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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
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
The
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.
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
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
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
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.