Duncan Miller

Our individual lives are so short, and geological time so long, that it is difficult to comprehend ‘deep time’, the most awe inspiring aspect of geology. Geologists often seem to work in units of a million years, as though that is the basic unit for the passage of time on Earth. So let’s make some effort to comprehend geological time – after all it is what makes geology tick.

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.