1. Plate Tectonics
Chapter 17

2. Drifting Continents
Section 17.1

3. Continental Drift
German scientist Alfred Wegener proposed the idea of continental drift which stated that Earth’s continents had once been joined as a single landmass
Called this landmass Pangea, greek for “all the Earth”
Wegener proposed Pangea broke apart million years ago

4. Evidence from Rock Formations
As Pangea broke apart, mountain ranges would have fractured
So similar rock types should be found on opposite sides of the Atlantic Ocean
Appalachian Mountains (US) share similar rocks in Greenland and Europe
All older than 200 million years

5. Evidence from Fossils
Similar fossils of different plants and animals have been found all over the world on widely separated continents
Fossils are older than 200 million years

6. Ancient Climatic Evidence
Rocks provide clues to past environments and climates
Evidence of coal was found on Antarctica
Coal forms from dead swamp plants
Glacial deposits found on Africa, India, Antarctica

7. A Rejected Hypothesis
Wegener’s continental drift hypothesis was not widely accepted until the 1960s
Wegener could not prove how or why the continents moved before his death in 1930

8. Seafloor Spreading
Section 17.2

9. Help from Technology
Most people thought oceanic crust was unchanging and older than continental crust
Echo-sounding technology (SONAR) sent sound waves to the ocean floor to measure depth
A magnetometer is a device that detects small changes in magnetic fields

10. Ocean Floor Topography
Maps made from data showed vast, underwater mountain chains and ocean ridges
Also discovered earthquakes and volcanoes along the ridges

11. Ocean Floor Topography Cont…
There are also deep- sea trenches, which are long, narrow depressions
The deepest trench, the Mariana Trench, is in the Pacific ocean and is 11 km deep

12. Magnetism
Rocks (like basalt) with iron provide record of Earth’s magnetic field
The study of the magnetic record is paleomagnetism
Magnetic Reversals or changes in Earth’s magnetic field were found in basalt flows
Normal polarity is when the magnetic field is aligned with earth’s present field
Opposite is called reversed

13. Magnetic Symmetry
As data was collected, scientists noticed a series of positive and negative stripes parallel to ocean ridges
The patter was identical on each side of the ridge

14. Seafloor Spreading
Seafloor spreading states that the new ocean crust is formed at ocean ridges and destroyed at deep sea trenches
Magma is less dense than oceanic crust so its forced up, fills the gap and hardens
As spreading continues, magma forces up and hardens as well
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15. Theory of Plate Tectonics
Section 17.3

16. Theory of Plate Tectonics
The theory states that Earth’s crust and rigid upper mantel are broken into enormous slabs called plates
Plates move in different directions and at different rates

17. Plate Boundaries
Tectonic plates interact at places called plate boundaries
Plates either come together (converge), move away from each other (diverge) or slide past each other

18. Divergent Boundaries
Places where two tectonic plates move apart from one another are called divergent boundaries
Most are found on the seafloor where they form ocean ridges and where seafloor spreading occurs
Over millions of years, seafloor spreading at divergent boundaries causes the ocean basin to grow wider

19. Convergent Boundaries
Places where two tectonic plates are moving toward each other are convergent boundaries
There are three types of convergent boundaries
At oceanic-oceanic boundaries, one plate descends beneath the other in a process called subduction
Arcs of volcanic islands often form around this boundary

20. Convergent Boundaries Cont…
At oceanic-continental convergence boundaries, subduction
Creates a volcanic mountain range rather than a volcano island arc

21. Convergent Boundaries Cont…
The third convergent boundary is continental-continental
Continental plates are pulled into subduction zones but are too buoyant to be forced under
The colliding edges crumple and uplift the form a mountain range

22. Transform Boundaries
Transform boundaries occur where two plates slide horizontally past each other, usually seen as long faults
At divergent boundaries new crust is formed
At convergent boundaries old crust is destroyed
At transform boundaries crust is deformed or fractured
*San Andreas Fault- Most studied fault in the world; produces CA earthquakes