Relative age dating and cross cutting relationships
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? If you are a member, we ask that you confirm your identity by entering in your email.You will then be sent a link via email to verify your account.We know that the curb was originally straight when it was first constructed. This work is licensed under a Creative Commons Attribution-Share Alike 4.0 International License.The fault cut the curb and is thus younger than the curb itself. This work is licensed under a Creative Commons Attribution-Share Alike 4.0 International License. Based on the principles of superposition and cross-cutting relationships, what are the relative ages of these rocks and events? 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. 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).Use superposition to determine which is older: the road or the lava flow? 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.
Through use of techniques (which were developed during the 20th century; see Section 2), they were able to later assign dates in years before the preset to important events in Earth's history.
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.
But, before that, they relied upon a different approach to first determine the sequence of important events in Earth's past: Relative age dating has to do with determining the temporal ordering of events in Earth's past.
Geologists employ a handful of simple principles in relative age dating; two of the most important of these are are the principles of Just as uniformitarianism is the key underlying assumption of geology, the science's most fundamental principle is superposition, developed by Danish anatomist Nicholas Steno (1638-1686) in the 17th century.