TIME
June 25, 2020Duncan Miller
Consider a rare, long-lived human life span of 100 years. There would be 10 000 of those in a million years. A physical example is to image a hypothetically rapid rate of erosion of something substantial like Table Mountain. The summit of Table Mountain is just over one kilometre above sea-level. If each year you were to shave off one millimetre, the size of a large grain of sand, from the top of Table Mountain within a million years it would have been eroded down to sea-level, not down to Tafelberg Road but to Camps Bay beach. (Fortunately it doesn’t erode that quickly.) You could play this game horizontally too. There are a million millimetres in a kilometre. Measure a straight stretch of road a kilometre long, then imagine marking it off in millimetres. It would take you a loooong time. Or take a large sheet of paper, one metre square, and rule lines one millimetre apart to make small squares. When you have a thousand rows of squares down and across, you will have a million small squares. If you have time on your hands, you could start numbering them…
Now that you have got to grips with the enormity of a million
years, let’s think of the age of the Earth, which is 4 560 ± 50 million
years. (No, we are not going to argue about how geologists know this. It
depends on the same well-understood physics that makes nuclear power stations
work, so is beyond rational argument.) Cape Town geologist John Rogers likes to
illustrate the age of the Earth by stretching out a 4,5 metre measuring
tape on the floor. There are a thousand millimetres to a metre; so
4,5 metres are four and a half thousand millimetres. If that represents
the age of the Earth, each millimetre on the tape represents one million years
– a whole, rapidly eroding Table Mountain in each millimetre. Or you can think
of it as 4 500 km marked off in millimetres, with each millimetre
representing one year. That’s Cape Town to Windhoek three times, in millimetres
representing one year each.
A window into deep time – a petrographic thin section in crossed polarized light of part of the 4 560 million year old Korra-Korrabes chondritic meteorite from Namibia (width of field of view 1 mm)
That piece of Cape Granite in your hand solidified 540 million years ago. That’s an “awefully” long time ago. But in Barberton there are rocks dating to 3 500 million years. And in our museum and university collections there are meteorites that are 4 560 million years old, the oldest things any human being will ever hold in the hand.
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