1. Earth ~200 million years ago

2. The Geologic Time Scale *Fossils *Correlation *Calibrated with
Based on
*Fossils
*Correlation
Later
*Calibrated with
radiometric dating

4. The Continental Drift Hypothesis
Proposed by Alfred Wegener in 1915.
Supercontinent Pangaea started to break up about 200 million years ago.
Continents "drifted" to their present positions.
Continents "plowed" through the ocean crust.

5. Continental Drift: Evidence
Geographic fit of South America and Africa
Fossils match across oceans
Rock types and structures match across oceans
Ancient glacial features

6. the continents, especially using
Continental
Drift:
Evidence
Tight fit of
the continents, especially using
continental shelves.

7. Continental Drift: Evidence
Fossil critters and plants

8. Continental Drift: Evidence
Correlation of mountains with nearly
identical rocks and structures

9. Continental Drift: Evidence Glacial features of the same age
restore to a
tight polar
distribution.

10. Continental Drift: Reactions
Received well in Europe and southern hemisphere.
Rejected in U.S., where scientists staunchly preferred induction (incremental progress built on observation) over what they perceived as speculative deduction.
Lack of a suitable mechanism crippled continental drift’s widespread acceptance.
Conflict remained unresolved because seafloors were almost completely unexplored.

11. The Rise of Plate Tectonics
WW II and the Cold War: Military Spending
U.S. Navy mapped seafloor with echo sounding (sonar) to find and hide submarines. Generalized maps showed:
oceanic ridges—submerged mountain ranges
fracture zones—cracks perpendicular to ridges
trenches—narrow, deep gashes
abyssal plains—vast flat areas
seamounts—drowned undersea islands
Dredged rocks of the seafloor included only basalt, gabbro, and serpentinite—no continental materials.

13. The Rise of Plate Tectonics
Marine geologists found that seafloor magnetism has a striped pattern completely unlike patterns on land.
Mason & Raff, 1961
Black: normal polarity
White: reversed polarity
Both: very magnetic

14. The Rise of Plate Tectonics
Hypothesis: Stripes indicate periodic reversal of the direction of Earth’s magnetic field.
To test this hypothesis, scientists determined the eruptive ages AND the polarity of young basalts using the newly developed technique of K-Ar radiometric dating.
The study validated the reversal hypothesis...

15. The Rise of Plate Tectonics
And then ( ) geologists realized that the patterns are SYMMETRICAL across oceanic ridges.
The K-Ar dates
show the youngest
rocks at the ridge.

16. The Rise of Plate Tectonics
Meanwhile, U.S. military developed new, advanced seismometers to monitor Soviet nuclear tests.
By the late 1950s, seismometers had been deployed in over 40 allied countries and was recording 24 hrs/day, 365 days/year.
Besides the occasional nuclear test, it recorded every moderate to large earthquake on the planet. With these high-precision data, seismologists found that activity happens in narrow bands.

17. Bands of seismicity—chiefly at trenches and oceanic ridges

18. The Theory of Plate Tectonics
“group authorship” in
Earth’s outer shell is broken into thin, curved plates that move laterally atop a weaker underlying layer.
Most earthquakes and volcanic eruptions happen at plate boundaries.
Three types of relative motions between plates:
divergent convergent transform

19. Tectonic Plates on Modern Earth

21. Divergent boundaries: Chiefly at oceanic ridges
(aka spreading centers)

22. How magnetic reversals form at a spreading center

23. Divergent boundaries also can rip apart (“rift”) continents

24. continent could lead to formation of
How rifting of a
continent could lead to formation of
oceanic lithosphere.
e.g., East Africa Rift
e.g., Red Sea
e.g., Atlantic Ocean

25. Pangea was ripped apart by such continental
Presumably,
Pangea was ripped apart by such continental
rifting & drifting.

26. Subduction zones form at convergent boundaries if at least one side has oceanic (denser) material.
Modern examples: Andes, Cascades
Major features: trench, biggest EQs, explosive volcanoes

27. Another subduction zone—this one with oceanic material on both sides.
Modern example: Japan

28. Earthquake depth indicates subduction zones

29. Collison zones form where both sides of a convergent boundary consist of continental (buoyant) material.
Modern example: Himalayas
This probably used to be a subduction zone,
but all the oceanic material was subducted.

31. Most transform boundaries are in the oceans.
Some, like the one in California, cut continents.
The PAC-NA plate boundary is MUCH more complex than this diagram shows.

32. Hotspots, such as the one under Hawaii,
have validated plate tectonic theory.

33. Two related ideas are widely accepted:
Why do the plates move?
Two related ideas are widely accepted:
Slab pull: Denser, colder plate sinks at subduction zone, pulls rest of plate behind it.
Mantle convection: Hotter mantle material rises beneath divergent boundaries, cooler material sinks at subduction zones.
So: moving plates, EQs, & volcanic eruptions are due to Earth’s loss of internal heat.

34. How does convection work
How does convection work? No one knows—but they aren’t afraid to propose models!
Whole-mantle convection
Two mantle convection cells
Complex convection

35. Field Trip Briefing The California subduction zone (9’ seismic line)
Subduction to transform (Atwater animation)
Faults of the Bay Area (SF-SJ maps)
Rock types we’ll encounter

37. Landslide north of Mussel Rock
Occurred between 2 am and 8 am, 2/21/05