes Why Geologists Date Lecture Goals : A) Why date? B) Absolute versus Relative Dating Techniques C) Specific types of dating: Fission track, Paleomagnetism Textbook reference: Levin 7 th edition (2003), Chapter 1; Levin 8 th edition (2006), Chapter 2 and 3 (plus stuff from Doug’s personal experience) A) Why Date? Geologists date geological materials in order to sort out the sequence of events responsible for their formation. In other words, we date to put a time frame on events in Earth history . We have already started to do this in the GY 112 labs, specifically the Week 2 exercises. Using the principles that you used then, the cartoon below should be relatively easy to sort out: Both of these cartoons allow you to "date" the order of events, but only in a relative sense (e.g., A occurred before B which occurred before C etc). For this reason, this is considered to be a relative dating technique. In contrast, methods that provide an actual number for an age (e.g., 365.2 MA), are considered to be Absolute dating techniques. B) Absolute versus Relative Dating There are several relative dating techniques that geologists use (including the sequence of events approach above), but the most useful is Paleontology the study of fossil life. In fact, in this course, we will spend a considerable amount of time discussing paleontology (some would say way too much time). But this simply reflects the importance of " beasties *" in Earth history. *Note: Dr Haywick, who is not a paleontologist regards all past and present living things as "beasties". This includes all fossils, all insects, all animals, most GY 112 students and anything else that either walked, crawled, swam, floated, flew etc. Other geologists (including my friend Dr Clark who is a paleontologist) probably find offence with my use of this term. Too bad.
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GY 112 Lecture Notes D. Haywick (2006) 2 We won't go into detail about paleontology just now (give it about a week), but here is how fossils can help you date rocks. Consider the following cartoon: You will be able to determine that layer A is older than layer B and C, but that is all you can say. However, if layer A contains a specific type of fossil, say a snail, you may be able to put a better date on that layer. The argument goes, if a sedimentary rock contains a fossil, the sediment that comprises that rock must be about the same age as the fossil it contains. So if you can date the fossil, you can date the rock. Now most fossils cannot be absolutely dated with radiometric or other techniques (see the next section), but paleontologists have amassed a lot of data about the various beasties that have been around on this planet. They have a pretty good "sequence of fossils" that can allow geologists to date sedimentary rock layers relatively accurately (but not yet in an absolute sense). So let's go back to our cartoon. If layer A contains a snail, you can ask a qualified paleontologist (say Dr. Clark) about what it is and when it lived. If you're lucky,
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This note was uploaded on 02/04/2012 for the course GLY 112 taught by Professor Haywick during the Spring '12 term at S. Alabama.

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112lect7 - GY 112 Lecture Notes D. Haywick (2006) 1 GY 112...

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