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Physical Geography
Earth Science
Plate Tectonics and Continental Drift
Continental Drift
As far back as 1620, Francis Bacon spotted that the west coast of Africa and the
east coast of South America looked as if they would fit together, like pieces of a
jigsaw. Between then and 1912 other people identified further similarities
between other continental coastlines, but it was only in 1912 that Alfred Wegener
published a theory to explain why the Earth looked like a huge jigsaw.
He suggested that a very long time ago all the land that covered the Earth had
been joined together into one huge continent. He named this landmass,
Pangaea, and suggested that millions of years ago this supercontinent had
somehow broken up. The different parts had then drifted into the present day
positions.
His evidence for this was drawn for several sources, covering geology, biology,
geography and other sciences.
Other discoveries added to his list of evidence. Coal had been found in the
Antarctic, but coal only forms under warm, wet conditions. The same problem
applied to the English coal fields. The present climate in Britain could never have
produced coal. One of two things could explain this. Either the Antarctic and
Britain had always been were they are today, but with a warm, wet climate, or the
Antarctic and Britain had once been much nearer the equator.
It seemed impossible to Wegener that the present day location of Antarctica had
ever had a climate that could form coal. Unless the Earth had changed its orbit
around the sun, the climate at the poles would always have been the same as it
is today; very cold. If the climate hadn't changed, then the only answer had to be
that the land had moved after the coal was produced.
Wegener's theory was ignored by most of the scientists of his day. This was
partly because Wegener himself wasn't a famous scientist, but also because he
couldn't explain why or how the surface of the planet moved. Remember, in 1912
there were no satellite images to study, none of the advanced rock dating
techniques we use today and, less than 100 years earlier, the accepted view was
that God created the world a few thousand years ago. The idea that the surface
of the Earth was moving was a bit too radical for many people to believe!
The origins of Plate Tectonics
In 1948 a scientist called Ewing was examining a group of islands in the Atlantic
Ocean. He discovered that the islands were the highest points along a mountain
range hidden below the sea. The mountain range was huge, and much to his
surprise he discovered it was made of young volcanic rocks, not ancient ones as
everyone would have expected. He named this ridge, the Mid-Atlantic Ridge.
Ewing's work was followed up by an examination of the rocks to either side of this
mountain range. It was already known that from time to time the Earth has
reversed it's magnetic field, the north and south poles swapping over. It was also
known that as volcanic rocks cool, all the minute particles of iron in them record
the magnetic field around them, preserving the position of the north and south
poles like a fossil. Scientists discovered that the rocks not only showed bands of
north and south magnetic fields, but that the patterns on either side of the
mountains were almost identical. This was a very important discovery because it
showed that the rocks had been flowing out from either side of the mountains.
In 1962 another scientist called Hess made a further discovery. He proved that
the newest rocks under the Atlantic were next to the Mid Atlantic Ridge, and that
the oldest ones were near the coast of the USA. From this he concluded that the
sea floor was slowly moving outwards from the Mid Atlantic Ridge, and moving
towards the USA coast. The sea floor was spreading out from the middle, by up
to 5cm every year!
Cross -section of the Mid-Atlantic Ridge showing bands of north and south polarity radiating out
from the center of the ridge
Now then, if the sea floor is getting wider, it would seem that the Earth itself must
be getting bigger, but scientists knew that the Earth was staying the same size.
The solution was found when it was discovered that parts of the Earth's crust
were being destroyed along the edges of the Pacific Ocean. Along the pacific
coast, submarines had found amazingly deep trenches and it seemed that the
Earth's surface was slowly moving into these trenches and vanishing as fast as
new surface was being created along the ocean ridges.
These new discoveries provided the information Alfred Wegener had been
unable to discover; an explanation of how and why the continents moved.
Plate Tectonics
The theory of Plate Tectonics, as this new discovery was named, suggested that
the crust of the Earth is split up into seven large plates and a few smaller ones,
all of which are able to slowly move around on the Earth's surface. They float on
the semi-molten mantle rocks, and are moved around by convection currents
within the very hot rocks.
The plates are divided into type types; continental and oceanic.
Continental crust is made of older and less dense rocks such as granites. The
most abundant minerals in continental crust are Aluminium and Silicon. From the
chemical symbols of these two elements ( si and al ) a new name was created for
these rocks, sial.
Sial is generally 35 to 70 km thick and mostly over 1500 million years old.
Oceanic crust is made of younger rocks like the ones found along the Mid
Atlantic Ridge. The most common rocks are basaltic ones, which are more dense
than the ones found in Continental crust. The two main minerals in these rocks
are silicon and magnesium ( si and ma ), so this type of rock is called sima.
Sima is only about 6 to 10km thick and has an average age of a mere 200 million
years.
How do the plates move?
The centre of the Earth is very hot, and some of this heat moves outwards into
the mantle. This heating isn't the same all over the planet so some areas become
hotter than others. Heat, like water, tries to level itself out, so heat energy moves
from hot places to places that are cooler. As the rocks slowly move below the
Earths surface, they drag the crust that lies over them. This causes the
continents to move.
At the edges of the plates we see the ridges and trenches that gave the first
clues to Plate Tectonics. There are a few rules that control what happens along
these edges, or plate boundaries as they are called.
The Rules of Plate Tectonics
1
Continental crust is less dense, or lighter, than Oceanic crust so it
doesn't sink. It is never destroyed and is permanent.
Oceanic crust is the heavier of the two crustal types, so it will sink
2 below Continental crust. It is constantly being formed and destroyed
at ocean ridges and trenches.
3
Continental crust can carry on beyond the edges of the land and
finally end far below the sea. This explains why the edges of all the
continents don't have deep trenches right up against their coastlines.
Plates can never overlap. This means that they must either collide
4 and both be pushed up to form mountains, or one of the plates must
be pushed down (subducted) into the mantle and be destroyed.
There can never be gaps between plates, so if two plates move
5 apart, as in the middle of the Atlantic, new rock will be formed to fill
the space.
We know the Earth isn't getting bigger or smaller, so the amount of
6 new crust being formed must be the same as the amount being
destroyed.
Plate movement is very slow. This is partly why Wegener's original
ideas were ignored. Nobody could 'see' the continents moving.
7 When the plates make a sudden movement we call it an
Earthquake, and it's just about the only time we are directly aware of
the plates moving.
Activities
(a) Read through the lesson, then create a list of the main evidence suggesting
that Continents have moved around the globe.
(b) Why do you think that there are coal deposits in Britain, despite the fact that
Britain is too far north to have a warm, wet climate?
(c) The sea floor is always spreading, but the Earth isn't getting bigger. Why is
this?
(d) Why do you think that there aren't deep trenches along the edges of all the
continents?