Geologic time is such a vast concept that it’s helpful to convert it to something more pedestrian just to get oriented. I like rainfall. [usgs geologic-time-spiral infographic]
* The total precipitation that falls on the world in one year is about one meter of rain, the height of a golden retriever.
* The total amount of rain that has fallen on the world since the industrial revolution began is about 200 meters, the height of Hoover Dam.
* The amount of rain that has fallen on the world since the time of Moses is enough to fill up all the oceans.
* The amount of rain that has fallen on the world since the Ice Age ended is enough to fill up all the oceans four times.
* The amount of rain that has fallen on the world since the dinosaurs died is enough to fill up all the oceans 20,000 times—or the entire volume of the earth three times.
* The amount of rain that has fallen on the world since coal formed is enough to fill up the earth 15 times.
* The amount of rain that has fallen on the world since oxygen formed is enough to fill the earth 100 times.
The experiments that assign specific numbers of years to geologic layers are almost as simple as this science-fair project, although not quite, and they are just as reliable. Not everyone agrees with this assessment, of course. Geologic time does contravene certain religious beliefs, a notorious difficulty with the subject that is very regrettable, since it doesn’t contravene the religious beliefs that count. But it’s probably more significant that the experiments, simple though they may be, involve obscure facts about rocks, a knowledge of physical law, and the assumption that this law was the same in the ancient past as it is now. None of this is obvious, much less interesting, to the average person. If you go to the supermarket and engage the checkout clerk in a conversation about the Paleozoic Era, radioactivity, or the disappearance of the megafauna, you’ll be met with a smile, whereupon you’ll probably be escorted from the building as a lunatic. However, the time scales do come from something concrete that can be explained simply.
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