Relative age dating and cross cutting relationships
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.Microscopic cross-cutting relations are those that require study by magnification or other close scrutiny.For example, penetration of a fossil shell by the drilling action of a boring organism is an example of such a relation.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? is simple, intuitive, and is the basis for relative age dating.It states that rocks positioned below other rocks are older than the rocks above.
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.
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).
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.
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).