A TIMELINE FOR THE PLANET click for Home page
These maps show a succession of two or even three different supercontinents, stretching back over a thousand million years. Not surprisingly the evidence gets increasingly uncertain as we go back in time. However scientists believe that there were several more even earlier supercontinents – at more or less 500 million year intervals – stretching back until the planet was young.
We’ve all heard of Continental Drift by now, with the
different land masses wandering all over the globe. To begin with it was just theory. But now satellites can actually measure the
movement. On average it’s about 15
millimetres a year, or about the speed that your fingernails grow. Some are moving much faster. The Indian landmass, for example, is
galloping northwards at around 2 centimetres a year; and a huge chunk of it has
already buried itself beneath the underbelly of
Geologists and Palaeo-geographers have worked out how the landmasses have moved in the past – how they have clumped together to form super-continents, and then split up again and gone their separate ways.
These maps don’t show the actual shape of the
continents in times past. Neither could
they. A lot of land has certainly been
eroded away since, and more will have been deeply buried under other land – as
These images simply show the believed disposition of present land masses at the time.
There are thought to have been at least 3 supercontinents, even earlier than the ones we depict below, stretching back more than 2½ thousand million years. This was the heyday of bacteria, when they were the only life around.
The hard evidence for these early supercontinents is
long gone. But every 500 million years
or so geologists have spotted signs of mountain building episodes, as the
individual landmasses came together.
Mountain building is happening today in various places. The most spectacular is the
This is the ancient supercontinent of Rodinia, which came together around 1100 million years ago. It’s the earliest for which there’s any hard evidence. To be honest, there seems to be only just enough to be sure that it actually existed.
It was the late Proterozoic when life was still entirely microscopic – and of course very much all underwater.
The orange patches are mountain-building areas, and the hatched areas are where there are signs of rifting. As we keep saying, don’t expect things to be simple in this game.
The rest of the maps all come from Christopher R. Scotese’s Paleo mapping project (www.scotese.com).
This is the very late Proterozoic when, according to Scotese, a new supercontinent was being formed called Pan(n)otia (I’ve seen it spelled both ways). It’s the first I’d heard of it, and a report in New Scientist (20.10.07) suggests that Panotia only ever comprised part of the total land anyway.
Apparently it was complete around 550 million years ago. However instead of going on to join the rest of the land to produce a proper supercontinent, it seems to have broken up again during the Cambrian (as we’ll see).
The very late Proterozoic is also the time of the terrible Varangerian ice age. Signs of glaciation have been found on almost every continent.
This is the world around the time of the great Cambrian explosion (more) around 500 million years ago. That was when large creatures suddenly appeared on the scene, apparently out of nowhere.
Panotia was well on the way to breaking up.
This is the Ordovician Period 460 million years ago. Not a lot happens on the plate tectonics front in a mere 40 million years – although maybe the landmasses are beginning to get a little closer again.
Note the external ocean (more). It’s the same as today’s Pacific, rechristened as the Panthalassa Ocean.
The mid Ordovician is the time of the Great Diversification, when sea-floor-dwelling filter feeders exploded into the greatest diversification of all time (more). Shortly after, round about the late Ordovician, the plants invaded the land. It was also one of the coldest times in Earth history, unless you believe the full blown ‘Snowball Earth’ theory (more).
This is the middle Silurian, 435 million years ago. There are now definite signs that the next supercontinent, Pangaea, is beginning to build.
Not shown on this map are the increasing areas of shallow sea around the margins of the continents (I got this from another map). I’ve read that this is what you would expect at this stage.
It is around the time that the animals invaded the land. High sea levels would be just what was needed to give them a leg up!
We have a bigger gap here. This is the time of the great supercontinent of Pangaea.
The amount of shallow sea has dropped, as the sea-floor gets old and cold. The Pacific/Panthalassa Ocean is of course still there. And we still have one large internal ocean, the Tethys Sea.
These were hard times. The large land mass generated widespread deserts, and the oxygen level fell.
Shortly before this had come one of the greatest mass extinctions of all time, the P-T event (more).
This is the early Jurassic. We can see that Pangaea is beginning to break up.
The climate was much warmer than today, and stayed so until the K-T extinction that killed off (most of) the dinosaurs.
The early Jurassic was the start of the dinosaurs’ heyday – shortly after the Triassic extinction had killed off most of the competition. Primitive mammals found a niche for themselves too, but it was a pretty insignificant one.
This is the late Cretaceous. The break-up of Pangaea is complete. The Atlantic Ocean is beginning to open up, and the map is beginning to look recognisable.
The Cretaceous climate was still balmy, right up to the poles. Dinosaurs and palm trees lived in both polar regions.
Sea level was 100-200 metres higher than today, which created plenty of shallow seas as the continental margins were flooded. They provided plenty of channels whereby warm water could be transported towards the poles.
This is today’s map. The Atlantic Ocean is still growing at the moment. But fairly soon (in geological terms) it will start to shrink again. It will develop its own ‘ring of fire’, as the old tired ocean floor starts to get pushed under the continental crust (more).
The past 60 million years or so have been a period of fairly frenetic continental collision and mountain-building. India hit Asia. Spain hit France. Italy hit France & Switzerland. Greece and Turkey hit the Balkan region. Arabia hit Iran and Australia hit Indonesia. Together these collisions finally extinguished the once-great Tethys Ocean.
And this is the world predicted for 250 million years hence. It represents the final stages of the building of the next supercontinent – or possibly the early stages of its break-up. Australia/Antarctica are still/again separate from the main land mass.
The north Atlantic has closed up again, though in a different way from the past. In particular Britain and Scandinavia have drifted north, and joined northern Russia on the edge of the Pacific (or possibly the Arctic/Pacific) Ocean. The south Atlantic has become a large inland sea and India and south-east Asia join up.
The main weapon in the palaeo-mapper’s armoury is ‘palaeo-magnetism’.
Most if not all rocks are very slightly magnetic. They contain minute specks of iron oxide, and these act like little compasses. (If this reminds you of magnetic tape or computer media then full marks. But don’t push the comparison too far.)
If the rock should get melted then these little compasses are free to align themselves with the Earth’s magnetic field – just like your hiking compass does (not quite actually as we’ll see). When the rock solidifies again these little compasses get locked in position. They provide a recording of where North was at the time of solidification. This recording is permanent. The Earth’s field is far too weak to overwrite it, unless the rock should get re-melted.
Something else important happens at the same time. The atomic clock gets reset (more). This means that the mappers also have the date on which the ‘recording’ was taken.
Equally important, the recordings tell the mappers the latitude of the place where the recording was taken.
It works like this. An ordinary compass pretends that the Earth’s field runs along the ground. But it only actually does this near the Equator. Everywhere else it dips downwards. And the further north or south you are, the greater the angle of ‘dip’. At the poles, the field dips straight down vertically, and an ordinary compass is useless.
An ordinary compass only measures the ‘horizontal component’ of the Earth’s field, because that’s what most of us want. Its dial is supported on a single pin bearing. It you put the compass on its side the dial slips off. But you can also get ‘dip’ compasses. Their dials are properly supported like a bicycle wheel. You can put them on their sides and measure the ‘vertical component’, or the angle of dip. With these you can work out your latitude without bothering with a map. Likewise the angle of dip, as recorded by a rock sample, tells the geologists what the latitude was when the sample last solidified.
Any large land mass will have many sites where the rock has suffered re-melting, and made its recordings. Hopefully the recordings will cover a wide range of dates. This enables the mappers to plot the movement of the land mass, north and south. If the recordings are reasonably dispersed, then the mappers can also plot the rotation of the land mass as it waltzes about the globe.
Unfortunately there’s no such simple tool for measuring movement east and west. Getting a handle on this involves detailed detective work and much trial and error.
However the geologists can often tell when landmasses came together and split apart and this helps a great deal. For example, if they find a geological structure in Antarctica that is identical to one they’ve already seen in Canada (I think I have the example right) then they must once have been adjoined. With any luck, they can also estimate when this was.
But that’s not all. The palaeontologists can often identify fossils that are identical on parts of two widely-separated masses. They must once have been adjoined too. And again the palaeontolgists can often estimate when this was.
It has taken many decades for Scotese to come up with his definitive maps. And not all palaeo-mappers agree with him even now.
© C B Pease, January 08