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.

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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. 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.

Geologic Age Dating Explained

Since, all sedimentary rock is formed through the gradual accumulation of sediment at the surface over time, and since the principle of superposition tells us that newer sediment is deposited on top of older sediment, the same must also be true for fossils contained within the sediment. Although this principle is generally applied to relative dating it is also the basis for evolution. Principles of Relative The Principle of Superposition tells us that deeper layers of rock are older than shallower layers Relative dating utilizes six fundamental principles to determine the relative age of a formation or event.


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Or, think about a stack of old magazines or newspapers that might be sitting in your home or garage: Use superposition to determine which is older: How do you know? The principle of cross-cutting relationships states that a rock unit or other geological feature, such as a fault that is cut by another rock unit or feature must be older than the rock unit or feature that does the cutting. Imagine cutting a slice of bread from a whole loaf.

When investigating rocks in the field, geologists commonly observe features such as igneous intrusions or faults that cut through other rocks. Because these features are the ones doing the cutting, we know that they are younger than the rocks that they cut into. Have a look at the photographs below, which show the curb of a road in a neighborhood in Hollister, California.

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You can see that the curb is offset: As it turns out, the famous San Andreas fault runs below the curb at this location, which has caused the curb to be broken and displaced. We know that the curb was originally straight when it was first constructed. The fault cut the curb and is thus younger than the curb itself. A curb in Hollister, California that is offset by the San Andreas fault.

The cartoon below shows an imaginary sequence of rocks and geological events labeled A-I.

Relative Dating of Rock Layers

Using the principles of superposition and cross-cutting relationships, can you reconstruct the geological history of this place, at least based upon the information you have available? An imaginary cross-section, showing a series of rock layers and geological events A-I. A is a fault. B-F are sedimentary rock layers. G and H are both igneous intrusions.


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Finally, I is an erosional surface. Based on the principles of superposition and cross-cutting relationships, what are the relative ages of these rocks and events?

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Second, we observe that rock layer H which is an igneous intrusion cuts into rock layers B-F. It is therefore younger than B-F. Third, we observe that the fault A cuts across and displaces rock layers B-F. Fourth, we see that G, another igneous intrusion, cuts across A-H; it is therefore younger than all of these note that G is not displaced by A, the fault.

Finally, we note an erosional surface, I, at the top of the sequence and immediately below the corn field that cuts both A and G.

Geologic Age Dating Explained - Kids Discover

I is therefore younger than both A and G. Putting this all together, we can determine the relative ages of these rock layers and geological events:. Given the information available, we cannot resolve whether H is older than A or, vice versa. This problem could be resolved, however, if we were to observe A cutting across H i. What geological principle states that rocks at the bottom of a sequence are older than the rocks above?

What dating approach is used to evaluate the ordering of past geological events?