112lect17 - 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 Archean Geology Lecture Goals : A) Time frame (the Archean and earlier) B) Rocks and tectonic elements (shield/platform/craton) C) Tectonics and paleogeography Textbook reference: Levin 7 th edition (2003), Chapter 6; Levin 8 th edition (2006), Chapter 8 A) Time frame Up until comparatively recently (start of the 20 th century), most geologists focused their discussion of geological time on two eons; the “Precambrian” and the Phanerozoic. We now officially recognize 4 eons and the Precambrian is just used as a come-all term for all time before the Phanerozoic: Eon Time Phanerozoic 550 MA to 0 MA Proterozoic 2.5 GA to 550 MA Archean 3.96 GA to 2.5 GA Hadean 4.6 GA to 3.96 GA The “youngest” of these two new eons, the Archean , was first introduced by field geologists working in areas where very old rocks cropped out. One of these geologists was Sir William Logan (pictured at left; ess.nrcan.gc.ca ) one of the most respected geologists working with the Geological Survey of Canada. His study area was the Canadian Shield. Logan was able to identify two major types of rocks; 1) layered sedimentary and volcanic rocks and 2) highly metamorphosed granitic gneisses (see image at the top of the next page from http://www.trailcanada.com/images/canadian-shield.jpg). Logan found evidence that the gneisses mostly underlayed the layered rocks and hence, they had to be older than the layered rocks. The layered rocks were known to be Proterozoic in age. The underlying gneisses were classified as Archean in age. Some portions of the Canadian Shield, including the area where Logan worked, could be easily classified in this fashion. Other parts could not. The Hadean Eon represents the earliest period of time in the Earth’s history. It starts at the formation of the Earth (about 4.6 billion years B.P.) and
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GY 112 Lecture Notes D. Haywick (2006) 2 lasts until 3.960 GA. This is the age of the oldest rocks that we have found (so far) on the Earth (Northwestern Canada). In other words, the Hadean represents the period of the Earth’s history devoid of any rocks. The upper limit of the Hadean (3.960 GA) varies somewhat according to who you are, where you are and which book your are reading. I frequently use a date of 4.1 GA (the age of Australian zircons) as the termination of the Hadean. There is now evidence that other Aussie zircons might be 4.4 billion years old. There is even variability within
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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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112lect17 - GY 112 Lecture Notes D. Haywick (2006) 1 GY 112...

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