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Supercontinents

When I was young we were fascinated by the mysterious ancient supercontinent of Gondwanaland. It existed way back in the mists of time, when the world was young and the dinosaurs were King.

Geological and Biological timescales Early supercontinents Effect on climate Continental Cycle

Later we learned that Gondwanaland was small beer, comprising only about half the planet’s land area (basically today’s southern hemisphere). The rest of the land resided in other equally large landmasses, with names such as Laurentia and Angara.

Even more disappointing, it was all relatively recent. The real supercontinent, that really was around when the world was young, was Pangaea. And Pangaea really did comprise virtually all the planet’s land, and the dinosaurs had yet to appear.

Now we know that even Pangaea was no more than ‘yesterday’ in the life of our planet. The dinosaurs may be yet to come, but a great deal had already happened on the ‘life’ front.

Geological and Biological timescales

However before we go any further, we’ll consider the difference between geological and biological timescales. It’s important because the geological time scale is far longer than the biological timescale.

This diagram comes from a Geological Museum booklet from 1981. The main part shows the explosive build up of life from about 1000 million years ago. The red strip shows episodes of mountain building. These are caused by land masses crashing together, and indicate a supercontinent in the making. The blue strip shows how, at the time, they thought the build-up of atmospheric oxygen was going. In fact the oxygen issue is still controversial today.

Five hundred million years ago, life had only just discovered the ‘big is beautiful’ concept, with the Ediacara fauna followed closely by the Great Cambrian explosion. Animals, trees, giant dragonflies and so on, were all still to come. Let alone dinosaurs, birds and mammals. Before around 500 million years ago, even ‘advanced’ life existed only on a microscopic scale.

And advanced life of any kind existed entirely underwater. The dry land was either completely bare, or ruled by organisms such as bacteria and algae.

But on a geological timescale, 500 million years is nothing. Geologists can produce chapter and verse of how, for example, the Atlantic Ocean has opened and closed several times – at around 500 million years a throw. Incidentally, each time it opened up, Scotland found itself with another chunk added on. The Great Glen (Loch Ness etc.) is only the latest of the geological ‘faults’ that show this.

(The truth is actually a good deal more complicated than I’ve just made out. The continents were doing a stately dance around each other, under the influence of plate tectonics. So the continents on the other side of the ‘Atlantic’ weren’t always the same. And geologists give the Atlantic’s previous incarnations different names. But if you don’t want to get bogged down in detail, it’s perfectly reasonable just to remember the simple story.)

The continental cycle

The most recent supercontinent is Pangaea, which lasted from about 250 to 150 million years ago. That’s from shortly before the dinosaurs appeared to the middle of their heyday.

There was an even earlier supercontinent called Rodinia. Not surprisingly, there’s much less known about Rodinia. But it seems to have existed around 1100 million years ago. This was somewhere around the time of the terrible Varangerian ice age.

However, as the diagram suggests, geologists now believe that there’s a coming together of land masses every 500 million years or so. They stay together for about 100 million years and then they break apart again.

How do they know? Well you won’t be too surprised to learn that, as the landmasses come together, there’s a spate of mountain building – as is happening today before our eyes. India is slamming against Asia, burying itself underneath, and pushing up the Himalayas.

Incidentally we mustn’t confuse this kind of mountain building with what is going on all around the Pacific. If there are volcanoes then we have oceanic crust burying itself more. There are no volcanoes in the Himalayas.

Likewise, as the landmasses are split apart, there’s a spate of ‘rifting’. Again we’re lucky. There’s an example of that too, in Africa’s Great Rift Valley. In 100 million year’s time there may be an ocean separating East Africa off from the rest.

The evidence for earlier supercontinents tenuous, but convincing to geologists. They have detected signs of a worldwide spate of this mountain building – followed a hundred million years later by a worldwide spate of rifting. And, as we’ve mentioned, these episodes seem to have happened roughly every 500 million years.

Early supercontinents

It seems unlikely that the earlier episodes succeeded in assembling all the land into single supercontinents. Large land masses are a sign that the plate tectonics process is beginning to get tired. The land probably assembled into large islands, and then broke apart again into smaller ones.

Effect on climate

The state of the supercontinent cycle has a huge effect on the climate, the constitution of the atmosphere and even on the amount of land there is (Click here for more on sea levels). It’s extremely complicated, and we can’t afford to go into it. However several mass extinctions have been attributed to the stage of the supercontinent cycle.