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.
