1. Continental Drift and the theory of Plate Tectonics

2. 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

3. 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

4. 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:

5. 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

6. 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

7. 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 north-west Europe are similar
Similar glacial deposits are found in Antarctica, South America and India, which are now thousands of km apart.

8. Subsequent discoveries that support the theory of continental drift
The two discoveries we are now going to look at are:
Sea floor spreading
Palaeomagnetism

9. Sea floor spreading Harry Hammond Hess
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

10. 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

11. 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

12. Is Wegener theory of continental drift now accepted?
Yes
Continental drift is now widely accepted in the scientific community
Palaeomagnetism suggests continents do move
Sea floor spreading shows how new oceanic crust is formed

13. 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!

14. 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

15. 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

16. Thankyou for listening! Photo Courtesy of Chloe