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The earth is approximately 4.5 billion years old and from the beginning to the present geologists from around the world have created a Geologic time scale of the earth’s history. the term EON on the geologic time scale is a grand division or the largest time unit of the geologic time scale a magnitude above the era. The ERA denotes the age of life that lived with in the era thus Cenozoic means “recent life” Mesozoic means “middle life” and Paleozoic means “ancient life All the terms are from the Greek language”. The PERIODS were named by early geologist from around the world. They were named after land marks, people or descriptive terms from Latin or Greek.
©GEOLOGY AS A SCIENCE
Uniformitarianism The earth has been around for a long time, about 4.5 billion years or so. And the processes that created the earth as we know it today have been at work for the same period of time and this belief is called uniformitarianism. Let me explain we know erosion is caused by wind or water well uniformitarianism says in effect that erosion 4.5 billion years ago was also caused by wind and water and that all geologic processes at work then are the same today. The law of superposition When sediments are laid down whether they be sand, clay pebbles or silt they form beds of sediment and divisions between them called bedding planes like you see when you go to the Grand Canyon in Arizona. The beds can be very thick or very thin. They can be deposited by the wind or laid down in an ocean or a flood plain setting. They can be red, white, grey, or what ever. But one thing they all have in common is that under normal circumstances the bed that overlies another bed is younger than the bed beneath it. This is called the law of superposition. Geologist uses this law of superposition to determine the relative age of events recorded in the rocks. Example if you find a dinosaur bone in a layer of rock and just beneath it you find a another layer of rock with petrified wood then you know under normal circumstances the petrified wood must be older than the dinosaur bone. And it most cases this is true but I have used the term under normal circumstances a lot that means there are exceptions to the law of superposition and the exceptions are when beds and bedding planes are over turned by faulting and folding of rock layers then in this case you must determine whether or not faulting or folding has taken place before you determine the relative positions of the beds or bedding planes. Unconformities Because of time, earth movements and erosion unconformities can exist between bedding planes. For example suppose you observe beds below lying at an angel and the beds on top are horizontal this is called an angular unconformity and this is formed when the beds were originally deposited in a horizontal position were tilted or folded and then eroded down and then other beds were deposited on top in a horizontal position. A disconformity exists when two parallel beds are deposited one on top of the other with an erosion surface between. A well marked surface between two formations is called a contact.
Geologic Resources - Resources for geoscientists, engineers, and related professionals.
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