1. Forces that Shape the Earth
State Correlation 4b

2. Plate Tectonics
1912 – German scientist Alfred Wegener Proposed two ideas that are known as Continental Drift Theory
Earth’s continents were once joined in a single large landmass called Pangaea that broke apart
Continents have drifted to their current location

3. Continental Drift
It has taken the continents about 225 million years since the breakup of Pangaea to move to their present locations. 3

4. Evidence for Continental Drift
Puzzle-like fit of the continents
Similar fossils have been found on different continents
Arctic areas & glacier deposits in tropical areas
Similar rock structures are found on different continents

6. Seafloor Spreading
Wegner’s theory was not widely accepted because no one could explain how or why continents moved.
Using sound waves scientist discovered underwater mountain ranges called mid-ocean ridges
In 1960, Harry Hess suggested the theory of seafloor spreading

7. Mid-Ocean Ridges
Form underwater mountain chains where the ocean crust moves apart allowing magma to rise forming new ocean crust
The East Pacific Rise is just one of the many mid-ocean ridges that wind beneath Earth’s oceans.

8. What Is Sea-Floor Spreading?
Sea-floor spreading occurs when
hot, less dense material below Earth’s crust rises toward the surface of the mid-ocean ridge
flows sideways carrying the carrying the seafloor away from the ridge in both directions
makes new ocean crust
Works like a conveyer belt
Older rock is found
farther from the ridge

9. Growing an Ocean
Because of sea-floor spreading, the distance between Europe and North America is increasing by a few centimeters per year. 9

10. Theory of Plate Tectonics
Explains the formation, movement, and subduction of Earth’s plates.
Claims that Earth’s plates are always in motion
Plates move
slowly
at different rates
in different directions

11. Structure of the Earth The Earth is made of 4 layers Inner Core
Outer Core
Mantle
Crust

12. Crust Mostly solid rocky outer layer of Earth
includes:
Oceans crust: under the ocean & more dense
Continental crust: surface of earth & less dense
Earth’s crust & upper mantle are broken into large plates
Together the crust and plates make up the lithosphere
Lithosphere is the outermost solid, rocky surface of the Earth that floats on top of the asthenosphere
Brittle, rigid, & has fault lines

13. Lithospheric Plates 13

14. Mantle Solid layer of hot rock
Upper mantle melts rocks, forming a substance called magma which flows like a thick liquid
The asthenosphere is the mechanically weak deforming region of the upper mantle that is ductile, plastic, and free-flowing
Convection currents occur here due to uneven heating

15. Convection Currents in the Mantle
Convection currents can cause plates to move away from each other or towards each other
Convection currents:
hot less dense rock rises to the lithosphere & spreads out cooling & shrinking in volume
The cooler more dense rock sinks allowing the hot less dense rock to rise and take its place
As convection currents move the plastic rocks sideways large portions of the crust move called lithospheric plates

16. Lithosphere & Asthenosphere
Compare & Contrast
Lithosphere & Asthenosphere
Using a double bubble

17. Plate Boundaries
Movement of Earth plates are responsible for most major geological events and landforms
Volcanoes, earthquakes, mountain formation
Plate Boundaries are where edges of plates interact
3 Types of plate boundaries
Divergent Plate Boundaries
Convergent Plate Boundaries
Transform Plate Boundaries

18. Plate Boundaries
Different type of plate movement occurs along each type of boundary. 18

19. Divergent Plate Boundaries
Forms when 2 plates move away from each other
Two types:
2 oceanic
2 continental

20. Divergent Boundaries: 2 ocean plates
Form undersea volcanic mountain ranges called mid-ocean ridges.
Ex. Mid-Atlantic Ridge

21. Divergent Boundaries: 2 continental plates
When two continental plates pull apart a rift valley forms
Ex. Great Rift Valley in Africa

22. Convergent Plate Boundaries
Boundaries that form when 2 plates collide or come together
3 Types
Oceanic & Continental
Two Oceanic
Two Continental

23. Oceanic & Continental
The denser oceanic plate is forced below the less dense continental plate
The movement of one plate under another forms a subduction zone
Oceanic plate melts as it subducts into the mantle forcing hot magma & gas up to the surface of the continent
Forms a deep-ocean trench & a long chain of continental volcanic mountains
Ex. : Mt. St. Helens

24. Two Oceanic Plates
The less dense ocean plate is forced down into the mantle forming a deep ocean trench that sinks & melts
Magma & gas push through the ocean floor forming a chain volcanoes
If volcanoes become large enough it rises above sea level & makes a volcanic island arc
Ex. Mariana Trench & Mariana Island Arc in the Western Pacific & Japanese islands

25. Volcanoes and Plate Boundaries
Volcanic belts form along the boundaries of Earth’s plates. 25

26. Two Continental Plates
First, the seafloor between the continents is forced below one of the continents
Then, when the plates collide the continent crusts buckle, thicken, & rises up forming mountain ranges
Compressional forces squeeze plates together causing Folding & Faulting
Ex. Himalaya & Appalachian Mountains

27. Folding

28. Faulting

29. Transform Plate Boundaries
2 plates grind or slide past each other without creating or destroying the lithosphere
Form Strike-slip faults
Earthquakes occur at transform boundaries
San Andreas Fault is an example
North America & Pacific plates slide past each other in a N – S direction creating earthquakes

30. Earthquakes
Faults are large cracks in the Earth’s crust where movement occurs
At Strike-slip faults rocks on either side of the fault are under pressure & can get locked together
When too much pressure builds up, the rocks suddenly slide past each other releasing the pressure
The violent shaking of the Earth’s crust is known as an earthquake

31. New Madrid Fault