Relative age dating and cross cutting relationships
An imaginary cross-section, showing a series of rock layers and geological events (A-I).
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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.
Based upon such compound cross-cutting relations it can be seen that the fault is older than the unconformity which in turn is older than the dike.
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 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.It may surprise you to learn that geologists were able to determine much of the history of the Earth and its life without knowing anything about the actual ages of the rocks that they studied.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.
Microscopic cross-cutting relations are those that require study by magnification or other close scrutiny.