1. Sea Floor Spreading and Continental Drift

2. Continental Drift
In 1915, Alfred Wegener ( ), a German meteorologist, wrote a book titled The Origin of Continents and Oceans.

3. Continental Drift Wegener proposed the idea of Pangaea
A supercontinent that drifted
apart
Wegener had some evidence
to support his theory which he
called Continental Drift

4. Evidence of Continental Drift
The continents fit together like a puzzle
There are fossils of ancient animals and plants on different continents and there were no land bridges
The mountain ranges on different continents align with one another
Climate changes: Tropical plant fossils found in non-tropical regions

5. Pangaea and its break-up

6. Fossil Evidence of Continental Drift

7. Sea Floor Spreading
In the early 1960s, Henry Hess ( ), a geologist and former commander of a Navy ship equipped with an echo sounder, used the profile of the sea floor to propose that it was spreading at the mid- ocean ridges.

8. Sea Floor Spreading
Hot fluid from the mantle (called magma) enters the rise or valley and cools, creating new sea floor (also called oceanic crust).

9. Sea Floor Spreading
The advent of Sonar allowed for mapping of the ocean floor.
Hess, using sonar, discovered significant depth differences along ocean bottom
Hess explained the difference by proposing mountain ranges and Rift Valleys.
He stated that the ocean floor must be spreading at the Rift Valleys, creating the depth differences

10. Mapping the Mid-Ocean Ridge
The mid-ocean ridge is the longest chain of mountains in the world.
Scientists mapped the mid-ocean ridge using sonar.
Sonar is a device that bounces sound waves off underwater objects and records it.

11. Evidence of Sea Floor Spreading
Age of the crust at mid ocean ridges
Older crust further away from the ridge, young crust at the ridge
Magnetic reversals over geological time
Alternating patterns that align on either side of the ridge
Pillow rocks at rift valleys
Evidence of magma rising

12. In the 1960s scientists found evidence that new material is erupting along the mid-ocean ridge.
The presence of pillow shaped rocks has showed that molten material has erupted again and again.
Pillow rocks are created when magma cools quickly at a mid ocean ridge.
Pillow Rocks

13. Magnetic Patterns
Over geologic time, the magnetic polarity of Earth has switched.
Scientists believe the poles switch because of a magnetic interaction between the planet’s inner and outer core.

14. Magnetic Reversals in Crust
Evidence in the rocks shows that Earth’s magnetic poles have reversed.
The iron bits line up in an alternating pattern.
The alternating pattern demonstrates crust movement

15. Age of the Crust
Drilling into the sea floor shows evidence of the ages of the rocks
The closer you get to the mid-ocean ridge the younger the rock. The farther you get, the older the rock.
This demonstrates a movement away from the ocean ridge

16. Age of the crust

17. Forces that shape the earth

18. Ocean Floor Spreading

19. Convection at Mid-Ocean Ridge
At a mid-ocean ridge, molten material rises from the mantle and erupts.
The molten material spreads out, pushing the older rock away on either side of the ridge.
This process continually adds to the ocean floor

20. Subduction in Deep-Ocean Trenches
As the ocean spreads apart, it also plunges into deep water canyons called deep- ocean trenches.
Subduction is the process by which the ocean floor sinks beneath a deep-ocean trench and back into the mantle.

21. Subduction
The processes of subduction and sea-floor spreading can change the size and shape of the oceans.
Because of these processes, the ocean floor is renewed about every 200 million years.

22. Subduction in the Oceans
The Pacific Ocean is shrinking! This is due to the fact that a deep-ocean trench is swallowing more crust than the mid-ocean ridge can produce.
The Atlantic Ocean however, is expanding!

23. How Plates Move
The Lithosphere is separated into 14 major plates and 32 minor plates
These plates move due to the convection currents occurring beneath them
The direction of their movement will have an effect on the surface of our planet

24. Plate map

25. Three Types of Plate Boundaries
Divergent
Convergent
Transform

26. Describing Plate Boundaries
There are three main kinds of plate boundaries:
divergent
convergent
transform

27. Plate Boundaries Place where 2 lithospheric plates meet
Transform Boundaries
Place where 2 plates are sliding past each other in opposite directions
Shearing stresses are caused
Divergent Boundary
Place where 2 plates are moving away from each other.
Creates Rift Valleys and new crust
Occur mostly at ocean floor

28. Convergent Boundaries
Boundary where 2 plates move towards each other and collide.
There are 3 types of convergent boundaries
They can occur as follows:
Ocean crust to ocean crust
Ocean crust to continental crust
Continental crust to continental crust

29. Convergent Boundaries
Ocean to Ocean crust collide
Volcanic Island Arcs created
Ocean to Continental crust collide
Volcanoes are created at the Subduction zones
Continental and Continental crust collide
Mountain ranges form due to similar densities of the continental crust (no Subduction)

30. Plate Boundary animations
Plate Boundaries