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How are Geological Periods Determined?

Scene from the Jurassic period

Q: Who determines the duration of geological time periods such as the Jurassic beginning approximately 205 million years ago and ending approximately 144 million years ago? How are the length of these geological periods and the faunal stages within in them determined?

From Mike (October 2009)

Reply by Dr Jan Zalasiewicz (University of Leicester)

Geological time – coping with (near) infinity

That’s a nice question. The geological periods basically reflect the natural patterns and changes of Earth history. Geologists of Victorian times recognized that successive stratal successions were characterized by different fossil assemblages: former biological dynasties, if you like. Such dynasties could end abruptly, as in the simultaneous demise of the ammonites and belemnites in the sea, and the dinosaurs on land. It was (and remains) sensible to use that biological revolution to mark the boundary between two major time divisions, the Cretaceous and Tertiary periods.

The Victorian geologists, though, had essentially no idea when this transition took place. Working out how many million years ago this was came later, once radioactivity was discovered and – very soon after – exploited as a means of radiometrically dating events in the Earth’s past. Geochronologists are even today refining the numerical calibration of the geological time scale, the error bars here now typically being of the order of one or two percent (and diminishing).

The geological Periods can be further subdivided, into Epochs and Ages. At the finest levels, most day-to-day practical time-slicing (or geological correlation, to be more correct) is still done using fossils. Some fossils aren’t very good for this - dinosaurs, for instance, which are large and rare and geographically restricted. The small fry are much better. Graptolites in the Silurian spread through the oceans and evolved quickly, and individual graptolite biozones represent, on average, well under a million years of time. And using ammonites in the Jurassic, even finer time resolution has been achieved.

The picture of Earth history is becoming ever more detailed, as such (and many other) geological dating techniques are discovered and applied to the rocks. From this history we can decipher how the Earth functioned in the past – and, of course, how it may evolve in the future, now that humanity itself has become a geological force comparable with the great forces of nature. It has been suggested, indeed, that we are now living in a new epoch of time – the Anthropocene. Time will tell.