Callan Bentley, Karen Layou, Russ Kohrs, Shelley Jaye, Matt Affolter, and Brian Ricketts and Charlene Estrada
Since 4.54 byr is a large chunk of time, geologists have divided it into more manageable chunks by creating a time scale. The commonly accepted time scale comes from the International Commission on Stratigraphy (Figure 3.1). It is continually revised as new research fine-tunes numbers between time scale divisions. The one in Figure 3.1 is the most up-to-date at the time of this writing and will be referenced throughout this manual. The divisions on the time scale are often based on significant events that have taken place tectonically, biologically, or climatically. The numerical ages are derived from the radiometric dating of rocks, minerals, and fossils.
Geologic time is first divided into s; these are the Hadean, Archean, Proterozoic, and Phanerozoic. The first three eons are often called the Precambrian, which we’ll call a “super” eon. The eons are subdivided into eras, and eras are subdivided into periods, periods into epochs, and epochs into ages.
Figure 3.1 – The geologic time scale subdivided by eon, era, system, series, epoch, and stage. What do you observe about each period of time? It is simple; they are color-coded. We will use these standard colors throughout this lab manual. The golden spikes at age boundaries indicate a specific place used to define the age boundary. Image credit: International Commission on Stratigraphy.Geologists use abbreviations to refer to the different portions of geologic time (Table 3.2), particularly on geologic maps. A few of these abbreviations may seem puzzling because the abbreviation isn’t always the first letter of the name. Carboniferous got the “C”, so Cambrian got a C with a slash through it “Ꞓ“. This left Cretaceous with a “K.” Tertiary had originally received the “T”, which left Triassic to be symbolized with a T that had an R subscript. In 2003, “Tertiary” was stricken from the geologic timescale and replaced by the Paleogene and Neogene. You will still see “T” on older maps for Tertiary, and some geologists still use the term today. So, be careful! There were already some M’s and P’s; so, Proterozoic eras were assigned as X, Y, and Z.Geologists use two methods for dating events in Earth’s history. The first is called relative dating, meaning how events relate to each other in time, or more plainly, they figure out the sequence of events (what came first, second, third, etc.). Relative dating has no regard for numerical ages. The second method is absolute dating, where geologists use radioactive isotopes to figure out the numerical age of a rock or mineral.
