Geological time

Wow, if all goes well this should be two posts in two days!  I hope you are as impressed as I am.

As I promised in my last post, I am going to explain the other part of my thesis title ‘Ichthyosaurs of the Late Jurassic’, namely the Late Jurassic part.  Many will be familiar with the latter word from Michael Crichton’s wonderful book and the groundbreaking-CGI-including, Spielberg-directed film from 1993 (ironically most of the dinosaurs are from the Cretaceous, but who cares?).

I will start by saying that geological time is vast, and vastly different to what we humans are used to dealing with.  The processes of rock making (usually) are so slow that talking about them on the human time scale of days, years or even millennia quickly becomes cumbersome.  I will be discussing these at a later date.  When dealing with the immense timespans inherent in geology, actually getting an age in the first place for your rock is nigh on impossible itself.

Much geology, and certainly the relevant part for my PhD, works on a scale of millions of years.  For instance, the Jurassic is defined as the period between 199.6±0.6–145.5±4.0 million years ago (Ogg, Ogg and Gradstein 2008).  The units are usually shortened to ‘mya’ or ‘Ma’ (for mega-annum).  As you can see from the upper bound, there can be quite a lot of uncertainty: 8 Ma, but this is still only 5.5% difference.  The Earth is currently believed to be about 4 500 Ma (4.5 billion years or Ga) old.  Life itself is thought to have arisen ~3 800 Ma, originally as single-celled organisms.  More complex life is first found from ~600 Ma whereas the first animals with hard shells are from ~550 Ma.

The history of the Earth with chronostratigraphic divisions based upon Ogg et al. (2008). Key events are also present and will be added to over time. (Click to view larger version.)

To try and coherently explain the length of time involved, it is usually transferred to the timescale of a year: ‘if the whole of Earth history was compressed into one year…’  The crux of this is that you see that life has been around for a long time (since the end of February), but spent most of that time be very simple and solitary.  Complex life doesn’t really manifest itself until the last ¹⁄₇ th of Earth’s history (13 November).  The dinosaurs appear on 11 December and become extinct 15 days later.  Humans appear within the last four hours and I was born less than a second before twelve midnight!

In terms of life, most study is done on the most recent 542 Ma.  This is the time since the ‘Cambrian Explosion’ where there was a surge in diversity and increase in preservation potential due to the evolution of hard body parts, such as shells and skeletons.  My study will be focussed on the Jurassic Period (200–146 Ma) particularly to last 20 Ma.  I will also look at ichthyosaurs from their whole span in the Mesozoic era (~240–~90 Ma).

References

OGG, J. G., OGG, G. and GRADSTEIN, F. M. 2008. The Concise Geologic Time Scale. Cambridge University Press, New York, 184 pp.

Source data for the chart above can be found here.