Unravelling the geological time scale

Posted by Don Pinnock on 5 February 2013

The film Jurassic Park, based on Michael Crichton’s extraordinary 1990 novel, gave us accessible dinosaurs and spawned an industry in cuddly T-Rexes, Dino Babies and fierce plastic models essential to every toy box. While the word Jurassic has remained in everyone’s verbal toolkit ever after, few knew what it meant other than some time very long ago.

In truth, it’s still a puzzle. Try asking anyone who knows their geological time scale and you’ll soon be confounded by eras, periods, epochs, a blizzard of strange names ending in -zoic, -cene, -ian or worse and mind-numbing depths of time. I had the time scale on my wall for years and never managed to master it.

Recently I found out why: it’s chaotic, anarchic, nationalistic and a battleground of scientific ideas. And to you and me the names don’t mean a thing. How do you get your head around titles like Cenozoic, Oligocene, Ordovician or Holocene? The only one that makes sense is Hadean, used to describe the wild fiery origins of our protoplanet. To a soft-skinned mammal it would surely have been Hell. Oops, I’ve just discovered it’s been renamed Archaeozoic.

Anyway, zoic or not, Hades isn’t a unitary state but deeply divided, starting 4,56 billion years ago with a time named, appropriately, Cryptic. A mere billion years later we’re into the Isuan, followed every half a billion or so years by Swazian, Randian, Huronian and Animikean.

There just have to be stories behind such freaky names. Somewhere, sometime, there were undoubtedly people with the mad gleam of discovery in their eyes, staring at a piece of rock in their dust-covered hands or peering in triumph through a microscope and thinking: ‘I will name this epoch Pusgillian or maybe Podolskian’ (they really exist).

Here’s a quick sketch to get on with. It leaves out more than it tells, but it’s the best I can do in a few paragraphs.

Back whenever, space bits bumped into one another in a sort of cosmic dustbin until about five billion years ago when there was enough stuck together to be a planet. Life began some one billion years later. It was a brave start, but didn’t leave fossils (probably because life was pretty insubstantial). They appeared only 3,4 billion or so years ago.

Geological history is based on major events in the Earth’s past as recorded in the rocks and is divided into four eons, 10 eras, each of which are divided into a number of periods which, in turn, are divided into epochs and ages. Then there are supereons, but let’s not go there. I’m sure you can see the creeping complications.

In a nutshell, some chunks of time are named after rock types and others after creatures that fossicked around in those periods. But, I hear you ask, where did the funny names come from? You may think they have deep, cosmic significance, but a few examples of their invention suggest a degree of, um, latitude.

Quaternary was applied by someone chipping away in the Seine Valley in 1829, but he got it wrong, evidently. Tertiary was borrowed from an Italian who used it for something else. Cretaceous (from chalk) was coined in 1882 to describe chalk layers in northern France. Carboniferous was named in 1822 by WD Conybeare and was a useful double entendre (it also has links to ‘coal measures’).

Triassic (from triad) was used to lump together three other sequences in Germany, Jurassic was named because it hangs out in the Jura Mountains in Switzerland, Devonian because it was found in Devon, Cambrian because it was christened in Wales, which used to be known as Cambria, and Permian because it was found in the Perm Krai region of Russia. Silurian and Ordovician are named after two ancient Welsh tribes for no good reason other than that they sounded nice. Mesozoic was simply a label invented by geologist John Phillips in 1841. And so it goes – all good science, of course.

These days things have tightened up a bit. That’s an understatement. The guys who do the naming are the International Commission on Stratigraphy, which is part of the International Union of Geological Sciences. They recently named a sandwich of time between the Cryogenian (when Earth was more or less a snowball) and the Cambrian (when complex life took off) Ediacaran, after some hills in South Australia.

The problem was that some wanted to call it the Vendian, for reasons I couldn’t discover. The decision took three international ballots and 15 years to reach a conclusion. Approached about what we mere mortals might view as a considerable waste of time, Richard Fortey of the Natural History Museum in London conceded a bit of strife. ‘Obviously they have been fighting in the geological community,’ he said, ‘but it’s good the name is now settled.’

Are you still tracking?

Okay, in case not, here’s the most concise summary of the ages of the Earth. An eon is a timescale of half a billion years or more (there are four). An era is several hundred million years (there are 10). A period is the time it took for any system of rocks to form. An epoch is tens of millions of years and an age is generally (but, sorry, not always) millions of years.

Right now we are in the Subatlantic Age of the Holocene Epoch of the Quaternary Period in the Cenozoic Era of the Phanerozoic Eon. Now you know. Whether that improves your appreciation of the landscape on your next trip beyond the city limits depends on how well you memorised your Earth times table.

Some other name roots (All from Greek)

Archean – original
Phanerozoic – visible life
Proterozoic – before animal life
Paleoarchean – old ancient
Neoarchean – new ancient
Paleozoic – old life
Mesozoic – middle life
Cenozoic – new/recent life

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