An Online Guide to Relative Dating

Geology, or the study of rocks, can be a fascinating way to learn about the Earth's history. Relative dating is a key component to unlocking the mysteries of the Earth's past. Four principles of stratigraphy give geologists ways to understand rock layers, including when and how they were created. The Grand Canyon acts as a modern testament to stratigraphy and relative dating. Though radiometric dating offers a modern way to date rocks, the principles of stratigraphy remain a tried and true way to learn about rock layers.

Stratigraphy, or the scientific study of rock layers, includes relative dating. When geologists study layers of rocks, they get a glimpse of the past. Determining the precise way that sedimentary rocks have layered throughout time gives them insight into how natural events have occurred. In some ways, it is a natural record of the Earth's history. To understand stratigraphy and relative dating, four basic principles must be mastered. These concepts have also built scientists' perception of the Earth and its development since the beginning of time. The principles are as follows:

The Principle of Original Horizontality is a relatively simple concept: it states that sediments laid down on the Earth's surface will form even horizontal layers. This means that it can be assumed that non-horizontal layers were somehow disturbed after they were originally laid, as with tilting or folding.

The Principle of Lateral Continuity supposes that rock layers extend for some degree over the Earth's surface. The distance can include meters or kilometers. In stratigraphy terms, this allows layers to be compared against each other.

The Principle of Superimposition states that the oldest rocks will be at the bottom of all layers. In cases were disturbances are suspected, geologists will have to determine what the original layering was by attempting to date the rocks. One easy way geologists can determine if young rocks have been moved to the bottom is by examining the layer for broken and faulted pieces. This can indicate the rocks have been turned, twisted and moved by natural forces, such as continental shifts.

The Principle of Faunal Succession notes that fossils will change in a specific way through different stratigraphic layers. These changes are consistent around the world, and allow geologists to determine what stratigraphic layers they might be examining by simply looking at fossils.

The relative dating of rocks and fossils can be determined by using the Principle of Superimposition. To understand the events in one region as compared to another, however, geologists have to stratigraphically correlate them before examining them. In order to accomplish this, geologists may use one of the concepts listed above: the Principle of Lateral Continuity or the Principle of Faunal Succession. These principles would be able to provide information not only on separate layers, but on the initial layer, as well.

Possibly one of the greatest stratigraphic rock formations is the Grand Canyon. The rocks that combine to create this natural landmark are millions of years old and provide a rich account of the Earth's history. Walls in the Grand Canyon where rocks have not been disturbed are impressive examples of original horizontality. Other areas that have been disturbed and tilted are examples of angular unconformity. Areas of the Grand Canyon that span several hundred kilometers from opposite ends and have not been disturbed are suitable examples of lateral continuity. In areas where rocks from the Proterozoic period are on the lowest layer and younger rocks on the upper layer, the Principle of Superimposition can be seen. Every layer of sedimentary rock in the Grand Canyon contains fossils, which adhere to the Principle of Faunal Succession. Their appearance is consistent, and can be compared to other parts of the world that contain sedimentary rock and fossils that appear the same way.

Biostratigraphy uses fossils to understand correlations between layers. Species, as compared to the Earth's history, live for a relatively small amount of time before they either become extinct or evolve into another animal. With this key fact in mind, rock layers can be examined. If a geologist comes across several rock layers with fossils featuring the same species, it can be assumed that the layers were made well within a few million years of each other. This, in turn, can help with dating the rock layers themselves. An even better way to date rock layers is to identify those species or organisms that have been known to die out or evolve quickly. This can narrow down time frames considerably. It shouldn't be assumed that all rock layers can be represented accurately, however. Though the four principles of relative dating can provide much insight into relative time, natural processes, such as erosion can significantly affect attempts at relative dating.

Radiometric dating has given modern geologists a way to verify their relative dating techniques. Using this practice, geologists can test the accuracy of their sedimentary sequences, in ways other than using fossil records. The radiometric dating, however, can also help provide more information about fossils used during the process. Absolute dating of rocks can also be used as a way to mark strata that has undergone the process of relative dating. The correlation of these two can give information about rocks and fossils that could not be determined before being compared and cross-referenced.

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