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Plate Tectonics

Plate Tectonics is ‘the wandering of the continents’.  It was originally hugely controversial, as we can well imagine. 

Internal and External oceans   Dance of the continents   Sea Level   When did it start?   Mafic rocks   Types of rock


I remember a letter in New Scientist, from a geologist who was taking his finals at about the time that N.S first hit the news-stands – and plate tectonics was hitting the headlines as a seriously plausible tale.  However the students were told that any mention of it would court instant failure.   I would hate to have been their lecturers a few months or years later!


The controversy has totally evaporated now.  Plate tectonics is no longer theory.  It’s ‘fact’.  Satellites can even measure the slow-but-inexorable widening of the Atlantic and other oceans.


There’s a lot of material available these days on Plate Tectonics.  So we won’t go into it.  But we will set the scene a bit.  This diagram of “The Ring of Fire” comes from ‘Earth Story’ by Lamb & Sington (BBC Books).  It depicts the volcanoes and earthquake-prone regions all round the Pacific.   All this activity is generated by tectonic subduction zones.  These show that the Pacific Ocean is shrinking as the surrounding plates converge on it.


The Atlantic doesn’t have subduction zones because, as we’ve just seen, it is still expanding.


During the planet’s early ‘molten’ phase, virtually all the really dense (heavy) material sank to the centre to form the Earth’s Core.  This was overwhelmingly iron.  And it’s the sloshing of this still-partly-molten iron that provides the magnetic field that allows compasses to work. .  The Earths’ field also protects us from many harmful rays emanating from the Sun.


Now by rights our planet should have been stone cold within a few hundred million years.  Indeed the physicists initially refused to believe that the Earth was anything like as old as the geologists were insisting it was.  Then they discovered radioactivity and all was explained. 


Most of the radioactive elements sank to the core too, because they are even denser than iron.  And their slow decay has been providing enough heat to slow down the cooling, and to keep the planet simmering away gently from that day to this. 


Above the core, there’s the Earth’s Mantle.  It comprises some 3 thousand miles (nearly 5,000 km) of almost-molten rock.  Think ‘glass’, because molten rock behaves much like glass.  If you’ve worked with molten glass, or seen it on television, you will know that it is not at all like water.  Instead of melting suddenly, it softens gradually as it’s heated.  So it can be almost solid – to the extent that it would break if you hit it.  But it still flow slowly if left to itself.


The mantle is so nearly solid that you wouldn’t believe it.  We can illustrate this.  Northern Europe still hasn’t finished ‘bouncing back’ after a huge weight of ice was lifted from it at the end of the Ice Age some ten thousand years ago.


However the mantle is still seething very very slowly; heated from below and cooled from above.  


Now think ‘pan of boiling jam’, slowed down millions of times. 


The Earth’s crust is the scum on the top of the jam pan – except that the crust is solid.  It’s carried over the surface by the jam/glass seething underneath.   Where one ‘plate’ of scum meets another one, one gets forced downwards and the other is forced upwards. 


That’s not quite the whole story.  Continental crust, the land, is genuinely scum.  It’s not actually frothy, but it’s less dense than the rest and always comes out on top.  So it’s permanent.  Continental crust may get re-molten, or otherwise reworked. But little of it ever gets reclaimed by the mantle.  Some land has been found that’s nearly as old as the planet (more).   And there’s probably plenty more equally old, just waiting to be found. 


But the planet doesn’t sport too much continental crust.


Most of the crust is ‘oceanic’ crust.  It’s made of straight mantle material (not quite as we’ll see, but near enough for the moment).   And it’s temporary.  Oceanic crust gets spewed out in some places and sinks back down in others.  We don’t see any of this because it all happens under the sea.  This picture comes from Lamb and Sington.


Fresh oceanic crust is created along the mid-oceanic ridges.  Look at a map of the Atlantic and you will see one stretching from Iceland right down to the Antarctic Ocean.  This Mid-Atlantic Ridge is one long volcano. 


Oceanic crust is destroyed at ‘subduction zones’.  As we’ve seen, the Pacific has subduction zones all round it, hence its ‘Ring of Fire’.  A subduction zone is characterised by a deep water trench, as one piece of oceanic crust sinks below another.  And by mountains with volcanoes in. 


The reason for the volcanoes is that the the crust drags millions of years worth of sediment down with it, deep into the Earth’s interior.  It brings down a great deal of water as well.  This sediment is lightweight stuff which melts easily. The water lowers the melting point of the rock above (don’t ask me how).  This helps the sediment to find its way up to the surface, still molten, where it emerges as a volcano.  The molten sediment melts some of the surrounding rock on the way up.  And this comes out too.


Naturally the sediment contains a a great deal of carbon – the remains of a huge number of dead creatures.  This emerges as the dreaded carbon dioxide.   Indeed serious episodes of volcanism have often been implicated in mass extinctions.


As we’ve seen, continental crust is lighter than oceanic crust.  And when it gets forced down it doesn’t go nearly as deep.  Little if any material actually gets melted, and we don’t get volcanoes.  Much of northern India, for example, has already buried itself under the underbelly of Asia – and forced up the Himalayas and Tibet.   There are no volcanoes in the Himalayas or Tibet.

Internal and External Oceans

For the past thousand million years at least (possibly a lot longer than that) our planet has comprised an ‘external’ ocean plus assorted continental masses, and an assortment of ‘internal’ oceans. 


The external ocean is of course the Pacific, which grows and shrinks a bit, but which never disappears.  Don’t ask though whether it has always been in the same place.  What with all the seething going on underneath, this is almost certainly a question with no answer.


The internal oceans are all the rest.  They come and go, as the continents join up and split apart.

The Dance of the Continents

The following is almost certainly a travesty of the truth – as comprehensible descriptions of natural events so often are.  But I find it useful, and I hope you do too.


The continental cycle comprises a stately dance between four great land masses, plus a number of smaller ones.  They ‘advance and retire’ over a period of 500 million years or so (more).  But at the same time they swing around and perhaps change places with other dancers.  So it’s not necessarily the same pieces that come together each time. 


But sometimes it is; which is why Scotland steals an extra chunk off North America at each retiring.

Sea level

Sea level is one thing that’s very much affected.  And nothing to do with ice ages either!  The very sea floor rises (and the continents sink) during continental break-up.  So the oceans overflow and swamp a lot of low-lying land.  This produces vast areas of shallow sea and freshwater swamp. 


The experts tell us that the sea level also rises while the bits are coming together again.  It’s while Pangaea was coming together that life invaded the land, and the Carboniferous coal forests were being laid down.


At other times, the sea level falls; and these lush moist havens for wildlife shrink.  According to one theory I’ve read, we are in such a time now, although there are still quite a lot of shallow seas around.  Certainly the coastline in parts of Africa is so precipitous that tankers waiting to dock in ports can’t anchor.  Even quite close inshore the water is simply too deep.  Instead they have to drift around, trying not to use their engines too much.


As the next supercontinent starts to form, in a couple of hundred million years’ time,  we can look forward to there being much less land for us to live on.


It’s the plate tectonics that piles the land up again as fast as erosion wears it away.  One day the planet really will go stone cold, and the plate tectonics will stop.   Erosion will then win, and we’ll end up with nothing sticking up above the sea at all.   But I don’t think we need to hold our breath!

When did plate tectonics start?

For a long time, scientists believed that the young Earth was far too hot and bubbly for anything like plate tectonics to have been possible.  But then the famous Isua terrain was found.  It blew the geologists’ minds because it was very nearly 4 thousand million years old.  Not only that, but it incorporated clear evidence of running water.  But there was more.  Later some preserved oceanic crust was uncovered in the same area.  And it was much the same age.  


So now it seems clear that plate tectonics, very much like today’s, has been going strong since the earliest times (more).

Mafic rocks

‘Mafic’ is a concept that’s important for plate tectonics, though possibly not for much else.


If this section loses you, you might like to skip to the next one, and then maybe come back to this topic.


 A mafic rock is a dense (heavy) rock.  The name comes from magnesium (Ma) and iron (Fe with the ‘e’ missing).  The magnesium bit is a puzzle because magnesium is one of the least dense (lightest) of metals.  But iron is the densest bulk material on the planet.  Anything denser only occurs in small quantities.  (You will have to ask the cosmologists why this is.  It has to do with exploding stars.)


As we’ve seen, the Earth’s core is almost pure iron.  Above it comes some 3 thousand miles (nearly 5,000 km) of mantle.  This is made of the most mafic rock of all, with the highest concentration of iron and other heavy metals.


As the mantle rock gradually extrudes its way up to the surface, at the spreading centres, it leaves some of the really dense material behind.  So the basalt that forms the ocean floor is less mafic than the mantle material that it came from.   This picture shows a lump of basalt.  If you compare it with the sample of granite, shown below, you will see that the granite is much more ‘crystalline’.


Incidentally the ‘komatiite’ that comprised the early ocean floor is even more mafic than basalt.  So whatever process favours the lighter material as the mantle rock works its way up, was less effective in the early days.


We mentioned earlier the processes that created the continental crust out of the oceanic basalt.  This process removes even more of the dense materials.  The continental rocks vary in maficity, but they are all less mafic than basalt. 


Silica is not particularly dense at all.  So sand is about the least dense of the materials that make up normal rock.


Sedimentary rock normally contains a lot of sand, so it’s generally the least mafic.

The different types of rock.

The first thing to say  about rock is that it is normally a glorious hotchpotch of different ‘salts’ .  Indeed, table salt (sodium chloride) makes an excellent rock, except that it’s soluble.  There are towns built on strata of rock salt, and they tend to have subsidence problems.


We won’t go into the chemistry of it, but a crystal of rock comprises a metal (magnesium, iron, calcium, sodium, you-name-it), combined with oxygen and either carbon or silicon.   Aficionados of the Periodic Table will not be too surprised at this.  They will know that silicon is in the same column as carbon, one row down. 


So really there are just two different kinds of rock, carbonate rock and silicate rock.  This is a gross over-simplification as we’ll see.  But it’s an important distinction just the same.


The most important exception to this simple story is straight silica.  Think glass, or beach sand.  Crystals of silica provide the sparkle to granite and its cousins, and it’s what gets left behind on the beach when the waves have ground the rock to smithereens, and have washed all the less durable components away.


Carbonate rock is chalk and limestone.  It comes from the carbon dioxide in the atmosphere, and it’s formed by sea creatures.  They grab dissolved carbon dioxide from the water, combine it with dissolved calcium and make their shells with it.   Sometimes you can see these shells very clearly. This picture shows a particularly prolific example.   I’ve read that there are also ‘inorganic’ processes that can produce limestone, but I don’t understand them to be significant players on today’s stage.


The real backbone of the Earth’s crust is silicate rock.  Silicate rock is essentially variations on the theme of granite.   Whereas carbonate rock is normally (always?) made of calcium carbonate, silicate rock can incorporate the entire gamut of different metals.  So granite has a lot of cousins.


But that’s not all.  Plate tectonic processes often bury surface rocks deep underground, where they experience great pressures and heat.  Not enough to melt them, as we’ve seen, but enough to ‘alter’ them quite a lot.  ‘Altered’ is a strange word to find in geological jargon.  But you get used to it.  We’ve already seen that much of northern India has buried itself under the Himalayas and Tibet.   When Tibet eventually wears down India’s rocks, now altered,  will re-emerge at the surface.


So in real life you end up with even more different rock types.  However they are all based on the above two


A very useful altered form of limestone is marble.  In fact there’s a limit to how much you can cook carbonate rock, before it disintegrates back into its constituent chemicals. Silicate rock can cope with much more punishment. 


There’s more.  These two rock types, together with their altered cousins, can be ground up into small pieces and deposited on the floors of lakes and rivers – to form sedimentary rock.  So sedimentary rock can be made of almost anything, including mud.  In general however, sedimentary material has had many of its heavier components washed away.  So sedimentary rock tends to be less dense than most.


© C B Pease, December 07