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Geologic Time Exercise
One of the most significant human discoveries relative to earth science is a comprehension of the magnitude of time. The Earth has a slow pulse; most geologic processes occur at imperceptible rates. Of course there are numerous exceptions - a landslide, a geyser, a flood - but by and large, most geologic phenomenon result from processes operating over long periods of time. It is only by perceiving the meaning of deep time that you can come to an understanding of how geologic forces can generate the large scale changes that we will be investigating in this course. Geologists always spew names that refer to times in the past; they often do this without thinking. For instance, I can tell you that here in eastern Indiana the bedrock is Upper Ordovician (~ 445 million years or mya) in age. But what do these names and times REALLY represent?
Because we think of time from our human frame-of-reference, it is difficult for us to perceive the "abyss of time." Our notion of time tends to range from the minutes and seconds it takes to fulfill some task to the several decades (numerous or few) that we have to be alive. Few among us know anything about our forbearers 300 years in the past; forget about 1000 years. From our primate-grounded perspective, we seldom see changes to the face of the earth. Yet, change is occurring all around us. Knowing that tectonic plates move at the rate that your fingernails grow may be a useful analogy. But then, consider how long your fingernails would be if you could grow them for 150 million years.
In order to help you start thinking about the monstrous depth of planetary time, I would like you to complete an exercise comparing geologic time to human time. In order to do this I want you to complete a geologic time-scale metaphor that turns a description of geologic time into something that you CAN readily visualize. To do this you will first s
1.) Select an item that can be measured in some way to use as your "measuring stick" (an inch, a gallon of gas, an acre).
2.) Assign your measuring stick a unit value of 1 year. Think of what a year really is...all those sunrises, days spent doing things, sunsets, passing of the seasons, etc. Think of how much you have done in a year; how different and how changed you are. Now think in this same way about 10 years. This, of course is our normal way of perceiving time.
4.) Now, using the unit value you have assigned to be one year, generate a true-to-scale metaphor for the following geological events. You may use the geological time scale below or the one that appears in your book (p. 290).
The height of the last Pleistocene Ice Age - 20 thousand years ago (kya)
First human ancestor in the Pliocene - 3 mya
The Beginning of the Miocene - 24 mya
The Cretaceous - Tertiary extinction (goodbye dinosaurs) - 65 mya
The end-Permian extinction - 248 mya
Deposition of limestone in a shallow tropical ocean near 10 degrees south latitude in what is now Richmond, Indiana - 445 mya
The first hard-shelled organisms (Cambrian - PreCambrian boundary) - 540 mya
The evidence of the first multi-cellular life - 900 mya
The evidence for oxygen buildup in the atmosphere - 2.2 billion years ago (or, bya)
The oldest evidence for life on Earth - 3.8 bya
The origin of the earth - 4.6 bya
5.) The general equation used to generate numbers in your true-to-scale metaphor will be:
Known age of past event in years times your unit value divided by one year)
Example:
1 foot = 1 year
What length is equivalent to the time since the start of the Cambrian (540 my)?
540 X 1,000,000 years * 1 foot/1 year = 540 x 1,000,000 feet = 102,272.72 miles or ~ 4 times around the earth.
6.) You may write out your metaphor in any manner that you wish. Please feel free to draw it. (See football field example below, but, unlike this example, make yours expand the time scale, not shrink it down).
Below is an example of a time-scale metaphor. (Please note that this one starts with the maximum value of the yardstick - the football field - assigned to the age of the origin of the earth. Thus, events after the origin of the earth are smaller than the football field. In our example, we are starting with a small unit value and are expanding it to see how big it gets).
The football field analogy accessible at: http://www.uky.edu/KGS/education/images/timefld.gif
Here is another example: http://www.uky.edu/KGS/education/geologictimescale.pdf
