Plate tectonics: The main features are:

The Earth's surface is made up of a series of large plates
(like pieces of a giant jigsaw puzzle).

These plates are in constant motion travelling at a few
centimetres per year.

The ocean floors are continually moving, spreading from the
centre and sinking at the edges.

Convection
currents beneath the plates move the plates in different
directions.

The source of heat driving the convection currents is
radioactive decay which is happening deep in the Earth.

The edges of these plates, where they move against each other, are
sites of intense geologic activity, such as earthquakes, volcanoes, and
mountain building.

Plate tectonics is a relatively new theory and it wasn't until the
1960's that Geologists, with the help of ocean
surveys, began to understand what goes on beneath our feet.

Where is the Evidence for Plate Tectonics?

The continents seem
to fit together like a giant jigsaw
puzzle:

If you look at a map, Africa seems to snuggle nicely
into the east coast of South America and the Caribbean sea. In 1912
a German Scientist called Alfred Wegener proposed that these two
continents were once joined together then somehow drifted
apart.  He proposed that all the continents were once stuck
together as one big land mass called Pangea. He believed that Pangea
was intact until about 200 million years ago

Click here for
some evidence of this theory

CONTINENTAL DRIFT

The idea that continents can drift about is called, not
surprisingly, CONTINENTAL DRIFT.

When Wegener first put forward the idea in 1912 people thought he
was nuts. His big problem was that he knew the continents had
drifted but he couldn't explain how they drifted. The old (AND VERY
WRONG!!) theory before this time was the "Contraction theory" which
suggested that the planet was once a molten ball and in the process
of cooling the surface cracked and folded up on itself.  The
big problem with this idea was that all mountain ranges should be
approximately the same age, and this was known not to be true.
Wegener's explanation was that as the continents moved, the leading
edge of the continent would encounter resistance and thus compress
and fold upwards forming mountains near the leading edges of the
drifting continents. Wegener also suggested that India drifted
northward into the Asia forming the Himalayas and of course Mount
Everest.

SEA FLOOR SPREADING

It is hard to imagine that these great big solid slabs of rock could
wander around the globe. Scientists needed a clue as to how the continents
drifted. The discovery of the chain of
mountains that lie under the oceans was the clue that they were
waiting for.

PLATES ARE CREATED:
In the diagram below you can
see that the continental crust is beginning to separate creating a
diverging plate boundary. When a divergence occurs within a
continent it is called rifting. A plume
of hot magma rises from deep within the mantle pushing up the crust
and causing pressure forcing the continent to break and separate.
Lava flows and earthquakes would be seen. In the diagram below you
can see that the continental crust is beginning to separate creating
a diverging plate boundary. When a divergence occurs within a
continent it is called rifting. A plume
of hot magma rises from deep within the mantle pushing up the crust
and causing pressure forcing the continent to break and separate.
Lava flows and earthquakes would be seen.

This is an example of a divergent
plate boundary (where the plates move away from each other). The
Atlantic Ocean was created by this process. The mid-Atlantic Ridge
is an area where new sea floor is being created.

As the rift valley expands two
continental plates have been constructed from the original one. The
molten rock continues to push the crust apart creating new crust as
it does.

As the rift valley expands, water
collects forming a sea. The Mid-Atlantic Ridge is now 2,000 metres
above the adjacent sea floor, which is at a depth of about 6,000
metres below sea level.

The sea floor
continues to spread and the plates get bigger and bigger. This
process can be seen all over the world and produces about 17 square
kilometres of new plate every year.

Click here for the
Scotland story

It's important that we look at the evidence for
sea floor spreading. So click here

Picture the following in your mind:

You have a nine piece jigsaw (now there's a challenge).

The piece in the middle starts to grow.

It gets bigger and bigger.

What do you think will happen to the puzzle? ANSWER

Now let's think back to our plates being created at the mid-ocean
ridges, it seems to be a good idea but if this is the only type of plate
movement then the world would get bigger and bigger. In fact the world has
remained the same size. So if plates are being created at the mid-ocean
ridges then they must be being consumed somewhere else in the world.

PLATES ARE DESTROYED (SUBDUCTION):

This is a convergent plate
boundary, the plates move towards each other. The amount of
crust on the surface of the earth remains relatively constant.
Therefore, when plates diverge (separate) and form new crust
in one area, the plates must converge (come together) in
another area and be destroyed. An example of this is the Nazca plate
being subducted under the South American plate to form the Andes
Mountain Chain.

Here we can see the oceanic plate moving from left to right. The
top layer of the mantle and the crust (all called the lithosphere)
sinks beneath the continent. A deep ocean trench is formed. Water
gets carried down with the oceanic crust and the rocks begin to heat
up as they travel slowly into the earth. Water is then driven off
triggering the formation of pools of molten rock which slowly rises.
The plate moves downwards at a rate of a few centimetres per year.
The molten rock can take tens of thousands of years to then either:

Solidify slowly underground as intrusive igneous
rock such as granite.

or

Reach the surface and erupt as lava flows. Cooling
rapidly to form extrusive igneous rock such as
basalt.

The floor of the Easter Pacific is moving towards South America at a
rate of 9 centimetres per year. It might not seem much but over the past
10 million years the Pacific crust has been subducted under South America
and has sunk nearly 1000 kilometres into the Earth's interior.

Types of Convergent Boundaries

The example above showed what happened when the dense oceanic plate
subducts under a lighter continental plate (ie, oceanic - continental).
Two other types of subduction can take place:

When two oceanic plate meet each other
(oceanic-oceanic) this often results in the formation of an island
arc system. As the subducting oceanic crust melts as it goes deeper
into the Earth, the newly-created magma rises to the surface and
forms volcanoes. If the activity continues, the volcano may grow
tall enough to breech the surface of the ocean creating an island.

The key to subduction seems to be water which acts as a kind of
lubricant as the heavier plate slips underneath the lighter plate.

I must not forget to mention the Himalayas
and Mount Everest because this is the third example of plate
movement

Millions of years
ago India and an ancient ocean called the Tethys Ocean were sat on a
tectonic plate. This plate was moving northwards towards Asia at a
rate of 10 centimetres per year. The Tethys oceanic crust was being
subducted under the Asian Continent. The ocean got progressively
smaller until about 55 milion years ago when India 'hit' Asia. There
was no more ocean left to lubricate the subduction and so the
plates welled up to form the High Plateau of Tibet and the
Himalayan Mountains. The continental crust under Tibet is over
70 kilometres thick. North of Katmandu, the capital of Nepal, is a
deep gorge in the Himalayas. the rock here is made of schist and
granite with contorted and folded layers of marine sediments which
were deposited by the Tethys ocean over 60 million years ago.

The fourth type of plate movement involves plates
sliding past one another without the construction or destruction of crust.
This boundary is called a conservation zone because plate is neither
created nor destroyed An example of such a   boundary is the San
Andreas fault in California. The force needed to move billions of
tonnes of rock is unimaginable. When plates move some of the energy is
released as earthquakes.

PLATE TECTONICS AND EARTH STRUCTURE WEB LINKS

Plate
Tectonics   Excellent source of volcanic information.

Plate
Tectonics and People    A comprehensive site dealing
with the effects that plate tectonics has on
humans.     ( Edited version on this site -
click here)

Plate
Tectonics    Excellent explanation of plate tectonics
and  plate boundaries.

Plate Tectonics,
TSAW Project   Good source for general information.

Earth's
Interior and Plates.

Continental Drift
Reconstructions

Webquest: Plate
Tectonics Contains lists of questions dealing with plate
tectonics.

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