the nature and action of PLATE TECTONICS
9. More on Plate Tectonics
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8. Crustal Tectonic
Plates and their movement
How the Earth's crust is split into large sections
called tectonic plates is described. Their movement and effects at plate
boundaries are explained e.g. earthquakes, volcanoes, mountain building,
ocean ridges/trenches, subduction (part of the rock cycle). Evidence to support the theory plate
tectonic movement (Wegener's theory of continental drift) is also described e.g. fossil similarities, magnet
continent shapes seem to fit into each other etc.
Plates of the crust and their movement - unstoppable
the basic ideas and evidence -
see also section 9.
for more on plate tectonics and earthquakes.
What can happen when tectonic plates meet or part
The 'compact' diagram
Fig 2. Plate Tectonics above gives the
"2nd Big Picture View"
view of plate tectonics
and the situations at (1) to (4)
will be referred to
throughout the answer
notes of sections 8. and 9.
The Earth's surface is very uneven
with all its mountains and valleys and at one time scientists thought this was
due to shrinkage of the Earth's surface (crust) as it has become cooler over
millions of years. We know this is not true, apart from erosion and the effects
of rivers, all the major geological features of the Earth can be explained by
the science of plate tectonics. This theory is much better at explaining
certain geological events and patterns observed in terms of eg mountain ranges
and deep ocean trenches.
There were features of the
Earth's geology and fossil record which could not be explained e.g.
Similar fossils of extinct
plants and animals were found on opposite sides of the Atlantic Ocean
It had been suggested that
these continents were once linked by land bridges that had sunk, or
washed away or just covered in water as the Earth cooled and contracted
and sea levels rose.
Also, the fact that
geographically the continent of Africa seem to fit quite neatly into the
shore line of the South America continent.
PLATE TECTONICS THEORY: The
Earth's lithosphere (the crust and the upper part of the mantle) is cracked into
a number of large pieces called tectonic plates, you can think of them as
giant rock rafts floating on the 'plastic' mantle. These plates (like big
rock rafts) are less dense than the mantle and so float on it and constantly
move at relative speeds of a few centimetres per year as a result of
convection currents within the Earth's mantle. The convection currents are driven by heat released by natural
radioactive processes in the mantle. This is what is meant by 'Continental
Drift'. Earthquakes and/or volcanic eruptions occur at the plate boundaries between
tectonic plates exemplified by the volcanic 'Ring of Fire' in the Pacific Ocean
and the earthquake zone of the San Andreas Fault on the west coast of North
America. This geologically violent activity happens when plates collide, move
away from each other and when one plates sub-ducts below another.
Where plates meet or move apart you get
volcanic activity. Where plates pass each other you tend to get earthquake
activity. For more details on this see section
plate tectonics, subduction zones,
effects of plate movement
Fig 10. A greatly simplified WORLD MAP
OF MAJOR PLATES of the Earth's crust
and some regions of specific
The Earth's lithosphere is the crust
and the upper part of the mantle. The Earth’s lithosphere is divided into plates
meaning they are divided into sections that meet at plate boundaries
(situations (1) to (4)
all represent plate boundary regions).
The plates effectively float on the more dense mantle material and move at speeds of 1-4
cm/year relative to each other. This plate movement means that most
parts of the Earth's crust are in very different locations from millions of
years ago e.g. in the UK, limestone cliffs from warm shallow seas and seams of
coal, the remains of tropical forests suggest that at one time we were in a
warmer climate a bit nearer the equator! The crust is the lightest rock of the three layers of the Earth. The
crust plate material under continents tends to be thicker and made of lighter 'granites' but
oceanic crust is a thinner but more denser 'basalt' type rock.
Plate movement, refer on the
2. diagram to (2)
the mantle heat is generated by radioactive decay of longer lived isotopes and is
transferred by conduction and convection throughout the mantle. This heat causes huge 'plumes' or
currents of hot 'plastic' magma to rise and these convection currents
in the mantle 'drive' the tectonic plates of the crust when they reach the
- (ii) If the crust is thin and weak
e.g. on the mid-ocean sea-bed, the hotter less dense and more 'runny' magma can break through and spread out on
either side forming new crust when the sea water it cools.
8(c) Where the plates of the Earth meet is called a plate boundary.
- Some of the evidence which is used to ‘map out’ the
plate boundaries ...
- bands of earthquake activity - the place origin of an
earthquake can be calculated from the readings of seismographic stations
around the world
- bands of volcanoes e.g. the 'Ring of Fire' in the Pacific
- more recent mountain ranges
- deep ocean trenches near continental plate edges
- mid-ocean ridges which can now be accurately mapped with
modern echo sounding techniques.
11. The 'reconstruction' of the super continent PANGAEA
It is now recognised that all the great
continental plates we now believe exist were once joined in one giant super
continent called Pangaea. Its structure has been worked out from geological data eg rock type
sequences, fossil sequence, mapping deep ocean trenches etc.
8(d) At one time it was believed that the major
features of the earth's surface were the
result of the shrinking of the crust as the Earth cooled
down following its formation. Wegener's theory of crustal movement
('continental drift') was not generally accepted until more than
50 years after it was proposed, so why not?
(right - Pangaea) and NOW (left - today's continents and oceans)
Some of the
crustal movement or ‘continental drift’ i.e. plate movement on a large scale over millions of
years in which land masses, once joined as a 'super-continent' (known as PANGAEA
- shown above on right), move apart by
several thousand kilometres is outlined below.
From 1912 onwards the German scientist Wegener
(1880-1930) first proposed the theory of continental drift and a previous
super-continent (Pangaea) which broke apart into 'smaller' continents, still
moving today with considerable evidence.
it was hotly disputed and rejected by most scientists of his day.
Wegener based his theory of 'continental drift' on the
following observations and ideas.
(i) The coastlines of South America (eastern)
and Africa (western) seem to snugly fit into each other, so maybe they had
once been part of a super-continent.
(ii) He knew of lists of similar fossils of
plants and animals that were found on the east coast of South America and in
the southern parts of Africa (see Fig 12. below).
(iii) He found other cases of similar fossil
sequences from land masses on opposite sides of other oceans.
(iv) The sequences of rock layers on opposite
sides of oceans matched each other.
(v) He assembled lots of evidence that plants
and animals on opposite sides of oceans were often strikingly similar. For
example the marsupials in Australia and South America are very similar and
look alike, but so did the flatworms that parasitized them!
(vi) Fossils seemed to be in the wrong place
eg why were fossils of tropical plants found coal and shales in the Arctic
and Antarctic polar regions.
(vii) From the evidence he accumulated
Wegener proposed that a giant super-continent had once existed, which we
know call Pangaea, and this super-continent broke up into sections over
millions of years, and he was dead right! even if he didn't know the
mechanism of continental drift! He also proposed that mountain ranges are
formed when continents collide, spot on again Wegener! All of this can now
be explained by the science of plate tectonics, but all his theories were
initially rejected before, during after the First World War..
Wegener's theory was
generally rejected for several reasons e.g.
(i) National prejudice (at least in Great
Britain and France), he was German and the
1st World War was going on, and perhaps he was mentally unstable and subject to
delusions, he was cruelly ridiculed by British scientists!
(ii) He was a meteorologist (weather
scientist), geophysicist and polar explorer, and NOT a geologist
The mechanism of continental 'drift' could not be explained or the 'timescale' appreciated.
(iv) Other scientists explained the fossil
similarities were due to land bridges in the past that plants and animals
could migrate across from one continent to another. These inter-continental
land bridges had eroded away or sunk beneath the seas and oceans. They also
queried 'why can't this continental drift be detected'?
It can be know, even at a rate of 1 - 4
cm per year, modern laser surveying instruments can measure these minute
changes. They do so in Iceland where the Mid-Atlantic ridge goes right
through the island and monitoring instruments have set up to measure the
rate at which the North American plate is moving away from the Eurasian
(v) Wegener conceived the idea that the
continents barging their way through ocean bed powered by tidal forces and
the Earth's rotation, but scientists considered the force was not great
enough and it was considered that Wegener had used inaccurate data in his
calculations of how fast the continents could move apart.
A modern view of the evidence for 'continental
drift' which supports the theory of plate tectonics
Despite the faults of Wegener's theory and
calculations his basic idea was correct and the continents are drifting
apart, and continue to do so today and they all originate from a once
It was only the development of sonar echo-sounding,
and other sonar technology, during and after
the 2nd World War that the oceans were finally 'mapped out' in the 1950's - 60's and
the recognition that deep ocean trenches existed and the mid-Atlantic ridge give
evidence of sea floor spreading. This was linked with data from the crucial
development of radioisotope dating and magnetic recording techniques. So by the
late 1960s the theory of tectonic plates was well endorsed by most
scientists and back up by increasingly diverse and more accurate data.
The Mid-Atlantic Ridge, running the whole
of the Atlantic Ocean, has been the focus of much scientific research.
In the 1960s scientists investigating the
Mid-Atlantic Ridge found evidence that molten rock (magma) rises up
through the sea floor. On emerging into the cold water it solidifies and
forms undersea ridges and mountains that are roughly symmetrical on
either side. As the magma wells up it spreads on either side of the
ridge, causing two sections of the old AND new crust to move apart at
about 10 cm/year.
- The basic evidence for supporting the theory
of continental drift, explained by plate tectonics, is no different than
Wegener's observations in the early 20th century. The difference between now and then is that
we have more accurate rock dating data from radioisotope analysis and the
sonar mapping of ocean beds, particularly deep ocean trenches.
- Several continent shapes seem to fit into each other
South America and Africa.
- From the map below you can see that Africa
seems to fit into the contours of North America and South America.
- Different continents have similar ancient mountain ranges
made of the same rocks formed in the same sequence, and of the same age, but now geographically
far apart. Sometimes a mountain band in the same country is 'broken' into
two displaced sections by side-ways plate movement e.g. granite hills in the
Great Glen of northern Scotland.
12. The geological rock and fossil evidence of the past link South America and Africa
- Rock types and
fossils, and their sequence and age, are very
similar in South America and South Africa up to about 200 million years ago and
then the sequences diverge as the continents parted.
- After 200 million years the fossil record
changes, the continents are now completely separated and the sequences both
fossils (from different evolutionary paths) and sedimentary rocks become
- Animals on different continents seem
to have a common ancestor e.g. llama in South America and the camel in Africa.
Reversal Patterns - Paleomagnetism science
- Bands of rock on either side of a mid-ocean ridge show the
same pattern of .....
- ... the N-S poles of the Earth's magnetic field 'flip
around' every so often, and this is called magnetic
- ... the direction of N-S pole reversal is 'trapped' in
new rocks formed as magma from the mid-ocean ridge rises up and is
exposed to the cold ocean water, and so cools and solidifies.
- The 'flips' happen over about 1000 years? but millions of years elapse
between each magnetic reversals.
- BUT what is the origin of this 'magnetic
- The explanation lies in the iron-rich
minerals in the magma that record the direction of the Earth's magnetic field
at the time when the rising magma solidified.
- The iron minerals in the cooling magma are very slightly
magnetised by the Earth's magnetic field and so on solidification the rocks
preserve an imprint of the direction of the Earth's magnetic field of the
- When the rock crystals
set, the iron atoms in the minerals act as tiny magnets, and they will
align themselves in the current direction of the Earth's magnetic field
and remain permanently set in that direction when the solid rock forms.
- Just like iron
filings scattered around a bar magnet line up in particular directions.
- But the direction of the magnetisation i.e.
the polarity, is determined by the direction of the Earth's magnetic field.
- Every so often the Earth's magnetic field
flips round e.g. N=>S to S<=N, so you get bands of rock with 'normal
polarity' followed by bands of rock with 'reversed polarity'/
- Matching magnetic reversal patterns
in oceanic crust occur in bands of stripes parallel to oceanic
ridges and on both sides of the Atlantic Ocean!
- These bands mirror each other and match
the periodic reversals of the Earth's magnetic field and so support
the concept of sea floor spreading.
- The pattern of polarity changes can be used to
estimate the different sections of the Earth's crust and track the very slow
movement of the tectonic plates.
- Geological studies of glaciated areas in east South America
match those in South and West Africa.
sedimentary rocks seem to be in the wrong place! Coal from hot swampy forests and coral limestone from warm
shallow seas can be found in Northern countries like Scotland and in
the extreme cold of Antarctica near the South Pole!
- So, how did they get there? Answer,
continental drift .....!!!!