Geological time is challenging to visualise.

You might say it is unimaginably vast. Here is a handy diagram showing major events that have occurred since the formation of the Earth. Take a moment to look at it and see if you can visualise what even a million years means.

Geological time shown in a circle

Note: Ma = million years, Ga = billion years = 1000 million years.

Tricky, huh? If we think about this in terms of percentages, it becomes really clear that the earlier parts in earth history are vast, compared to the bits where, for example, animals have been around:

Geological time shown in a circle

If you want to explore these relationships a little further, you can find a live version of this chart at this location.

An analogy

That still doesn't really help us get our heads around this, though, does it? Perhaps a better way of achieveing this is to imagine the geological timescale as a 24 hour clock. If we do this, and map the geological column against our 24 hour clock, we find:

Period
Start
End
Major Events
Hadean 0:00:00 4:10:26 Life may well have been around here, but we don't have definitive evidence this was the case, as you will learn in the first lecture
Archean 4:10:26 10:57:23 Our earliest fossil evidence of life lies in this period, at around 5.45
Proterozoic
10:57:23
21:10:20
 Complex cells - i.e. Eukaryotes - present by 16.00, multicellular life by 18.30
Cambrian
21:10:20
21:27:08
 Extant phyla appear in the Cambrian explosion shortly after 21.10
Ordovician
21:27:08
21:41:06
 The first complex ecosystems on land start to form by 21.41
Silurian
21:41:06
21:49:46
 Fossil evidence of plants and animals on land appears
Devonian
21:49:46
22:07:33
 Vertebrates venture onto land; boney fish in the seas
Carboniferous
22:07:33
22:26:24
 Most of our coal dates from here
Permian
22:26:24
22:41:26
 Aminiotes ( reptiles, birds, and mammals) successful
Triassic
22:41:26
22:57:31
 Dinosaurs appear.
Jurassic
10:57:31
23:14:27
 Dinosaurs, dinosaurs, dinosaurs
Cretaceous
23:14:27
23:39:30
 BOOM no more dinosaurs at 23:40
Paleocene
23:39:30
23:42:28
 Mammals can start to dominate
Eocene
23:42:28
23:49:23
 Mammal successful, climate warm and wet
Oligocene
23:49:23
23:52:48
 Continents approach current positions
Miocene
23:52:48
23:58:20
 The first Hominids evolve
Pliocene
23:58:20
23:59:11
 Getting colder...
Pleistocene
23:59:11
0:00:00
 Ice ages!
Holocene
0:00:00
0:00:00
Everyone you love, everyone you know, everyone you ever heard of, every human being who ever was, lived out their lives, in the last seconds before midnight.



Want to learn more?

This video - from and copyright of - PBS Eons, covers much of the same ground as the above table. It gives a good overview of what happened when in relation to the geological timescale:

The Geological Column

In the 19th Century, the succession of fossils in rocks was used to construct a timescale for the more recent rocks. With the advent of radiometric dating, we have been able to calibrate that to actual time in millions of years, and with more work, we have also filled in the older bits of the timescale. Which brings us to the geological column as we understand it today, for the period from 542 million years ago to today (if you want to play with an interactive version, click on the image - there is also an iPhone app):

The geological timescale

So, what now?

Well, geologists and those researching ancient life use the names - especially the periods - all the time, and it is really valuable for you to both know what order they come in, and roughly what years in earth history they refer to.

I'm afraid there is no easy way around this: it is something that will be really valuable to you if you are able to learn it. Certainly, these terms will crop up again and again over the course of this unit. Below is the full, official geological column.

You can always find the latest version of this at the website for The International Commission on Stratigraphy, who also own copyright of this image.

Mnemonics may help you to remeber these! Courtesy of the internet here are a few possible ones for the Eras, with these at the top:

  • Precambrian, Palaeozoic, Mesozoic, Cenozoic
  • Please pay my children!
  • Pizza places make chicken
  • Peter Piper milks cows

And some for the Periods (still a lot to remember, I'm afraid...):

  • Precambrian Cambrian, Ordovician, Silurian, Devonian, Carboniferous, Permian, Triassic, Jurassic, Cretaceous, Tertiary, Quaternary
  • Pregnant camels often sit down carefully, perhaps their joints creak terribly quickly
  • ---------
  • Cambrian, Ordovician, Silurian, Devonian, Mississippian & Pennsylvanian (=Carboniferous), Permian, Triassic, Jurassic, Cretaceous, Tertiary, Quaternary
  • Cold oysters seldom develop many precious pearls, their juices congeal too quickly
  • Come over some day, maybe play poker. Three jacks can take queens
  • Charlie Oliver Still Drives My Purple Plymouth To Jersey City Through Quicksand
  • Can old senators demand more political power than junior congressmen? Tough question!

Note that we don't really use tertiary any more (it is largely equivalent to the Cenozoic Era) - but many of these menomics predate that change!

That is it for this minisite, but there is a quiz on the Blackboard site for EART22101 which - if you get the answers right, will unlock an achievement to show you know the geological timescale!