Dating Rocks and Fossils Using Geologic Methods
Dating Fossils – How Are Fossils Dated?
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Despite seeming like a relatively stable place, the Earth's surface has changed dramatically over the past 4. Mountains have been built and eroded, continents and oceans have moved great distances, and the Earth has fluctuated from being extremely cold and almost completely covered with ice to being very warm and ice-free. These changes typically occur so slowly that they are barely detectable over the span of a human life, yet even at this instant, the Earth's surface is moving and changing.
As these changes have occurred, organisms have evolved, and remnants other methods of absolute dating some have been preserved as fossils. A fossil other methods of absolute dating be studied to determine what kind of organism it represents, how the organism lived, and how it was preserved. However, by itself a fossil has little meaning unless it is placed within some context.
The age of the fossil must be determined so it can be compared to other fossil species from the same time period. Understanding the ages of related fossil species helps scientists piece together the evolutionary history of a group of organisms. For example, based on the primate fossil record, scientists know that living primates evolved from fossil primates and that this evolutionary history took tens of millions of years.
By comparing fossils of different primate species, scientists can examine how features changed and how primates evolved through time. However, the age of each fossil primate needs to be determined so that fossils of the same age found in different parts of the world and fossils of different ages can be compared. There are three general approaches that allow scientists to date geological materials and answer the question: Relative dating puts geologic events in chronological order without requiring that a specific numerical age be assigned to each event.
Second, it is possible to determine the numerical age for fossils or earth materials. Numerical ages other methods of absolute dating the date of a geological event and can sometimes reveal quite precisely when a fossil species existed in time. Third, magnetism in rocks free online dating sites for usa be used to estimate the age of a fossil site. This method uses the orientation of the Earth's magnetic field, which has changed through time, to determine ages for fossils and rocks.
Geologists have established a set of principles that can be applied to sedimentary and volcanic rocks that are exposed at the Earth's surface to determine other methods of absolute dating relative ages of geological events preserved in the rock record. For example, in the rocks exposed in the walls of the Grand Canyon Figure 1 there are many horizontal layers, which are called strata.
The study of strata is called stratigraphyand using a few basic principles, it is possible to work out the relative ages of rocks. Individual rock layers, or strata, can be seen exposed in the wall of the Grand Canyon in Arizona, USA. Just as when they were deposited, the strata are mostly horizontal principle of original horizontality.
The layers of rock at the base other methods of absolute dating the canyon were deposited first, and are thus older than the layers of rock exposed at the top principle of superposition. In the Grand Canyon, the layers of strata are nearly horizontal. Most sediment is either laid down horizontally in bodies of water like the oceans, or other methods of absolute dating land on the margins of streams and rivers.
Each time a new layer of sediment is deposited it is laid down horizontally on top of an older layer. This is the principle of original horizontality: Thus, any deformations of strata Figures 2 and 3 must have occurred after the rock was deposited. The principles of stratigraphy help us understand the relative age of rock layers. Layers of other methods of absolute dating are deposited horizontally at the bottom of a lake principle of original horizontality.
Younger layers other methods of absolute dating deposited on top of older layers principle of superposition. Layers that cut across other layers are younger than the layers they cut through principle of cross-cutting relationships. The principle of superposition builds on the principle of original horizontality. The principle of superposition states that in an undeformed sequence of sedimentary rocks, each layer of rock is older than the one above it and younger than the one below it Figures 1 and 2.
Accordingly, the oldest rocks in a sequence are at the bottom and the youngest rocks are at the top. Sometimes sedimentary rocks are disturbed by events, such as fault movements, that cut across layers after the rocks were deposited. This is the principle of cross-cutting relationships. The principle states that any geologic features that cut across strata must have formed after the rocks they cut through Figures 2 and 3.
The sedimentary rock layers exposed in the cliffs at Zumaia, Spain, are now tilted close to vertical. According to the principle of original horizontality, these strata must have been deposited horizontally and then titled vertically after they were deposited. In addition to being tilted horizontally, the layers have been faulted dashed lines on figure. Applying the principle of cross-cutting relationships, this fault that offsets the layers of rock other methods of absolute dating have occurred after the strata were deposited.
The principles of original horizontality, superposition, and cross-cutting relationships allow events to be ordered at a single location. However, they do not reveal the relative ages of rocks preserved in two different areas. In this case, fossils can be useful tools for understanding the relative ages of rocks. Each fossil species reflects a unique period of time in Earth's history. The principle of faunal succession states that different fossil species always appear and disappear in the same order, and that once a fossil species goes extinct, it disappears and cannot reappear in younger rocks Figure 4.
The principle of faunal succession allows scientists to use the fossils to understand the relative age of rocks and fossils. Fossils occur for a distinct, limited interval of time. In the figure, that distinct age range for each fossil species is indicated by the grey arrows underlying the other methods of absolute dating of each fossil.
Absolute dating methods (ANT)
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