- Geology Online Subchapter
- The Rules for Relative Dating
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Alas Hutton was a poor writer or popularizer, so his ideal got little attention until Intellectual successor to James Hutton. Published Principles of Geology three-volume set, The first geology textbook popularized uniformitarianism. Although modern textbooks treat uniformitarianism as something unambiguusly good , it has it's downsides: The early uniformiatrians went far beyond the philosophy stated above.
They also maintained that all geologic processes happened at the same rate in the past as today, and that time was eternal or maybe cyclical. Hutton claimed that the Earth had, "no vestage of a beginning and no prospect of an end. Probable killer of the giant dinosaurs, for example. The biggest revelation was simply that the Earth had to be millions, not thousands of years old. Scroll to the bottom of this link for and example. Thus the notion of Geological time was born. Now the problem was how to measure it.
There are now two complimentary approaches: In which the order of the origin of various rock units is determined. Absolute or numerical dating: In which an acutal numerical chronologic age is established for rocks. More During the 19th century, however, geologists could only establish the relative ages of rock units.
The means to do so had already been provided by Steno and Hutton, but only provided rocks could be seen in direct association. Once again, the study of fossils provided the stimulus for a major innovation: Our understanding of fossils has a varied history: In classical antiquity, scholars knew of and were interested in the bones of ancient animals.
Roman Emperor Augustus publicly displayed ancient arms and armor plus the bones of "giants" and other mythical creatures at his estate in Capri. During the Middle Ages and Renaissance, learned people considered fossils to be "sports of nature" - mineral assembleages that coincidentally seemed to resemble living things.
Geology Online Subchapter
In Prodromus , Steno argued that fossils are the remains of organisms, using fossil shark teeth as example. In William Smith , a British civil engineer, added a fourth principle: Faunal succession , noting that different groups of fossil organisms were preserved in different rock units. By this means it became possible to say that one rock was the same age as another rock halfway around the world. However, you have to use the right fossils: Fossils of organisms that existed for short perios of geologic time but were geographically widespread.
Ammonites, Shelled cephalopods that evolved quickly so each species lasted only a few million years, but whose remains were distributed worldwide in many environments. Sphenodiscus lenticularis - an ammonite. Fossils of organisms that endured for long periods of geologic time but were linked to a specific environment. Lingula , a brachiopod living only in lagoonal mud-flats that has changed very little in the last million years.
Modern left and ancient right examples of the brachiopod Lingula. Of course microscopic index fossils can be superabundant, even in small rock samples, so they are particularly useful.
The Geologic Time Scale: Using Steno's, Hutton's, and Smith's principles, Geologists gradually developed, a standardized a Geologic Time scale developed. Link to it in simplified form or in its full glory.
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An internested heirarchy of larger and smaller time units. From most to least inclusive: Eon Era Period Epoch Time units had descriptive names, usually based on: The localities at which characteristic rocks were exposed E.
- Introduction to Physical Geology Syllabus.
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- Relative dating Law of superposition Law of horizontality Original;
Jurassic for Jura Mts. Characteristic rock types E.
The Rules for Relative Dating
Cretaceous for "creta" - Latin for Chalk Descriptions of fossil life found in them E. Eocene - "Dawn of the recent". Subsequent layers would follow the same pattern.
As sediment weathers and erodes from its source, and as long as it is does not encounter any physical barriers to its movement, the sediment will be deposited in all directions until it thins or fades into a different sediment type. For purposes of relative dating this principle is used to identify faults and erosional features within the rock record. The principle of cross-cutting states that any geologic feature that crosses other layers or rock must be younger then the material it cuts across. Using this principle any fault or igneous intrusion must be younger than all material it or layers it crosses.
Once a rock is lithified no other material can be incorporated within its internal structure. In order for any material to be included within in the rock it must have been present at the time the rock was lithified. For example, in order to get a pebble inside an igneous rock it must be incorporated when the igneous rock is still molten-- such as when lava flows over the surface. Therefore, the piece, or inclusion, must be older than the material it is included in. Lastly the Principle of Fossil Succession.
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Aside from single-celled bacteria, most living organism reside at or very near the Earth's surface either in continental or oceanic environments. As these organisms die they are deposited on the surface along with all other sediments. If conditions are right the remains of the dying organisms can then be preserved as fossils within the rock that formed from sediments that covered the remains.