Geological Time

We've all seen those diagrams that illustrate the various divisions of geological time; here is an example. But there's always a lot of information on them that a quizzer doesn't need to know. This page attempts to summarise what you do need to know.

Terminology

The first thing we need to remember is the names of the four main types of subdivision of geological time – the layers in the hierarchy. In increasing order of length, they are:

So several epochs make a period, several periods make an era, and several eras make an eon.

This gives us the acronym EPEE – which (as you know) is a type of sword. The problem, of course, is that it doesn't help us to distinguish between the three Es; so I use a second acronym: PRO – which tells me the second letter in each word – so it's ePoch, period, eRa, eOn. Now I just have to remember that this is in increasing order of length.

Epochs are sometimes divided into ages. But I don't remember ever hearing a quiz question about geological ages.

Eons

The Earth was created about 4.6 billion years ago, and the first life appeared about half a billion years later.

At one time there were only two eons, and they both started with P. Then the first one was divided into three and became a supereon. So we now have four eons (the first row gives the number of years ago for each one):

	> 4 billion		2.5 – 4 billion		0.54 – 2.5 billion		< 540 million
	Hadean		Archean		Proterozoic		Phanerozoic

The first three eons are now referred to as the Precambrian "supereon" (because the Cambrian is the first period in the Phanerozoic eon).

It was just before the start of the Phanerozoic eon that the first multi–celled animals appeared.

The first three eons (the ones that make up the Precambrian supereon) are subdivided as follows:

Eon	Eras
Hadean	Cryptic, Basin Groups, Nectarian, Early Imbrian
Archean	Eoarchean, Paleoarchean, Mesoarchean and Neoarchean (dawn, old, middle and new ancient)
Proterozoic	Paleoproterozoic, Mesoproterozoic and Neoproterozoic (old, middle and new early life)

The above table is adapted from Wikipedia, which says the Cryptic era is an informal term, used because there is very little information about this time. The term Basin Groups refers to the impact craters that are used to define this time period. The Nectarian era is named after a crater on the moon which (along with other great lunar basins) was formed at this time. The Early Imbrian era is named after the Mare Imbrium (Sea of Showers), also on the Moon, which is a sea of lava that occupies part of an impact crater that was formed at this time.

Eras of the Phanerozoic eon

The word "Phanerozoic" is derived from the Greek words meaning "visible life". The Phanerozoic eon is divided into three eras (as before, the first row gives the number of years ago for each one):

	254 – 541 million		66 to 252 million		< 66 million
	Paleozoic		Mesozoic		Cenozoic

No prizes for guessing (and here's a clue, if you haven't already revealed the answers) that the names of these three eras mean "old life", "middle life" and "new life".

These three eras were formerly known as the Primary, Secondary and Tertiary periods. The Tertiary was later split by adding a Quaternary period. The first three have since been replaces by the names we see in the table above, but the Quaternary remains as the third and last period in the Cenozoic era (see below).

The Paleozoic era

The six periods of the Paleozoic era are (earliest first):

490–540 mya	Name is from the Latin name for Wales	Cambrian
443–490 mya	Named in 1879 after another Welsh tribe, who lived in northern mid–Wales	Ordivician
417–443 mya	Named after the tribe that inhabited south–east Wales, where its fossil–bearing rocks were studied in the 1830s, in pre–Roman times	Silurian
360–417 mya	Named after the English county where its rocks were first studied	Devonian
300–360 mya	The name (the first of the modern 'system' names) reflects the fact that it's when many coal beds were formed around the world, including in the UK	Carboniferous
250–300 mya	Named after a part of Russia where typical strata were found in the mid–19th century	Permian

Here i before e except after c (for details, please refer to my Quiz Books page) helps us with a mnemonic:

CAMels Often Sit Down CARefully. PERhaps ...

This is obviously not complete; more will be revealed in the next section.

The 'boundary' between the Permian and Triassic periods (and the Paleozoic and Mesozoic eras) is marked by the Earth's most severe known extinction event, when some 57% of all biological families and 83% of all genera became extinct. The cause of this event is not clear; it may have been one or more large meteor impact events, massive volcanic eruptions, or climate change brought on by large releases of underwater methane, methane–producing microbes, or combustion of fossil fuels. The event is known as the Permian–Triassic extinction event, the P–Tr extinction, the P–T extinction, the End–Permian Extinction, or (colloquially) 'the Great Dying'.

The Mesozoic era

The three periods of the Mesozoic era are (earliest first):

200–250 mya	Name refers to the three typical rock layers that are found throughout north–western Europe	Triassic
145–200 mya	Named after a range of mountains in the Alps, where its limestone strata were first identified	Jurassic
65–145 mya	Name comes from the Latin for chalk	Cretaceous

Our mnemonic (courtesy of the aforementioned i before e except after c) continues (remembering that the previous section ended with Perhaps):

Their Joints CREak?

The Mesozoic is sometimes known as the Age of Reptiles; it's when the dinosaurs flourished. Dinosaurs first appeared in the middle of the Triassic period, and (as Michael Crichton and Steven Spielberg have reminded us) dominated during the Jurassic. They died out suddenly at the end of the Mesozoic era, at a point in time referred to as the Cretaceous–Tertiary or Cretaceous–Paleogene boundary – abbreviated to K–T or K–Pg. ('K' is the traditional abbreviation for the Cretaceous period, from the German word for chalk. As we saw above, the Mesozoic era was previously known as the Secondary period, and the Cenozoic as the Tertiary.)

The Cenozoic era

The three periods of the Cenozoic Era are subdivided into a total of seven epochs. At this stage, our mnemonic (originally from i before e (except after c), but amended by me) shifts from periods to epochs. I suppose the reasoning is that because this is the current era, we move to a new level of detail. The following table covers both the periods and the epochs of the Cenozoic era:

Period	Time	Meaning	Epoch
Paleogene	57–70 mya (approx)	Old new	Paleocene
	35–57 mya (approx)	New dawn	Eocene
	25–35 mya (approx)	Few new (new, but few molluscs!)	Oligocene
Neogene	6–25 mya (approx)	Less new	Miocene
Neogene	2.5–25 mya (approx)	Newer	Pliocene
Quaternary	12,000 – 2.5 million years ago	Newest	Pleistocene
Quaternary	< 12,000 years ago	Entirely new	Holocene

And the final line of our mnemonic is:

Painful! Early Oiling Might Prevent Permanent Harm.

The terms Primary, Secondary and Tertiary – previously used along with Quaternary for the periods of the Cenozoic era – are no longer used. They've been replaced by the Paleogene and the Neogene periods.

The Mnemonic in Full

Just to recap, here is the mnemonic in full:

Periods of the Paleozoic era	CAMels Often Sit Down CARefully. PERhaps ...
Periods of the Mesozoic era	... Their Joints CREak?
Epochs of the Cenozoic era	Painful! Early Oiling Might Prevent Permanent Harm

Where are we now?

By way of a recap, the following table gives the names of the eon, era, period, and epoch that we're currently living in:

		Meaning
Epoch	(the last 12,000 years)	Entirely recent	Holocene
Period	(the last 2.5 million years)	Fourth	Quaternary
Era	(the last 65 million years)	New life	Cenozoic
Eon	(the last 540 million years)	Visible life	Phanerozoic

Australopithecus, the earliest genus of ape from which humans are descended but chimpanzees and bonobos are not, was living in Africa in the mid–to–late Pliocene epoch. They began using stone tools about 3.3 million years ago, and this marked the beginning of the Stone Age.

The Pleistocene epoch began about 2.5 million years ago and ended about 12,000 years ago. This makes it about two hundred times as long as the Holocene (so far ...)

Homo habilis – "handy man" – lived in the early Pleistocene epoch – from about 2.3 to 1.4 million years ago. He was followed by Homo erectus, Neanderthal Man, and finally (about 200,000 years ago) Homo sapiens.

The last Ice Age began about 110,000 years ago, and ended about 10,000 years ago – making it roughly contemporaneous with the Pleistocene epoch. The Stone Age ended at around the same time; the Bronze Age began a little over 5,000 years ago, which was late in the Holocene epoch.

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