I've been reading a lot of "stuff" recently as part of a personal "hobby" project I'm playing with. Much of this "stuff" deals with the ancient past, so I keep running into terms like the Cambrian Period, the Mesozoic Era, and the Paleocene Epoch.
Most of the time this just washes over me, but then I started wondering what terms such as Eon, Epoch, Era, Period, and Time (in the geologic sense)
actually mean and how do they relate to each other? To cut a long story short, I did a bit of research and discovered that in the context of geology, an eon is defined as the largest division of geologic time, comprising two or more eras; an era is defined as a major division of geologic time composed of a number of periods; a period is the basic
unit of geologic time, during which some standard rock system is formed (a period comprises two or more epochs and is included with other periods in an era); and an epoch is defined as a sub-division of a geologic period during which a geologic series is formed. Finally, the term age is used to refer to some span of time that is shorter than an epoch and that is distinguished by some special feature, such as the Ice Age.
The thing is that I'm a visual person, so in order to get a better grasp on this I created a sort of time-line diagram (the one to the left of the image below). In fact you can't see much detail here, but if you click on this image you'll be presented with a larger, more detailed version (observe that the vertical scale in these illustrations is logarithmic. This provides a method for representing a large span of time while maintaining resolution at the more recent end of the scale.)
Two versions of the geologic timeline
(Click this image to view a larger, more detailed version)
Once I'd finished this first (left-hand) diagram, I was a happy camper . . . for about one day . . . until someone pointed out that the actual definitions of the names and times associated with these various geologic terms are somewhat fluid, because geologists are constantly making new discoveries that cause them to reassess and "tweak" things.
As an example of what I mean, it turns out that the little rapscallions have recently gone through a fairly thorough revision of the time scale (my representation of the new version is shown to the right of the above illustration). Personally, I was a little concerned about the Paleocene Epoch (as I'm sure you will understand), but it looks like this has not been revised away (at least, not yet). Having said this, there are some quite significant changes as follows:
- There is no longer a Tertiary Period. This used to stretch from 65 to 1.8 million years ago and encompass the Paleocene, Eocene,
Oligocene, Miocene, and Pliocene Epochs. (See also point 3 below).
- People are still wrestling with the subject of the Quaternary Period. This used to stretch from 1.8 million years ago to today and encompass the Pleistocene and Holocene Epochs, where the Holocene is the name given to the last 10,000 years or so; that is, since the end of the last major glacial event, or "Ice Age", to the present day. (See also point 3 below).
- An acceptable version of current consensus (the "latest-and-greatest" as it were) that integrates currently available stratigraphic and geochronologic information is known as the Geologic Time Scale 2004 (GTS2004). Boiled down, the current state-of-play
is as follows:
- The old Tertiary Period has been renamed the Paleogene Period, which is truncated at the end of the Oligocene Epoch (that is, the
old Tertiary Period used to encompass the Paleocene, Eocene, Oligocene, Miocene, and Pliocene Epochs, but the new
Paleogene Period encompasses only the Paleocene, Eocene, and Oligocene Epochs).
- The old Quaternary Period has been remaned the Neogene Period, and this now extends to encompass the Miocene and Pliocene Epochs (that is, the
old Quaternary Period used to encompass only the Pleistocene and Holocene Epochs, while the Neogene Period has been extended
to also encompass the Miocene and Pliocene Epochs).
- The Quaternary is now regarded as being a sub-period of the Neogene Period.
- The Holocene is now regarded as being a sub-epoch of the Pleistocene Epoch.
- A lot of the dates associated with the various Periods and Epochs have been "tweaked". In the right-hand side of the above illustration I've mostly rounded things to the nearest million years.
So is everything now tied down and fixed in stone (if you'll forgive the pun)? You wish! In fact, work is already well underway on the next revision of the Geologic Time Scale as I pen these words. I don't like change, I'm too young for all of this. How many of these diagrams am I going to have to draw?
Questions? Comments? Feel free to email me – Clive "Max" Maxfield – at email@example.com). And, of course, if you haven't already done so, don't forget to Sign Up for our weekly Programmable Logic DesignLine Newsletter.