Oh, wow, oh, oh, wow, oh, oh, wow.

Oh, wow.

Oh, James. Hello, you're listening to The Science of Everything podcast, episode 157, the geologic

timescale. I'm your host, James Fodor. Today we're going to talk about the history of

Earth, focusing, of course, on the geologic timescale, which is a way of assigning dates to

particular segments of rock and corresponding periods of history throughout the development of Earth.

And we're going to talk about the main periods in the geologic timescale and what major events

characterize those. We'll talk about the changing composition of the atmosphere, the supercontinent

cycle, the different phases in the

evolution of life, and also mass extinction events. Recommended pre-listening is the previous episode

156 fossils and dating methods. So just picking up from some of the things we discussed in that

episode, let's begin by talking about some terminology.

So you may recall from the previous episode that we distinguished between geochronology and chronostratigraphy, which is, again, quite confusing because both of them seem to be about

dating of rocks. But the difference is, effectively, whether we're referring to rock segments

or rock layers or periods of time.

And that's the main distinction that we introduced last time.

So a geochronologic unit is a period of time.

It's a period of time in which a particular unit of rock was formed.

A stratigraphic unit is the unit of rock.

It's the layers of rock that are identifiable and have a particular age and a distinctive

in some way so that they can be mapped and so forth.

So you've got your rocks and they've got your period of time in which the rock was formed.

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