A TIMELINE FOR THE PLANET click for Home Page

The oldest rocks on the planet

Isua

The oldest place on the Planet seems to be Greenland.

Several sites have been explored and dated. But the most thoroughly researched that I’ve seen is Isua in the west of the area. Considerable areas have been found there dating back nearly 4 thousand million years. These pictures were lifted from the BBC Programme “Earth Story”. The end of the geologist’s hammer gives the scale. I’ve seen various dates attributed to it at various times. The latest seems to be 3.8 kMy.

All we see is an agglomeration of small stones and pebbles. It may not look too exciting. There are plenty of places where you could see something very similar today. But this stuff was laid down when the planet was not much more than 500 million years old!

The thing that blew the geologists’ minds when they first found the Isua outcrop was this. Pebbles are fashioned by waves or running water. No other mechanism can produce them. And this means that there was both land and sea already around on this very young planet. In turn this means that the surface, at least, must already have been nearly as cool as it is today – even if just underneath it was still very much hotter. The geologists had simply not believed it possible that the planet was already settling down into old age as early as this.

You can see more clearly in the lower picture that the pebbles are glued together by black gunge. These are deposits of carbon. Many scientists are very excited by this carbon, because it seems to contain telltale evidence that it was produced by life. If life did indeed get going so very early, then that is extremely exciting. And huge amounts of work have been put in to try to establish whether this evidence is genuine or not. The story swings first one way and then the other. A lot seems to depend on what particular scientists want to believe. I shouldn’t be saying things like that. But scientists are human like the rest of us.

For my part, I’m quite happy with the idea that life got going as early as this. Many scientists see no reason why it couldn’t have got going a good deal earlier still. But whether this carbon proves it or not, is still anybody’s guess.

That’s not the end of the ‘Isua’ excitement. The same area also contains a piece of oceanic crust that is equally old, and tells an equally dramatic story about Archaean plate tectonics.

Isua may be the oldest bit of land surface found so far, but it is not oldest bit of crust to have been found. That honour goes to the Acasta gneiss. Acasta is a river in Canada, and gneiss is a form of heavily ‘altered’ granite. The Acasta gneiss is not surface rock. It has no real story to tell, except that there was granite, and therefore continental crust, around when it was formed. I’ve seen 4.3 thousand million years quoted for this..

There are vast areas of territory that are not very much younger than Isua. The centres of large land masses are favoured areas for really ancient real estate, because most of the trouble occurs around the edges of the continents. Both Australia and Africa can boast regions several thousands of million of years old.

Even the Acasta gneiss is not the oldest material. The real accolade goes to the zircons. And they have an equally exciting story to tell.