|Geologic column or time scale. The duration of each eon
increases substantially from left to right.
The black and white bars to the left in the Cenozoic and Mesozoic are the magnetic time scale, which we will see later in the course.
(Walker and others, 2014)
The geologic time column as we know it developed in the early 19th century. An older scheme had used Primary (oldest), Secondary, Tertiary, and Quaternary (youngest), and the new scheme retained Tertiary and Quaternary but replaced the others. Tertiary survived until the early 21st century (it is now the Paleogene and Neogene), but Quaternary is still present at the very top of the diagram. The time scale is depicted as a column of rock, with the oldest on the bottom and the youngest on the top (superposition). Where required the column can be displayed in sections, as in the depiction above.
When the time scale was developed, it relied entirely on relative dating, based on superposition. The observations in one location was extended to other locations by correlating the fossils. Small, rapidly evolving, common pelagic organisms make the best index fossils, but the original development was entirely pragmatic and empirical based on observations that fossil faunas occurred in a predictable, coherent fashion as show by Smith and his geologic map on England. The work predated Darwin's theory, and any accurate idea of how old the earth was.
Early quantitative attempts to develop an absolute age of the earth tried to use the build up of salt in the oceans, or the thermal cooling of the earth from an molten mass done by Lord Kelvin got huge numbers from the human perspective but woefully undershot the true age. When an internal heat source from radioactive decay was discovered, it also provided a way to get the actual, absolute ages. The time scale above combines fossil correlations with radiometric ages.
The first level of the time scale are the eons: Cenozoic (recent life), Mesozoic (middle life), Paleozoic (ancient life), and Precambrian. The tops of the Paleozoic and Mesozoic correspond to two of the greatest extinction events for life of earth, and it was this turnover of the fossils that led to the selection of the boundaries. The first abundant fossils with hard shells appear in the Cambrian, so when the time scale was developed in the early 1800's the Precambrian was largely without know fossils. While we know have firm evidence that life originated much earlier, the fossil evidence is much less robust that what appears in the Cambrian explosion. Going backward in time the eons become longer, because fewer rocks have been preserved.
The next level are the periods. Those in the Paleozoic are named for regions, mostly in England, where they were defined for the distinctive fossils. The Mesozoic eras are named for threefold division of the European Triassic, the French Jura mountains, and the chalky Cretaceous (often uses the letter K, from the German for chalk, which is where get got the older KT boundary acronym, and the current KP).
The next level are the epochs. Within the Cenozoic, these were originally established by Lyell based on the fraction of the fossils that were present. Eocene was "dawn of the recent" (Paleocene was added later), Miocene was "less recent", and so on.
The next level are the ages, which are frequently used in geological ocean0graphy because of the resolution with which we can date marine sediments.
The color palette shows age, and a quick glance will show the age distribution on a map. Most maps will not have all the time intervals represented, and might have to vary the assignments somewhat, but the Quaternary "dirt" is almost always shown in yellow.
1. Ma replaces m.y.B.P. (million years before present); for example the KP
boundary was at 65 Ma,
2. m.y. is a duration, like "5 m.y. in the early Cambrian."
Last revised 8/13/2016