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 absolute age dating 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. But, before that, they relied upon a different approach to first determine the sequence of important events in Earth's past: relative age dating. Very simply, relative age dating has to do with determining whether one geological or paleontological event happened before or after a second event. Did rock layer A form before or after rock layer B? Did trilobites live before or after the dinosaurs? 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 superposition and cross-cutting relationships.
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. Portrait of Nicholas Steno (public domain; Wikimedia Commons). The principle of superposition 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. 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. Taughannock Falls near Trumansburg, New York, illustrating the Principle of Superposition. Image by Jonathan R. Hendricks. This work is licensed under a Creative Commons Attribution-ShareAlike 4.0 International License. Superposition is observed not only in rocks, but also in our daily lives. Consider the trash in your kitchen garbage can. A rtic le was gener ated by G SA Conte nt Genera tor DEMO.
The trash at the bottom was thrown out earlier than the trash that lies above it; the trash at the bottom is therefore older (and likely smellier!). Or, think about a stack of old magazines or newspapers that might be sitting in your home or 2756&pyt=multi&po=6456&aff_sub5=SF_006OG000004lmDN garage: most likely, the newspapers at the bottom of the pile have dates on them that are older than the newspapers at the top of the pile. The photograph below was captured at Volcano National Park on the Big Island of Hawaii. Use superposition to determine which is older: the road or the lava flow? How do you know? A photograph from Volcano National Park, Big Island of Hawaii. Image by Jonathan R. Hendricks. This work is licensed under a Creative Commons Attribution-ShareAlike 4.0 International License. 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. 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. 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. 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. This post has been written with GSA Content Generator Demoversi on.