Geologic TimeConcepts and Principles Chapter 4 -How do we tell time in geology?...relative dating, absolute dating Absolute age dating: 4 methods: 1. Radiometric

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Geologic TimeConcepts and Principles Chapter 4 -How do we tell time in geology?...relative dating, absolute dating Absolute age dating: 4 methods: 1. Radiometric age dating: parentsdaughters.isotopes 2. Carbon 14 age dating: 3. Tree Rings: 4. Fission track dating -Basic principles: horizontality, superposition, cross cutting relationships -unconformities- 3 types. Slide 2 When looking down into the Grand Canyon, we are really looking all the way back to the early history of Earth Grand Canyon Slide 3 More than 1 billion years of history are preserved, like pages of a book, in the rock layers of the Grand Canyon Reading this rock book we learn that the area underwent episodes of mountain building advancing and retreating shallow seas We know these things by applying the principles of relative dating to the rocks and recognizing that present-day processes have operated throughout Earth history Grand Canyon Slide 4 We are obsessed with time, using clocks calendars appointment books Mostly we dont have enough of it. Our common time units are seconds hours days weeks months years What is time? Ancient history involves hundreds of years thousands of years But geologic time involves millions of years even billions of years Slide 5 Geologists use two different frames of reference when discussing geologic time Relative dating involves placing geologic events in a sequential order as determined from their position in the geologic record It does not tell us how long ago a particular event occurred only that one event preceded another For hundreds of years geologists have been using relative dating to establish a relative geologic time scale Concept of Geologic Time Slide 6 The relative geologic time scale has a sequence of eons eras periods epochs but no numbers indicating how long ago each of these times occurred Relative Geologic Time Scale Slide 7 The second frame of reference for geologic time is absolute dating Absolute dating results in specific dates for rock units or events expressed in years before the present It tells us how long ago a particular event occurred giving us numerical information about time Radiometric dating is the most common method of obtaining absolute ages Such dates are calculated from the natural rates of decay of various natural radioactive elements present in trace amounts in some rocks Concept of Geologic Time Slide 8 The discovery of radioactivity near the end of the 1800s allowed absolute ages to be accurately applied to the relative geologic time scale The geologic time scale is a dual scale a relative scale and an absolute scale Geologic Time Scale Slide 9 The concept and measurement of geologic time has changed through human history Early Christian theologians conceived of time as linear rather than circular James Usher (1581-1665) in Ireland calculated the age of Earth based on recorded history and genealogies in Genesis He announced that Earth was created on October 22, 4004 B.C. A century later it was considered heresy to say Earth was more than about 6000 years old. Changes in the Concept of Geologic Time Slide 10 During the 1700s and 1800s Earths age was estimated scientifically Georges Louis de Buffon (1707-1788) calculated how long Earth took to cool gradually from a molten beginning using melted iron balls of various diameters Extrapolating their cooling rate to an Earth-sized ball, he estimated Earth was 75,000 years old Changes in the Concept of Geologic Time Slide 11 Others used different techniques Using rates of deposition of various sediments and thickness of sedimentary rock in the crust gave estimates of

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