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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. This problem could be resolved, however, if we were to observe A cutting across H (i.e., the fault displacing the igneous intrusion).

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?

Say for example that a volcanic dike, or a fault, cuts across several sedimentary layers, or maybe through another volcanic rock type.

Pretty obvious that the dike came after the rocks it cuts through, right?

You can see that the curb is offset: the bottom half does not line up with the top half.

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.

Imagine cutting a slice of bread from a whole loaf.Because of cross-cutting relationships, the cut that divides the slice from the rest of the loaf is younger than the loaf itself (the loaf had to exist before it could be cut).When investigating rocks in the field, geologists commonly observe features such as igneous intrusions or faults that cut through other rocks.The image below shows a sequence of Devonian-aged (~380 Ma) rocks exposed at the magnificent waterfall at Taughannock Falls State Park in central New York.The rocks near the bottom of the waterfall were deposited first and the rocks above are subsequently younger and younger. This work is licensed under a Creative Commons Attribution-Share Alike 4.0 International License.Yet, you’ve heard the news: Earth is 4.6 billion years old. That corn cob found in an ancient Native American fire pit is 1,000 years old. Geologic age dating—assigning an age to materials—is an entire discipline of its own.

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