Learning Objectives

1. Continental drift was proposed by Alfred Wegner in the early 1900s based on the apparent fit of continental coastlines, similar fossil plants and animals on widely separated continents, distribution of Paleozoic glaciations and paleoclimatology, and apparent polar wandering.

2. Wegner proposed that all continents had once been connected in a supercontinent called Pangaea, that broke apart to form the present continents. Wegner thought the continents moved across stationary oceanic crust from a combination of centrifugal and gravitational force. His ideas received little support when proposed.

3. Paleomagnetism is the study of the ancient magnetic fields of the earth. Magnetized minerals preserve a record of the direction of the magnetic pole and their distance from it at the time of their formation. Paleomagnetic data revived interest in continental drift by demonstrating polar wandering and supporting the reconstruction of Pangaea.

4. Other recent evidence for continental drift includes better continental margin fits, similar rock contacts and age relations between continents when fitted together, glacial movements indicated by striations, and sources of boulders in ancient tills, and similar geologic sequences including metamorphic rocks in Brazil and Gabon.

5. The idea that the sea floor spread away from mid-oceanic ridges and was subducted beneath a continent or island arc as a result of mantle convection was proposed by Harry Hess in the early 1960s.

6. Sea-floor spreading explains processes at the mid-oceanic ridges as the result of rising mantle: the existence of the ridge itself, high heat flow, basaltic volcanism, a rift valley and shallow-focus earthquakes.

7. Sea-floor spreading explains processes at the oceanic trenches as the result of descending oceanic crust: existence of the trench itself, low heat flow, negative gravity anomalies, Benioff zones, and andesitic volcanism.

8. Sea-floor spreading explains the young age of the sea floor, loss of older oceanic crust, lack of pelagic sediment on the ridge crest, and increasingly older oceanic crust away from the ridge crest.

9. Plate tectonics is the theory that the earth's surface is divided into a few large, thick plates that move and change size. It combines the older ideas of continental drift and sea-floor spreading. Plates are formed by lithosphere (crust and uppermost mantle) and move on the asthenosphere to a depth of about 200 km. New lithosphere is added along the ridges at the trailing edge of the plate and lost to subduction. Plate boundaries are either diverging, converging or transform.

10. The Vine-Matthews hypothesis recognized that sea floor magnetic anomalies were symmetrical with respect to the mid-oceanic ridge crests and matched the pattern of magnetic reversals discovered previously in stacked lava flows. The hypothesis also accounted for the patterns by magnetization of basaltic dikes intruded into the ridge crests, split and moved away from the ridges. Based on the Vine-Matthews hypothesis, spreading rates are 1 to 6 cm/year. The hypothesis also allows prediction of the sea floor age based on magnetic anomalies that can be tested with samples recovered by deep-sea drilling.

11. Other tests of plate tectonics involve recognition that fractures off-setting mid-oceanic ridges are transform faults, that have earthquakes confined to the segments between ridge crests and first-motions opposite those predicted if the fractures were simple strike-slip faults.

12. Diverging plate boundaries experience extension that produces normal faults, shallow-focus earthquakes, rift valleys (grabens), basaltic volcanism, crust thinning, uplift, and creates new ocean basins. Whether rifting causes uplift, or vice versa is unclear. Thick accumulations of salt may mark initial flooding of the rift valley, bordered by uplifted, and exposed continental margins. Later stages of diverging boundaries produce passive continental margins.

13. Transform boundaries allow plates to slide past one another. These boundaries exhibit strike-slip motion and may connect two ridge segments, a ridge and a trench, or two trenches. The straight course of these faults resolves mechanical constraints caused by divergence along curved boundaries, and are always aligned parallel to the spreading direction.

14. Ocean-ocean convergence is characterized by andesitic to basaltic island arcs, Benioff zones, low heat flow, accretionary wedges, and forearc basins. The descending plate must reach 100 km to begin generating magma. The position of the trench and the hingeline of bending migrate seaward progressively onto the subducting plate.

15. Ocean-continent convergence exhibits an active continental margin, accretionary wedge, forearc basin, Benioff zone, and a magmatic arc associated with young mountain belts. A foreland basin may also be formed in response to the weight of the thrust sheets forming the mountain belt.

16. Continent-continent convergence passes through the stages exhibited by ocean-continent convergence, but results in a suture zone of young mountains in continental interiors marking the former subduction site, thickened continental crust, and shallow focus earthquakes.

17. Backarc spreading can move the position of an island arc or a small strip of continental crust. Sea floor formed by backarc spreading is apparently that found as ophiolites. Ridge crests, trenches, transform faults, and their associated features are capable of migration in response to changing mantle conditions. Plate size increases or decreases in response to spreading and subduction.

18. Plate tectonics explains consistently: distribution of basaltic and andesitic volcanoes, shallow-, intermediate-, and deep-focus earthquakes (Benioff zones), young mountain belts, mid-oceanic ridges, oceanic trenches, and fracture zones.

19. Mantle convection may be the result of plate motion, not the cause of it. Plate motion is caused by one, or a combination of the following three mechanisms: 1) ridge-push - sea floor and lithosphere slides down the slope of the lithosphere-asthenosphere boundary; 2) slab-pull - cold, sinking lithosphere being subducted pulls the plate away from the ridge crest; 3) trench-suction - horizontal pull of trench as it moves seaward from steeply subducting plate. Slab-pull accounts for most plate movement, and all rapid plate motion.

20. Mantle convection may result in mantle plumes. They are stationary with respect to moving plates and produce hot spots, such as Yellowstone, Iceland and the Hawaiian Islands. Mantle plumes may also be responsible for the initial fracturing of the lithosphere in a characteristic three-pronged pattern that begins divergence (e.g. Red Sea region). Aseismic ridges connect seamounts and guyots that may reflect plate passage over a hot spot.

21. Plate tectonics predicts that metallic ore bodies may be associated with diverging plate boundaries because of hot spring activity, and converging plate boundaries where metal-rich ophiolites have been brought to the surface, or ore-bearing magmas form island arcs or intrude continental crust. Mantle plumes are notable for their lack of ore deposits.