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Science
Geological Time

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Terminology
Eons
Eras of the Phanerozoic Eon
The Paleozoic Era
The Mesozoic Era
The Cenozoic Era
The Mnemonic in Full
Where Are We Now?

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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:

Click to show or hide the answer Click to show or hide the answer Click to show or hide the answer Click to show or hide the answer

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
Click to show or hide the answer Click to show or hide the answer Click to show or hide the answer Click to show or hide the answer

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
Click to show or hide the answer Click to show or hide the answer Click to show or hide the answer

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".

The Paleozoic era

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

490–540 mya Name is from the Latin name for Wales Click to show or hide the answer
443–490 mya Named in 1879 after another Welsh tribe, who lived in northern mid–Wales Click to show or hide the answer
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 Click to show or hide the answer
360–417 mya Named after the English county where its rocks were first studied Click to show or hide the answer
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 Click to show or hide the answer
250–300 mya Named after a part of Russia where typical strata were found in the mid–19th century Click to show or hide the answer

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 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 Click to show or hide the answer
145–200 mya Named after a range of mountains in the Alps, where its limestone strata were first identified Click to show or hide the answer
65–145 mya Name comes from the Latin for chalk Click to show or hide the answer

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 dinosaurs became extinct suddenly at the end of the Mesozoic era (referred to as the Cretaceous–Tertiary or K–T boundary).

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
Click to show or hide the answer 57–70 mya (approx) Old new Click to show or hide the answer
35–57 mya (approx) New dawn Click to show or hide the answer
25–35 mya (approx) Few new (new, but few molluscs!) Click to show or hide the answer
Click to show or hide the answer 6–25 mya (approx) Less new Click to show or hide the answer
2.5–25 mya (approx) Newer Click to show or hide the answer
Click to show or hide the answer 12,000 – 2.5 million years ago Newest Click to show or hide the answer
< 12,000 years ago Entirely new Click to show or hide the answer

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 Click to show or hide the answer
Period (the last 2.5 million years) Fourth Click to show or hide the answer
Era (the last 65 million years) New life Click to show or hide the answer
Eon (the last 540 million years) Visible life Click to show or hide the answer

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.

© Haydn Thompson 2017