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Continental Drift and the
theory of Plate Tectonics
What is Continental
Drift?
Definition: The gradual movement of the continents
across the earth's surface through geological time.
How is it caused?
Plate tectonics suggests that Earth's entire crust is
composed of a number of large plates that are constantly
moving in relation to each other
What is initial evidence
of continental drift?
Well, there’s four main areas, that
support the theory of continental fit, we
are going to look at:
• Continental fit
• Climatological
• Biological
• Geological
Continental fit:
If we look at a world map, it appears the
continents fit together like puzzle pieces
Continents that match include:
• South America and Africa
• Australia and Antarctica
• North America and Europe
We can see this puzzle like fit in these diagrams:
Climatological:
Wegener collected ancient climate information
to confirm his glacial data.
• The data from the sedimentary rocks showed a change in climate – which
can only be explained by continental drift
• The data of fossils of tropical plants were found in coal in Antarctica –
which means Antarctica must have previously been close to the equator
• Glacial Deposits were found in South Africa – meaning South Africa must've
been further away from the equator for water to have formed
Biological:
Biological evidence of continental drift includes:
• Dinosaur fossils were found in Australia's Dinosaur Cave of
dinosaurs adapted to colder climates – so Australia must’ve
been closer to the poles to create these colder climates
• Rocks in mild climates show scarring of much colder
climates
• Plant remains from humid swamps have been found in both
Antarctica and India
• Fossil formations have been found on either side of the
Atlantic
Geological:
Geological evidence of continental drift includes:
• The ‘same’ rocks have been found in south-east Brazil and
South Africa.
• The trends of the mountains in the eastern USA and northwest Europe are similar
• Similar glacial deposits are found in Antarctica, South
America and India, which are now thousands of km apart.
Subsequent discoveries
that support the theory
of continental drift
The two discoveries we are now going to look at
are:
• Sea floor spreading
• Palaeomagnetism
Sea floor spreading
Sea Floor Spreading = the formation of fresh
areas of Oceanic crust which occurs through the
upwelling of magma at Mid-Ocean Ridges
Harry Hammond Hess
• His time as a navy officer during World War 2 meant
he used sonar (also known as echo sounding) to map the
ocean floor across the North Pacific
• Everyone thought that the ground was completely flat
under the sea like this:
• However Harry Hess found that the oceans were
shallower in the middle and discovered Mid-Ocean
Ridges:
• He outlined a theory that explained that the continents
actually drift, this is now known as Sea Floor Spreading
• Hess believed that ocean trenches were the places
where the oceans grew from
Palaeomagnetism
Palaeomagnetism= the study of fossil magnetism in
rocks and the investigation of the Earths
magnetic field
• Frederick Vine and Drummond Matthews studied the
patterns of magnetic stripes on the ocean floor
• The magma rises from the mantle and cools to make
new crust which preserves a record of the polarity of
the Earths magnetic field
• They noticed that the stripes were symmetrical on
each side of the Mid-Ocean Ridge and this is the same
with the ages of the rocks where the youngest is
closest to the ridge and the oldest is furthest away
Sea floor spreading diagram
Why were scientists
sceptical of his ideas?
• Many people embraced the idea that the two continents
(South America and Africa) were a result of a land bridge
between the two continents
• Wegner actually specialised in meteorology and astronomy,
not geology – so his theories were not taken seriously
• His ideas were not concrete – scientists said
centrifugal force and tidal pull were not strong
enough to cause continental drift
Is Wegener theory of
continental drift now
accepted?
Yes
o Continental drift is now widely accepted in the
scientific community
o Palaeomagnetism suggests continents do move
o Sea floor spreading shows how new oceanic crust is formed
However is the theory of
continental drift as robust as it
may seem?
After all we’ve said there are actually some
cracks in Wegener's theory.
So we are going to look at another theory (we’ll call it expanding earth
theory) that may prove Wegener wrong: The theory is…
The size of the earth was much smaller than it is today and so all continents were
together as a single huge land mass (called Pangea) then and as earth continued
grow in size all these continents looked like as if they are moving apart whereas they
actually have just reached their current positions!
However how robust is
the argument of the
‘expanding earth’
• The continents do crash and bump into each other "willy-nilly“
according to GPS measurements so there definitely is continental
movement
• The young seafloor is evidence for spreading and subduction
• There is much more evidence of continental drift that does not rely
on theory
So, to conclude
• When Wegener first proposed the idea of continental drift, it was widely not
accepted
• However new discoveries (e.g. palaeomagnetism and sea floor spreading)
have since meant it is now widely accepted
• There are some other theories; however these are a lot less robust than the
theory of continental drift
Thankyou for listening!
Photo Courtesy of Chloe