1. Chapter 2
Plate Tectonics

2. Introduction 1795 1920/1930s Theory of Gradualism Continental Drift
Earth was in continuous, but gradual change, constantly decaying, renewing and repairing itself
Proposed by James Hutton
“Father of modern geology”
1920/1930s
Continental Drift
Idea that the continents once fit together like pieces of a giant jig-saw puzzle, to make one vast supercontinent - Pangaea (“all the land”). Later the fragmented into separate continents that drifted apart, moving slowly to their present positions
Proposed by Alfred Wegener
Highly rejected because Wegener couldn't explain why

3. Continental Drift Evidence: Mechanisms: Geometric fit of continents
Ancient glacial deposits
Fossils
Mechanisms:
Continents plow through the ocean floor

4. Sea-Floor Spreading 1960s Harry Hess/Robert Dietz Sea-Floor Spreading
As continents drift apart, new ocean floor forms between them
New material (crust) comes up, cools and pushes old to the side
As new oceanic crust is created it is pushed away from the boundary, continents move with the rest of the crust
Evidence:
Magnetic strips on ocean floor
Mechanism:
New volcanic ocean floor is being made at the ocean ridge
Material on both sides of ridge is moving away

5. Subduction 1960s Harry Hess/Robert Dietz Subduction
Production of new crust must be balanced by destruction of crust elsewhere
Crust plunges into the mantle along subduction zones – coincide with zones of concentrated earthquakes
Continents move toward each other when the old ocean floor between them sinks back down to the Earth’s interior

6. Theory of Plate Tectonics
Combines the ideas of continental drift, sea-floor spreading, and subduction
Lithosphere is broken into plates that are in motion
The plates float on the ductile asthenosphere
Explains origin and distribution of volcanoes, fault zones and mountain belts

7. Evidence of Plate Tectonics
Ocean floors have complex topography
Mountains and trenches
Sea floor gave evidence that there are plates
Mid Atlantic ridge

8. Evidence of Plate Tectonics
Volcanos occur in chains along continental margins and ocean trenches

9. Evidence of Plate Tectonics
Seismic events (earthquakes) are concentrated in lines and arcs coinciding with edges of continents, oceanic trenches, or the middle of ocean basins

10. Plate Driving Mechanisms
Ridge – Push
As material comes up from mid- ocean ridge
Plate gets pushed apart
Slab – Pull
- Weight of plate being subducted pulls it
Convection
Heat rising moves material along
Convection cells

11. Convection
Heat rises to surface by convection (heat fluid becomes less dense and rises)
Hot mantle rises up, cool mantle sinks down

13. Plate Boundaries
Neighboring plates interact at diverging, converging, and sliding boundaries
Continents move toward, away, and slide past each other in all directions

14. Convergent Plate Boundaries
Crust is destroyed at convergent plate boundaries
Plates are pushed towards each other, usually forming a subduction zone
Different results for all three types of collisions
Ocean-ocean
Ocean-continent
Continent-continent

15. Continent-continent Boundary
Continental crust is too light to sink down (can’t subduct)
Compression and folding uplift the plate edges to form mountains
High Mountains within continents
Himalayas Mts. in Asia

17. Ocean-continent boundary
Similar to ocean-ocean
Ocean crust is denser so it get subducted
Volcanic arc on land with high mountains landward, coastal trench
Ex. Andes Mts. in S. America

19. Ocean-ocean Boundary Older ocean crust is subducted under the younger
Volcanic arcs and oceanic trenches
Example: Japan & Aleutian Islands

21. Divergent Plate Boundaries
Occurs where plates are moving apart, as magma pushes up from the mantle, creating new crust
The farther the crust is from the rift,
the older it is

22. Seafloor Spreading
Forms mid-ocean ridges with new ocean floor
Ex. Mid-Atlantic Ridge
Rift Valley
Forms valleys and volcanoes
Ex. Red Sea (Saudi Arabia torn from Africa)

25. Transform Plate Boundaries
Plates slide or grind past each other
Fault zones with frequent earthquakes
Caused by friction – no space between the plates
Crust is conserved
Transform boundaries link other boundaries
Most widely distributed types of plate boundary
Ex. San Andres Fault - CA