But caution must be taken, as there may be situations in which the rules are not valid, so local factors must be understood before an interpretation can be made. There are a few simple rules for doing this. The simplest and most intuitive way of dating geological features is to look at the relationships between them. The principle becomes quite complex, however, given the uncertainties of fossilization (Paleontology), the localization of fossil types due to lateral changes in habitat (facies change in sedimentary strata), and that not all fossils may be found globally at the same time.19.2 Relative Dating Methods Relative Dating Principles Based on principles laid out by William Smith almost a hundred years before the publication of Charles Darwin's theory of evolution, the principles of succession were developed independently of evolutionary thought. As organisms exist at the same time period throughout the world, their presence or (sometimes) absence may be used to provide a relative age of the formations in which they are found. The principle of faunal succession is based on the appearance of fossils in sedimentary rocks. This principle allows sedimentary layers to be viewed as a form of vertical time line, a partial or complete record of the time elapsed from deposition of the lowest layer to deposition of the highest bed. Logically a younger layer cannot slip beneath a layer previously deposited. The principle of superposition states that a sedimentary rock layer in a tectonically undisturbed sequence is younger than the one beneath it and older than the one above it. Observation of modern marine and non-marine sediments in a wide variety of environments supports this generalization (although cross-bedding is inclined, the overall orientation of cross-bedded units is horizontal). The principle of Original Horizontality states that the deposition of sediments occurs as essentially horizontal beds. A fundamental principle of geology advanced by the 18th century Scottish physician and geologist James Hutton, is that "the present is the key to the past." In Hutton's words: "the past history of our globe must be explained by what can be seen to be happening now." The principle of uniformitarianism states that the geologic processes observed in operation that modify the Earth's crust at present have worked in much the same way over geologic time. As a result, xenoliths are older than the rock which contains them. These foreign bodies are picked up as magma or lava flows, and are incorporated, later to cool in the matrix. A similar situation with igneous rocks occurs when xenoliths are found. For example, in sedimentary rocks, it is common for gravel from an older formation to be ripped up and included in a newer layer. The principle of inclusions and components states that, with sedimentary rocks, if inclusions (or clasts) are found in a formation, then the inclusions must be older than the formation that contains them. Finding the key bed in these situations may help determine whether the fault is a normal fault or a thrust fault. Faults are younger than the rocks they cut accordingly, if a fault is found that penetrates some formations but not those on top of it, then the formations that were cut are older than the fault, and the ones that are not cut must be younger than the fault. The principle of cross-cutting relationships pertains to the formation of faults and the age of the sequences through which they cut. There are a number of different types of intrusions, including stocks, laccoliths, batholiths, sills and dikes. In geology, when an igneous intrusion cuts across a formation of sedimentary rock, it can be determined that the igneous intrusion is younger than the sedimentary rock. The principle of intrusive relationships concerns crosscutting intrusions. Many of these involve the ability to provide the relative ages of strata or the manner in which they were formed. There are a number of important principles in geology.
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