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The geology of the young Earth

The geology of the young earth is fascinating. It was both very similar and very different from that of today’s tired old planet. I had hoped to be able to give you a brief account of it. But I can’t find any comprehensible source material to plunder for it.

Archaean plate tectonics (and komatiite) the planet settles down

I’m told that there are no academic brownie points in producing anything suitable for us ordinary taxpayers. There aren’t even any brownie points, in this field anyway, for producing textbooks for the next generation of scientists. I find this rather shocking. There is no better way of obtaining a real understanding of a subject, than having to write an intelligible book about it. I speak from experience here.

So I’m reduced to trying to remember what my friend Will Diver, of the University of Plymouth, told me over ten years ago.

The big difference between then and now is the amount of heat trying to escape from the planet’s interior. Today’s planet is old and tired. Its insides have cooled dramatically since the early days. So there is no longer a great deal of heat trying to get out.

But the Archaean Earth was very much hotter. Indeed it blew the minds of the geologists when they first found the really ancient terrain at Isua in Greenland. They had expected the planet’s surface still to be hot and bubbly, and far too turbulent to produce any permanent land of any kind. But here they were, looking at 4 thousand million year old pebbles! This meant that the surface at least had indeed already cooled enough to last – and to last a very long time.

It could still have been far too hot for us. But there were rivers and streams of liquid water. The geologists could be quite certain of that, because running water is the only way you get pebbles.

And among the pebbles were controversial signs of life. There were deposits of carbon, and carbon is normally regarded as a sure sign of life. But the implication of life having got going so early is dynamite. And folk are studying the carbon with ever more sophisticated instruments. Sometimes the story is that this carbon is indeed ‘organic’. Sometimes the story is that it’s not.

My view, for what it’s worth, is that there’s no reason at all why life shouldn’t have got going this early. And therefore there’s no reason why these deposits shouldn’t be the remains of it.

Don’t be carried away though. The volcanoes and the earthquakes will have been far more common and far more terrible than anything we see today. Some land has survived from that early period, and a lot more has been found that’s not a huge amount younger. But things will have been far too exciting for anything more complex than bacteria to stand a chance.

Archaean plate tectonics

We cover modern plate tectonics elsewhere.

The thing about Archaean plate tectonics is that nobody expected it. As we’ve seen, the scientists didn’t expect there to have been any permanent land at all. First they discovered the oldest known decent sample of continental crust (Isua). Then an article in Science (23.3.07) described a piece of Archaean oceanic crust that has been found right up alongside the Isua territory. It’s very rare for oceanic crust to be preserved. But it can happen, if a piece gets pushed out on to the land. Anyway, this piece of ancient oceanic crust turns out to be as old as the Isua material. It suggests strongly that modern plate tectonics was already at work.

The processes must have been much more vigorous at that time, because the interior had much more heat to get rid of. And yet, as we’ve seen, the surface temperature was low enough for liquid water to exist. And the seething seems to have been going little if any faster than it does today.

How is this possible? The explanation that I was given is this. Almost all the heat that escapes from the interior now, and probably then too, finds its way out at the mid-ocean spreading centres. This picture comes from Lamb & Sington. It shows the hot magma welling up from the interior, and pushing the ocean floor apart. The heat is released as the cold seawater quenches the magma, and cools it into fresh ocean floor.

Today, we have huge plates and a fairly small length of spreading centre volcanoes. Only a small amount of heat escapes, as befits our old tired earth.

It has been worked out that huge amounts of extra heat could escape, without the surface being noticeably hotter, if we had much smaller plates, and therefore a much greater length of spreading centre. So instead of the large continents of today, the Archaean world will have comprised myriads of small islands. That is what the Archaean geologists were believing some ten years ago. As far as I know, they still do.

But how much land was there in those early days? Last time I read anything on this, there were two camps. One said that almost all the ‘continental crust’ on the planet was produced very early on. If they are right then there will have been almost as much as there is today. The other camp said that continental crust is produced gradually, as basalt is slowly converted into granite, by a process that I won’t bore you with (I don’t know it). If this camp is right, then there will have been very little land at the beginning of the Archaean, but quite a bit by the end.

During these times, the material that welled up wasn’t tired old basalt. It was komatiite (pronounced komati-ite from the Komati river in Africa where it was first found. One hears of molten rock flowing like water. Most of them don’t in fact, but komatiite really does. In Geological parlance, komatiite is even more ‘mafic’ than basalt, which is more mafic than granite, which is more mafic than sedimentary rocks (more).

The planet settles down

Gradually the planet cooled down, the amount of heat needing to escape got less, and the length of spreading centre needed to let it out got less too. By what process the one is believed to influence the other, I have no idea.

But by 500 million years later (3½ kMy ago) quite large chunks of Africa, Australia and Canada were together in single pieces. All these continents are veritable cornucopias of very ancient finds of various kinds. But the volcanoes and earthquakes were still pretty terrible. Conditions had still not settled down enough to enable anything more sophisticated than bacteria to survive.

By another thousand million years later, 2½ kMy ago, there are clear signs of the planet beginning to settle down quite seriously. And this represents the boundary between the Archaean and the Proterozoic.

There are signs of mountain building, and other evidence of land masses coming together. This is followed, around 100 million years later, by signs of rifting and the breaking apart of land masses. This is supercontinent type stuff, although whether all the land ever came together into a single mass (as it has done several times since) is doubtful.

In particular there are the redbeds. These are huge dunes of iron-rich sand (I think). But the important thing for us is this. They were developed on land. This means that the land masses must have been quite large, or the wind-blown sand would all have ended up in the sea. Red beds are found of all ages. But they start at the beginning of the Proterozoic.

By the end of the Proterozoic, a mere 500 million years or so ago, conditions had become seriously quiet. And large creatures were beginning to appear.