1. Chapter 2 Plate Tectonics: A Scientific Revolution Unfolds

2. Continental Drift: An Idea Before its Time
Alfred Wegener
First proposed his continental drift hypothesis in 1915 before the theory of Plate Tectonics
Continental drift hypothesis
Supercontinent called Pangaea began breaking apart about 200 million years ago
Continents "drifted" to present positions

3. Continental Drift: An Idea Before its Time, cont’d
Evidence used in support of continental drift hypothesis
Fit of the continents
Fossil evidence
Rock type and structural similarities
Paleoclimatic evidence

4. The Great Debate Objections to the continental drift hypothesis
Lack of a mechanism for moving continents
Wegener incorrectly suggested that continents broke through the ocean crust, much like ice breakers cut through ice
Strong opposition to the hypothesis from all areas of the scientific community

5. Continental Drift and Paleomagnetism
Renewed interest in continental drift initially came from rock magnetism
Magnetized minerals in rocks
Declinations show the direction to Earth’s magnetic poles when rocks formed
Inclinations provide a means of determining the rocks latitude of origin and how much it moved north or south since it was created.

6. Continental Drift and Paleomagnetism
Polar wandering
The apparent movement of the magnetic poles indicates that the continents have moved
Indicates Europe was much closer to the equator when coal-producing swamps existed

7. Continental Drift and Paleomagnetism, cont’d
Polar wandering
Curves for North America and Europe have similar paths but are separated by about 24 of longitude
Differences between the paths can be reconciled if the continents are placed next to one another

8. A Scientific Revolution Begins
During the 1950s and 1960s technological strides permitted extensive mapping of the ocean floor
Seafloor spreading hypothesis was proposed by Harry Hess in the early 1960s
Oceanic crust created and spread apart at shallow mid-oceanic ridges
Oceanic crust destroyed at deep trenches

9. A Scientific Revolution Begins, cont’d
Geomagnetic reversals
Earth's magnetic field periodically reverses polarity – the north magnetic pole becomes the south magnetic pole, and vice versa. This flips the remnant magnetic inclination of the rocks 180 degrees.
Dates of geomagnetic reversals determined from lava flows.

10. A Scientific Revolution Begins, cont’d
Geomagnetic reversals
Geomagnetic reversals are recorded in the ocean crust as alternating linear stripes of high and low magnetic anomalies (highs for normal polarity and lows for reversed polarity).
These linear anomalies parallel the mid-ocean ridges and are symmetrical with respect to them.
In 1963 Vine and Matthews tied the discovery of these “magnetic stripes” in the ocean crust near mid-ocean ridges to Hess’s concept of seafloor spreading.

11. A Scientific Revolution Begins, cont’d
Earthquakes and Subduction Zones
Wadati and Benioff mapped out the depths and locations of earthquakes along trenches and beneath arcuate trends of volcanoes that parallel them.
They found shallower earthquakes closer to the trench, and progressively deeper earthquakes towards the volcanic arcs and beyond them.
Postulated that the oceanic crust and upper-most mantle is sinking or subducting down into the rest of the mantle along these zones and is being destroyed.

12. Plate Tectonics: The New Paradigm
Earth’s major tectonic plates
Earth is divided into seven major lithospheric plates and several other minor plates that are relatively rigid.
Plates are composed of the oceanic and/or continental crust and the uppermost mantle.
Several plates include an entire continent plus a large area of seafloor.

13. Plate Tectonics: The New Paradigm, cont;d
Earth’s major tectonic plates cont.
Plates are in constant slow motion with respect to one another and are continually changing in shape and size.
Earthquakes, deformation and volcanic eruptions occur where these plates meet.
The interior of plates generally have fewer earthquakes, less deformation and less volcanic activity compared to the margins.

14. Plate Tectonics: The New Paradigm, cont’d
Plate boundaries
Interactions between individual plates occur along their boundaries
Types of plate boundaries
Divergent plate boundaries (constructive margins formed by extension)
Convergent plate boundaries (destructive margins formed by compression)
Transform fault boundaries (conservative margins formed by side-to-side shear)

15. Divergent Plate Boundaries
Most are located along the crests of oceanic ridges
Oceanic ridges and seafloor spreading
The seafloor is elevated along well-developed divergent plate boundaries, forming oceanic ridges
Geologic features include
crustal and lithospheric thinning
rift zones formed by extensional faulting
volcanic activity
shallow earthquakes

16. Divergent Plate Boundaries, cont’d
Continental rifting
Splits landmasses into two or more smaller segments along a continental rift
Examples include the East African rift valleys and the Rhine Valley in northern Europe
Produced by extensional forces acting on lithospheric plates

17. Convergent Plate Boundaries
Older portions of oceanic plates are returned to the mantle or subducted in these destructive plate margins
Surface expression of the descending plate is an ocean trench
Oceanic plate moves downward along subduction zones
Geologic features include
volcanic arcs
shallow-deep earthquakes
mountain belts

18. Types of Convergent Plate Boundaries
Oceanic-continental convergence
Denser oceanic slab sinks into the asthenosphere
Along the descending plate partial melting of mantle rock generates magma
Resulting volcanic mountain chain is called a continental volcanic arc (Andes and Cascades)

19. Types of Convergent Plate Boundaries, cont’d
Oceanic-oceanic convergence
When two oceanic slabs converge, one descends beneath the other
Often forms volcanoes on the ocean floor
If the volcanoes emerge as islands, a volcanic island arc is formed (Japan, Aleutian islands, Tonga islands)

20. Types of Convergent Plate Boundaries, cont’d
Continental-continental convergence
Continued subduction can bring two continents together
Less dense, buoyant continental lithosphere does not subduct
Resulting collision between two continental blocks produces mountains (Himalayas, Alps, Appalachians)

21. Transform Fault Boundaries
Plates slide past one another and no new lithosphere is created or destroyed
Most join two segments of a mid-ocean ridge along breaks in the oceanic crust known as fracture zones
A few (e.g. the San Andreas fault) cut through continental crust
Geologic Features include
strike slip faults
lack of volcanic activity
shallow earthquakes
inactive fracture zones which extend outward from oceanic transform faults (active fracture zones) and separate lithosphere of different age and density

22. Testing the Plate Tectonics Model
Hot spots and mantle plumes
Caused by rising plumes of heat or hot mantle material
Volcanoes and calderas can form over them (Hawaiian Island chain, Yellowstone caldera)
Mantle plumes
Long-lived structures that are fixed in one subsurface location while the plates drift over them
Some originate at great depth, perhaps at the core-mantle boundary

23. Measuring Plate Motion
Paleomagnetism and plate motions
Paleomagnetism stored in rocks on the ocean floor provides a method for determining plate motions
Both the direction and rate of seafloor spreading can be established

24. Measuring Plate Motion, cont’d
Measuring plate velocities from a fixed reference frame in space
Accomplished by establishing exact locations on opposite sides of a plate boundary and measuring relative motions
Two methods are used
Very Long Baseline Interferometry (VLBI)
Global Positioning System (GPS)

25. What Drives Plate Motions
Many, but not all researchers agree that convective flow in the mantle is the basic driving force of plate tectonics
Theory I: Causes elevation and density differences due to temperature variations, which create gravitational forces
Theory II: Causes frictional drag on the plates above the convecting mantle

26. End of Chapter 2