1. Evidence for Continental Drift
Jigsaw Puzzle fit of continents
Alfred Wegener during Greenland expedition

2. More evidence
Matching fossils on continents now located thousands of miles apart. Example = Mesosaurus, a freshwater reptile
Many others

3. Matching mountain ranges

4. More evidence Matching geologic structures including: Mountain chains
Ore deposits
Same rocks of same age

5. More evidence Climate change evidence
Glacial deposits at current equator
Fossilized palm trees in Greenland
Map shows why according to the placements of current continents within Pangaea

7. Wegener not believed Why? -
What could possibly force the continents to move across the ocean floor in this way. They would be crushed.
He was a meteorologist,
not a geologist

8. Developments 1950s and 1960s
World War II submarines using sonar found mountains under the oceans – the mid-ocean ridges
Sea floor drilling showed rocks younger than expected and youngest towards the center of the mid-ocean ridge
Theory of seafloor spreading suggested by Princeton professor Dr. Harry Hess

9. First look at the earth’s layers as shown here.
Seafloor spreading
First look at the earth’s layers as shown here.

10. Evidence for sea floor spreading
Alvin and other submersibles found cracks and lava showing spreading and volcanism at mid ocean ridges and odd life forms
Matthews and Vine’s survey of the Indian Ocean sea floor showed matching stripes of reversing polarities on either side of ridge – what caused these?
Lava spreading during alternating magnetic periods

11. The Earth’s magnetic field
It is produced by the outer core of the earth which is made of liquid iron and nickel
This moving magnetic material produces a moving magnetic field, which in turn produces a moving electric field. It is a dynamo!
Earth’s magnetic field varies over time and it protects us from cosmic radiation

12. Seafloor spreading and paleomagnetism

13. Pangaea revisited
By piecing together this information, we can see how the continents have moved over the past 200 million years, due to seafloor spreading

14. Plate tectonics
Sea floor spreading provides the driving mechanism for movement
However, it is not the continents that are moving, but the “plates” of lithosphere “floating” in effect on the asthenosphere
The lithosphere is made up of about 20 plates which move relative to each other in several ways
Let’s look at a generalized sketch

15. Confirmation of the Theory of Plate Tectonics
Evidence supporting the theory of plate tectonics:
Apparent Polar wandering: plate movement causes the apparent position of the magnetic poles to have shifted.
The paleomagnetic fields in the rocks would indicate a single pole until the continents drift apart.

16. Confirmation of Plate Tectonics Theory
Paleomagnetism: strips of alternating magnetic polarity at spreading regions.
The patterns of paleomagnetism support plate tectonic theory. The molten rocks at the spreading center take on the polarity of the planet while they are cooling. When Earth’s polarity reverses, the polarity of newly formed rock changes.

17. The Plates

18. This diagram shows the major Tectonic Plates.
Presenter: Point out the UK, sitting on the Eurasian Plate. Also the plate boundary between Africa and South America (note that it has the same shape as the coastlines in these countries).

19. Sea-Floor Spreading and Plate Boundaries
To view this animation, click “View” and then “Slide Show” on the top navigation bar.

20. Age of Oceanic Crust Courtesy of www.ngdc.noaa.gov
Ocean Ridges: This map shows the age of the oceanic crust. The red colouring shows the youngest ages, whilst the dark blue shows the oldest ages (around 200 million years old).
Presenter ask: Where are the Ocean Ridges located? I.e. where are the divergent boundaries?
Answer: The divergent boundaries are where the plates are pulling apart and new material is being produced. Therefore the Ocean ridges are in the middle of the red areas (the boundaries are in fact shown on the map). We can see a progression of the oceanic crust getting older away from the ocean ridges (like a conveyer belt).
Presenter: Before moving on to the next slide, point out Iceland. The divergent boundary runs straight through Iceland….
Courtesy of

21. Mid Atlantic Ridge
Mid Atlantic Ridge

22. Divergent Boundaries Spreading ridges
In plate tectonics, a divergent boundary is a linear feature that exists between two tectonic plates that are moving away from each other. These areas can form in the middle of continents or on the ocean floor.
As the plates pull apart, hot molten material can rise up this newly formed pathway to the surface - causing volcanic activity.
Presenter: Reiterate the process by going through the diagram, including the presence of mantle convection cells causing the plates to break apart and also as a source for new molten material.
Where a divergent boundary forms on a continent it is called a RIFT or CONTINENTAL RIFT, e.g. African Rift Valley.
Where a divergent boundary forms under the ocean it is called an OCEAN RIDGE.
Spreading ridges
As plates move apart new material is erupted to fill the gap

23. Converging Margins: India-Asia Collision
To view this animation, click “View” and then “Slide Show” on the top navigation bar.

24. Transform Faults and Seafloor Spreading
To view this animation, click “View” and then “Slide Show” on the top navigation bar.

25. What causes plate tectonics?
Convection in the mantle, as the plastic asthenosphere flows, carrying the plates with it.
This is probably aided by slab pull at subduction zones and ridge push at mid ocean ridges and rising plumes in the mantle
This diagram shows several different model hypotheses

26. “Plates” of lithosphere are moved around by the underlying hot mantle convection cells
How and Why do tectonic Plates move around?
The question of how tectonic plates are moved around the globe is answered by understanding mantle convection cells.
In the mantle hot material rises towards the lithosphere (like hot air rising out of an open oven - ever opened an oven door and felt the blast of hot air coming past your face?). The hot material reaches the base of the lithosphere where it cools and sinks back down through the mantle. The cool material is replaced by more hot material, and so on forming a large “convection cell” (as pictured in the diagram).
This slow but incessant movement in the mantle causes the rigid tectonic plates to move (float) around the earth surface (at an equally slow rate).