What geologic principles are used in relative age dating

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If in location B we find the same fossil assemblage Assemblage 2 in a rock unit, we may assume that they are of essentially the same age as in location A. Crosscutting Relations are those where one rock literally cuts across another, such as for example when igneous dikes and sills are emplaced in fractures within a pile of sedimentary rocks see picture at left.

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Obviously, the sedimentary rocks had to be there prior to emplacement of the igneous rocks, and thus they are older than the igneous rocks. Conversely, the igneous rocks are younger than the sedimentary rocks. Other examples of cross crutting relationships can be related to faults fault has to be younger than the rock it is found in and unconformities see below. Inclusions of one rock in another are a further way of determining relative age relationships.


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Inclusions are always older than the rock they are found in. Even if we did not see the igneous and metamorphic rocks in surface exposures, the fact that they occur in the brownish sediment unit indicates the presence of older ingneous and metamoprhic rocks that supply material to that unit.

If pebbles of this granite are found in the sedimentary sequence, then the granite must be older than the sediments. These principles continue to provide the basic framework within which geologists read the record of Earth history and determine relative ages. The principle of uniformitarianism states that physical processes we observe operating today also operated in the past, at roughly comparable rates, so the present is the key to the past.

The principle of original horizontality states that layers of sediment, when first deposited, are fairly horizontal because sediments accumulate on surfaces of low relief such as floodplains or the sea floor in a gravitational field. If sediments were deposited on a steep slope, they would likely slide downslope before they could be buried and lithified.

With this principle in mind, geologists conclude that examples of folds and tilted beds represent the consequences of deformation after deposition. The principle of superposition states that in a sequence of sedimentary rock layers, each layer must be younger than the one below, for a layer of sediment cannot accumulate unless there is already a substrate on which it can collect.

Thus, the layer at the bottom of a sequence is the oldest, and the layer at the top is the youngest. The principle of lateral continuity states that sediments generally accumulate in continuous sheets within a given region.

If today you find a sedimentary layer cut by a canyon, then you can assume that the layer once spanned the area that was later eroded by the river that formed the canyon. The principle of cross-cutting relations states that if one geologic feature cuts across another, the feature that has been cut is older.

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Geologic Principles for Deļ¬ning Relative Age ~ Learning Geology

For example, if an igneous dike cuts across a sequence of sedimentary beds, the beds must be older than the dike. If a fault cuts across and displaces layers of sedimentary rock, then the fault must be younger than the layers. But if a layer of sediment buries a fault, the sediment must be younger than the fault. Principle of baked contacts: The principle of inclusions states that a rock containing an inclusion fragment of another rock must be younger than the inclusion.

Relative dating

For example, a conglomerate containing pebbles of basalt is younger than the basalt, and a sill containing fragments of sandstone must be younger than the sandstone. Geologists apply geologic principles to determine the relative ages of rocks, structures, and other geologic features at a given location. Examples of geologic events include: The succession of events in order of relative age that have produced the rock, structure, and landscape of a region is called the geologic history of the region.

We can use these principles to determine relative ages of the features.