1. Lecture 3 Outline: Plate Tectonics – from Hypothesis to Theory
Learning Objectives:
What is the evidence for continental drift?
What is the evidence for seafloor spreading?
What is the evidence for subduction?
How does plate tectonics explain this evidence?

2. How was Plate Tectonics Discovered?
Using scientific method
how researchers work collectively over time to develop accurate and reliable explanations
By observing, hypothesizing, and testing

3. What is the Scientific Method?
Observe: gather information
Hypothesize: attempt to explain observations
Predict: use hypothesis to make testable predictions
Test: gather additional evidence from observations or experiments to support or refute hypothesis
Modify: modify hypothesis in light of new evidence
Repeat steps 3-5 until hypothesis consistent with all available evidence and conceivable tests

4. How does a Hypothesis become a Theory?
Hypothesis gradually gains widespread acceptance by repeated testing and modification
Theory = hypothesis that withstands scrutiny over time
predictions tested and shown to be accurate

5. What is a Theory?
Everyday “theory”: possible explanation or educated guess
Scientific “theory”: explanation consistent with available evidence
Example: Theory of Evolution
supported by large body of scientific evidence
succession of organisms in fossil record
genetic relationships among modern organisms
defined and observable mechanisms of variation and heredity

6. How was Plate Tectonics Discovered?
Plate tectonics too slow to directly observe
So how do we know plates move?
Three independent lines of evidence
Continental Drift
Seafloor Spreading
Subduction

7. What is Continental Drift?
Early geologists knew continents moved vertically
Fossil sea shells high above sea level
Uplift and subsidence following earthquakes
Can continents also move laterally?
Hypothesis proposed by Wegener (1920s)
Continents once joined as one land mass
Pangaea
Broke apart and fragments - modern continents - “drifted” to current locations

8. What was Wegener’s Evidence for Continental Drift?
Fit of coastlines
Distribution of fossils
Similar types/ages of rocks on widely separated coastlines

9. Why was Wegener’s Hypothesis of Continental Drift Rejected?
Wegener could not explain how continents move through solid rock of ocean floor
Discoveries about Earth’s magnetic field and mapping of ocean floor in 1950s provide mechanism for continental drift
Seafloor spreading

10. What was Discovered when the Seafloor was Mapped?
Major topographic features
Ridges
huge underwater mountain ranges through all ocean basins
Trenches
narrow but very deep, mostly encircle Pacific Ocean
Mapping sea floor from ships

11. What was Discovered when the Seafloor was Mapped?
ridge
trenches
trench
trench
ridge
ridge
ridge
trench

12. What was Discovered when the Seafloor was Mapped?
Age of seafloor shows orderly distribution
Rocks youngest along ridges
Progressively older away from ridges
No old oceanic crust
Oldest rocks on continents (~4 billion yrs) nearly as old as Earth
Nowhere on seafloor are rocks older than ~200 million years
Observations suggest ocean crust made at ridge
Proven by pattern of magnetism recorded in ocean crust

13. What was Discovered when the Seafloor was Mapped?
Youngest rock in red - oldest in blue
Symmetry in ages on either side of ridges

14. What is Earth’s Magnetic Field?
Generated by Earth’s rotation and molten iron in outer core
Two poles of opposite polarity
Magnetic poles near rotational (geographic) poles because field generated by rotation

15. What are Magnetic Reversals?
Magnetic field flips polarity (direction)
Normal: magnetic north near geographic North Pole
Reverse: magnetic north near geographic South Pole
Importance of this discovery not realized until seafloor mapped

16. How do Rocks Record Changes in Earth’s Magnetic Field?
Iron minerals align parallel to ambient field as they crystallize
Indicates polarity - direction to magnetic north
Polarity locked in as rock cools
Preserves record of magnetic field polarity at time rock formed
High temperature: magnetic minerals randomly oriented
During cooling: magnetic minerals align with field
After cooling: magnetic orientation recorded in rock will not change as long as rock remains cool – even if magnetic field changes

17. How are Magnetic Polarity Reversals Recorded on the Seafloor?
Bands of alternating normal and reversed polarity
Symmetric on either side of ridge
Ridge axis
Polarity along ridge south of Iceland
Red: rocks formed during current period of normal polarity
Other colors: rocks formed during earlier periods of normal polarity
White: rocks with reversed polarity

18. How Does Seafloor Spreading Explain these Observations?
Youngest rock at ridge formed during current period of normal polarity (N)
Slightly older rock further from ridge formed during last time field had reverse polarity (R)
Even older rock further from ridge formed during previous period of normal polarity (N)
Produces magnetization pattern symmetric with respect to ridge

19. How Does Seafloor Spreading Explain these Observations?
Seafloor spreading results in
Crust older with distance from ridge
Stripes of normal and reverse magnetization symmetric to ridge

20. How Does Seafloor Spreading Explain these Observations?
Provides mechanism for continental drift
Seafloor acts as conveyor belt to move continents
Amount of new oceanic crust produced at ridges must be balanced by equal amount destroyed elsewhere
Where is oceanic crust destroyed?

21. What is the Global Distribution of Earthquakes?
Not randomly distributed
Concentrated near trenches

22. How was Subduction Discovered?
Subduction = process that removes old oceanic crust
Discovered by relationship of deep earthquakes to trenches
Earthquakes deeper with distance from trench
Trench
Hot
Cold

23. How was Subduction Discovered?
Earthquakes along and “inland” of trench - deeper with distance
Mark descent of old oceanic crust into mantle
Process = subduction
Places where it occurs = subduction zones
trench T
Top left: Map view shows earthquakes become progressively deeper to west of Tonga trench
Remaining panels: Vertical cross sections showing descending earthquake belts T T

24. What is the Global Distribution of Volcanoes?
Volcanoes not randomly distributed either
Concentrated in chains (arcs) inland of trenches
Occur above depth where subducting crust causes melting in mantle
Pacific “Ring of Fire”
Earthquakes and volcanoes associated with subduction

25. How Does Plate Tectonics Explain Earthquakes and Volcanoes?
Plate Boundaries
Compare location of plate boundaries to global distribution of earthquakes and volcanoes
Earthquakes and volcanoes mostly at plate boundaries
convergence and divergence
Earthquakes
Volcanoes

26. What Happens Where Plates Diverge?
Ridge
new oceanic crust produced by upwelling of asthenosphere
seafloor spreading

27. What Happens Where Plates Converge?
Trench
denser oceanic crust sinks beneath lighter continental crust
subduction

28. How Fast are Plate Motions?
Average about 5 cm per year
Roughly rate fingernails grow
Not very fast, but over long periods of time plates move great distances
50 km per 1,000,000 yr
Portland - Salem
5000 km per 100,000,000 yr
Portland - Boston

29. How Fast are Plate Motions?
We can now directly measure plate motions using GPS

30. Questions for Review
What is the scientific method? What is the difference between a hypothesis and a scientific theory?
Explain the geologic evidence that led to the discovery of a) continental drift, b) sea floor spreading, and c) subduction. How does plate tectonics explain this evidence?
Why does Earth have a magnetic field? How do rocks provide a record of the magnetic field back through time? What evidence does this provide that shows seafloor spreading?
What causes plates to move?