Note: The content of this post is from answers to questions posed on Quora, written by Dylan Duverge. For the originally published content, Click Here.
Geologists, paleontologists, geophysicists and evolutionary biologists have worked together to construct, to the best of our ability, a record of the past, as revealed by careful geologic observation and evolutionary reconstruction. There are several principles and techniques that are used to construct the geologic time scale, outlined below.
- Geologic mapping and the law of superposition: Rocks are formed by layers of sediment or lava deposited one on top of the other over time. This means that deeper rocks are older than shallower rocks, as long as they haven’t been overturned by tectonic forces. By carefully examining and mapping rock outcrops, understanding the orientation of rock layers, and linking similar rock types over large areas, you can start to get an idea of how they are layered and which rock formations might be older than others. As an example, the image below is from a geologic map from an artistic team of geologic mappers (Dibblee and Minch) for an area in California (Bitterwater Valley). The youngest geology is the valley floor (very light beige), since it is loose sand deposited by streams. The bolder colors represent true sedimentary rocks, which get older from left to right (for the most part). The second image shows one of the first geologic profiles done by one of founders of modern geology (James Hutton) in England. There, the rocks get younger towards the right-hand side.
- Fossil discoveries: By discovering, cataloguing and identifying fossils, not just the cool dinosaurs, but especially the less “sexy” fossils like seashells (e.g., Ammonites, Brachiopods, Trilobytes, etc.), geologists can start to recognize distinct evolutionary time periods in the rock record. Especially useful fossils for reconstructing geologic time are “marker fossils,” which due to their distinctiveness, abundance, and brief occurrence in the past, can be traced across geologic environments all over the world. Geology and paleontology can in this way be used to determine the age of rocks relative to one another, but not an exact date.
- Radiometric (Carbon) Dating: One of the most important discoveries in the development of the geologic time scale was the ability to use the decay rate of isotopes in certain elements to backtrack when that element was formed. The most commonly known is carbon dating, whereby organic matter trapped in geologic formations can be sampled and given an approximate age. This ability very rapidly advanced our understanding of the geologic time scale. Today, additional “absolute” dating techniques are used that are useful for rock types that do not have carbon in them, like lavas and granites (e.g., luminescence dating), or rock formations that are too old for carbon dating (e.g., potassium-argon dating).
There have been likely been many techniques used to pinpoint the dividing lines between geologic time periods, but the aforementioned ones are the most critical. Below is the most illustrative representation of the geologic time scale I found in Google images. It doesn’t represent the length of the time periods well, but illustrates the correlation with different rock types and fossils, which is why I like it.