1. Plate Tectonics

2. Plate Tectonics and the Ocean Floor
Plate tectonics – Theory that the outermost portion of the Earth is composed of plates that move on a liquid molten layer (similar to icebergs floating on water)
Alfred Wegener a German meteorologist developed the theory of continental drift in 1912
Continental drift- Continents slowly drift across the globe

3. Evidence of Continental Drift
Pangaea – (pan= all; gaea= Earth)
A large landmass that the continents
fit together to form
Evidence
Matching rocks on different continents
Similar rocks form Appalachian mountains match rocks in British Isles
Distribution of organisms
Finding fossils of the same species on different continents
Glacial Activity in now tropical areas

4. The continents fit together

5. Fossils of same species found on different continents.

6. More on Continental Drift
As the seafloor spreads the lithosphere (continental and oceanic crust) moves in sections on a viscous liquid layer- asthenosphere
On average these tectonic plates move 1-5 inches/year

7. More on Continental drift
If new crust is being formed all the time why isn’t the earth covered in crust?
Because as new crust is formed old crust is destroyed
Let’s start by looking at the 2 types of crust
Both types of crust make up the outer layer of earth, called the lithosphere
Continental Crust
Granite
Lower density than continental
Oceanic Crust
Igneous rock + basalt
Higher density than continental

8. Continental drift
The crust sits on top of a viscous layer – the asthenophere
A Viscous substance is a thick liquid
What are some examples of a viscous substance that you are familiar with?
The asthenosphere is part of the mantle called the upper mantle

9. Convection
Convection is the heat transfer in a gas or liquid by the circulation of currents from one region to another
Convection cell is a circular-moving loop of matter (gas or liquid) involved in convection movement

10. Convection
Convection causes movement in the liquid mantle layer which the crust sits upon
As a result the crust slowly moves on this layer
Kind of like an iceberg floating on water
As this occurs the tectonic plates move slowly over the asthenosphere

11. Tectonic plates

12. Forming Crust
Mid-Ocean ridge- Underwater mountain range winds through all oceans
Seafloor spreading occurs at the ridge
This is where molten rock reaches the surface, cools, and forms new crust
As this new crust forms the magnetic minerals in the crust align in relation to Earth’s current magnetic field
Scientists can look at the sea floor to get a history of Earth’s magnetic reversals (we will talk about how this works in a minute)

13. Seafloor Spreading
Harry Hess ( )- left his depth recorder on while traveling as sea in the Navy. He noticed mountain ridges at the center of the ocean basins and deep, narrow trenches at the edges
Published his idea of sea floor spreading which suggested that new ocean crust is formed at ridges, split apart and moves away from ridges

14. Seafloor Spreading
It was confirmed that sea floor spreading does occur at mid-ocean ridge
Crust is also being destroyed at ocean trenches (we will talk about this in a minute)
This is called subduction

15. New crust is made at the mid-ocean ridge

16. Magnetic Field Reversals
Fredrick Vine and Drummond Matthews (1963)
Combined the pattern of magnetic sea floor stripes with sea floor spreading. Can see fluctuations in rock from normal polarity to reverse polarity.

17. Magnetic field reversals-
This is the piece of
Evidence that made
The continental drift
Theory widely accepted
Among scientists.
As the sea floor spreads
(at mid-atlantic ridge) the
Magnetism of the rocks
Formed reverses. These
Magnetic shifts can be
Observed in crust.

18. Being able to identify these magnetic pole reversals in the rock
Confirmed the theory of continental drift.

19. Plate Boundaries- 3 types
Divergent Boundaries
Convergent Boundaries
Ocean to Ocean
Ocean to Continental
Continental to Continental
Transform Boundaries

20. Divergent boundaries Plates are moving apart Ocean crust – Ocean crust
Sea floor created
Mid-Atlantic Ridge
Continental crust – Continental crust
Continental rifting
Continents are splitting apart
Red Sea, Gulf of California

21. This is how the Mid-Atlantic ridge started!

22. Convergent Boundaries
Plates are moving together
Ocean –Continent
Old sea floor destroyed
Subduction zone (continental arcs)
Subduction- A process in which one plate descends below another (volcanoes)
Ocean –Ocean
Subduction zone (island arcs) (volcanoes)
Continent –Continent
Collision Zone
Mountains form
Why does subduction occur? Hint: composition of plates

23. Examples of Convergent Plate boundaries
Continental/Oceanic

24. Transform Boundaries Plates are moving past each other
Oceanic
Continental
Both of these neither create nor form sea floor
Causes Earthquakes

26. What do we know about all of the locations of these active
volcanoes?