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Plate Tectonics Theory.
1620 - English philosopher Francis Bacon wrote about the long held view that
the land on either side of the Atlantic 'fitted together' and must be related.
1912 - Alfred Wenger used 300 years of topological and geological evidence to
put forward his theory that the continents were once all joined together in an
ancient supercontinent called Pangaea. He proposed that the continents had
subsequently 'drifted' apart to their present positions. Critics pointed out that
there was no mechanism by which the continents could move over an 'obviously'
solid Earth.
Wenger’s evidence:
1. Continental Fit
Some continents seem to fit together if placed side by side. Even better
fit if continental shelf is used.
2. Geological evidence
Rocks of the same age, type and formations are found in different parts
of the world. They clearly match when the continents are 'fitted' back
together. Similar evidence of glacial deposits has also been found
3. Climatological evidence
Places as far apart as Antarctica, North America and the UK contain
similar coal deposits formed in tropic climates in the Carboniferous
period. They must have drifted from their original locations.
4. Biological evidence
Identical plant and animal fossils are found in rocks millions of years old
and thousands of miles apart.
Some species like marsupials are found only in one place e.g. Australia
because they drifted apart before preditors that wiped them out
elsewhere migrated to those areas.
1948 - Survey of the Atlantic Ocean floor revealed a continuous ridge running
largely north to south. It was large - around 1000 km wide; reached heights of
2.5 km and was made up of volcanic rock. Similar submarine mountain ranges
were subsequently found in the Pacific Ocean.
1950s - magnetic surveys of the ocean floors shown very regular patterns of
palaeomagnetism striping either side of the ridges. The Earth's magnetic field
is 'recorded' in the rocks when they cool. As the stripes mirror themselves
either side of the ridge, it is clear that new rocks are being added equally on
both sides.
1950s and 60s - ages of the oceans floor were measured. Surveys suggested
that very young rocks (less than 1 million years) are found near the ridges and
older rocks (over 200 million years) are found near the continents.
But....
If the crust was pulling apart and being added to on the ridges, why was the
planet not getting larger?
Answer....
Crust must be being destroyed somewhere!
As no one could see it on the land, they looked in the sea. They discovered huge
trenches where large areas of the ocean floor was being subducted. Hence the
mechanism for sea floor spreading and continental drift was provided.
So, what is going on?
Current theory state....
Higher temperatures at the Earth's core and heat released by radioactive
decay of elements within the mantle help to create convection currents.
These currents exhibit a continuous circulatory motion within the
aethenosphere that causes the crustal plates to move.
They pull apart at speading ridges (under the oceans) and rift zones (on the
continents).
The thinner oceanic crust of the spreading ridges allows magma to escape and
new crust to form.
The oceanic crust is then pulled back down into the mantle at subduction zones.
The less dense and more buoyant continental crust is not consumed, which
explains why it is geologically more complex and contains much older rocks.
The Earth's surface is divided up into over 50 plates moving at different rates
and in different directions.