| Geological Time |
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Geological Time "The Earth is very old - present estimates put it at 4.54 billion years, ± 45 million. Most of the rocks we see today have been recycled many, many times. They have been down to the bottom of the deepest oceans, buried kilometers below the surface of the Earth, before being uplifted once again to form the very highest peaks of the Himalayas, the Andes, the Rockies or the Alps, where erosion starts them on their weary way again, back down to the sea. The cycle goes on unceasingly. It has done so for billions of years in the past and will continue for billions of years to the fixture. At the same time the continents have moved around the globe, effortlessly, like so many birds on migration - once buried under glaciers at the poles they soon find themselves passing the equator en route to another destination."
P229 Lewis, Cherry (2000) The Dating Game: One Man's Search for the Age of the Earth, Cambridge University Press, ISBN 0-521-89312-7 Geological time provides the scale for us to track the history of the earth from its beginning until today. Geological evidence suggests that earth formed around 4.5 billion years ago. Based on what we find in the fossil record, simple forms of life began about 3.8 billion years ago. However, complex life structures did not start to appear until 500 million years ago - or about in the last 11 percent of time. Modern humans, on the other hand have been around for about 1.8 million years or 4/10000th of the time earth has been around. Some of the bedrock in the Ruby Range in Montana dates back to around 2.7 billion years.
Why do we care about geological time? It allows us to put into context the events and history of life. It helps us to understand the environment under which living organisms evolved. It helps to document the changes which took place in the earth and the order in which they occurred. If you are going to understand what shaped a mountain, range or valley, grasping the sequence of events that had to occur in order for these features to be formed, is a major reason for understanding geological time.
Without understanding the geological time it is impossible to interpret the geological record. How do we know how old the rocks are? Dozens of methods were used to try and date the earth. This date has ranged from the creation of the earth as 0ct 23rd, 4004 BC by James Ussher of the Church of Ireland (he did this by chronologically calculating the life spans of all of the “begatters” in the bible.) In addition rates of deposition of sediments were used to calculate that the earth was somewhere between 20 and 100 million years old (using the concept of Uniformity of rate - one assumed that the observable rate of deposition of sediments was a given constant over time. By using the total thickness of "known" sediments - they extrapolated the age. Thermodynamics were used to estimate the amount of time it would take for a planet the size of the earth to cool down. None of these methods came close to what we know by using radioactive decay. Even this method required interpreting meteorite fragments and assuming they were formed at the same time as the earth, estimating the earth's age. (Radioactive dating works someone what like this. Complex elements such as uranium break down based on what is known as their half life. If we can assume that the element in it's pure form existed by itself, and then can determine the ratio of it's half life element, we can determine the amount of time it has taken to for that ratio to be formed.) We can also use radioactive dating to compare rocks that may be 100’s of miles away. For example the dykes in the Tobacco roots have been tied to the dike in at Mt. Moran in the Teton’s Mountain Range. In this way we can assume that the same geological event was going on at both places! Since the Tobacco root Dike has been dated to 765 mya the dike in Mt Moran is the same age. Now we can use Geological time to help us piece together what was going on back in the Protozoic Eon.
If you consider the current arguments about global warming and what causes it, you will find a tremendous amount of evidence locked in the geological record. Only by knowing the sequence and age of the rocks in which the evidence lies, will we be able to answer questions such as: "Has climate change and global warming occurred before?" "If so, why does it occur?" "What are the possible causes of climate change?" "Can the causes related to climate change cause extinction?"
There are at least 6 periods of mass extinction recorded in the geological record. During these periods of mass extinction up to 90 percent of life was eliminated. Not just the extinction of a single species. How can we discover what happened if we don't understand the order in which it happened?
There are many ways to portray geological time. Geological time is categorized by Eons, Eras, Periods then Epochs (from largest to smallest increments of time). Eons represent the largest time periods and generally include the Hadean (4.57 to 3.8 Billion years ago), the Archean (3.8 to 2.5 billion years ago) the Proterozoic (2.5 billion to 630 million years ago) and finally the Phanerozoic (630 million years ago until the present).
The difficultly in portraying geological time is the limitation of the media in which the scale of geological time must be presented. If we used a scale diagram and started in Seattle, Washington and ended in Miami, Florida (a distance of 3360 miles or 5408 kilometers), the history of modern man would only show up 1.3 miles or 2.13 kilometers from Miami.
In the mean time - a couple of good diagrams exist. You can view these at International Commission on Stratigraphy
Or by using your image search capable of your favorite search engine and entering the key words Geological Time Scale.
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