UNIT 14: PLATE TECTONICS
Terms in this set (29)
the theory that continents move freely over the Earth's surface, changing their positions relative to one another.
a hypothesis that the sea floor forms at the crest of the mid-oceanic ridge, then moves horizontally away from the ridge crest toward an oceanic trench.
a hypothetical supercontinent formed by fitting together the present continents.
Four lines of evidence derived from Wegener's study of the continents that support the theory of continental drift.
• the similarity of plant fossils of late Paleozoic age on several different continents;
• the similarity of rock sequences on five continents even though the locations are now widely separated and younger rocks at the five locations are dissimilar;
• evidence for late Paleozoic glaciations on continents of the southern hemisphere but not in the northern hemisphere; and
• paleoclimate evidence suggesting either polar wandering or continental drift.
Describe how paleomagnetic data support the theory of continental drift, rather than supporting polar wandering.
Paleomagnetic data from rocks of the same age on two different continents point to two different locations for the magnetic north pole. Unless there are different magnetic poles for each continent, it is most likely that the poles stood still and the continents moved.
the dimensions of the mid-oceanic ridge.
The mid-oceanic ridge is more than 80,000 km long and 1500-2500 km wide. It rises two to three kilometers above the ocean floor.
the three layers that make up the oceanic crust
The top layer of the oceanic crust consists of marine sediment with an average thickness of 0.5 km.
The second layer is about 1.5 km thick and consists mostly of pillowed and fractured basalt.
The third layer is 5 km thick and consists of parallel sheeted dikes in the upper section and sill-like gabbro bodies in the lower part.
are slivers of the oceanic crust and the upper mantle that were caught between converging plates and wedged upward onto continents.
Describe how basalt magma is formed at spreading centres.
Basaltic magma results from the presence of hot mantle rock rising under the ridge (see Figure 3.22 in the textbook).
What did Hess propose as the driving mechanism for seafloor spreading?
Hess proposed deep mantle convection as the driving mechanism for seafloor spreading.
the explanation developed by Vine and Matthews for magnetic anomalies on the sea floor.
Vine and Matthews hypothesized that tensional cracks that form in the rift valley of the mid-oceanic trench are filled in by basaltic magma, which forms intrusion dikes. In times of "normal" magnetic polarity, the dikes are normally polarized; in times of "reversed" polarity the normal polarity is reversed.
At what range of rates is the sea floor moving? How was this range determined?
Measured rates for seafloor motion range from one to six centimetres per year. Rates are determined by dating a reversal and then measuring the distance of that piece of sea floor from the rift valley of the ridge crest.
How can the hypothesis of seafloor motion and the Vine-Matthews hypothesis about the origin of magnetic anomalies be tested?
The congruence of the two hypotheses can be tested by dating a sample of igneous rock at some distance from the ridge crest. If the dated sample is the same age as that predicted for the sea floor at that location based on study of magnetic anomalies, then the hypotheses have been tested successfully.
The two hypotheses tested by the study of seismicity of fracture zones associated with the mid-oceanic ridge
The two hypotheses tested by the study of seismicity of fracture zones associated with the mid-oceanic ridge were 1) that earthquakes are evenly distributed along the entire length of the fracture zone; and 2) that the rocks on either side of the fracture zone move in opposite directions. (Figure 19.18 of the textbook)
three stages by which a divergent plate boundary located in the middle of the continent becomes a divergent plate boundary located in the middle of the sea floor.
formation of a rift valley, formation of a seafloor, and lastly the widening of the sea floor and formation of mid-oceanic ridge
formation of a rift valley.
- elevation of the crust.
- occurrence of shallow-focus earthquakes along normal faults.
- formation of a rift valley as a central graben
- occurrence of high heat flow and basaltic volcanism
formation of a seafloor
-separation of continental crust on the upper part of the plate .
-flooding of sea water into the linear basin.
-development of a series of fault blocks on the edges of the continents.
- erosion of uplifted edges of the continents, which fills the fault basin with continental sediment.
- continuation of volcanism, which builds true oceanic crust.
- evaporation of sea water, leaving a thick layer of rock salt overlying continental sediments.
widening of the sea floor and formation of mid-oceanic ridge
-widening of the sea, which tears the layer of rock salt.
- development of full-fledged mid-oceanic ridge.
- development of continental shelves and slopes.
- formation of deep continental rise.
What factor determines the spacing between the trench and island arc associated with a subduction zone?
The subduction angle determines the arc-trench spacing.
In what situation does a subduction trench not have an island arc associated with it?
No island arc is associated with a subduction trench when the subduction angle is very shallow and the top of the subduction plate never contacts the asthenosphere.
is the highly contorted, thrust-faulted, marine sediment close to a trench. It is formed by underthrusting of marine sediment that is scraped off the subducting oceanic plate. As underthrusting pushes the wedge upward, a forearc basin forms between the complex and the volcanic arc.
the process of backarc spreading.
is caused by the creation of new sea floor in the backarc region. It is usually attributed to the rising and lateral spreading of a large blob of hot mantle rock, called a mantle diaper.
transform plate boundary
occurs where adjacent plates slide horizontally past each other, neither creating nor destroying lithosphere. Transform boundaries occur along transform faults
Four features explained by plate tectonic theory:
• the distribution and composition of volcanoes.
• the distribution of earthquakes.
• the distribution of young mountains.
• the mid-oceanic ridge and oceanic trenches.
Possible driving mechanisms for plate motion:
• plates are pushed apart by intruding magma; con: tension cracks in the rift valley
• plates are carried by deep mantle convection currents; pro: would produce tension cracks in the rift valley region
• plates are pulled by the subducting leading edge of the plate; pro: explains tension cracks, explains large negative gravity anomalies, explains why some subducting plates are in tension;con: some subducting plates are in compression, while other plates subduct at very gentle angles
the three models of mantle convection
The three models of mantle convection are deep convection, shallow convection, and a two-tiered convection system with separate shallow and deep convection cells.
How might recent studies change our ideas about mantle convection?
Shallow convection is more compatible with a layered mantle than deep convection. Convection in the lower mantle is desirable as a mechanism of heat transfer from the hot core to the cool surface of the earth. However, if studies of the lower mantle can demonstrate that plates do penetrate the lower mantle below 670 km, then perhaps deep subduction does occur, and the two-tiered model is not correct.
the theory of mantle plumes
proposes that upward convection of hot mantle rock is confined to narrow plumes that extend like vertical pipes deep into the mantle. The downward sinking of cooled mantle rock takes place slowly throughout the rest of the mantle. The radial flow of mantle material outward from the top of plumes tends to break up the lithosphere and move the plates.
How are mid-plate volcanoes, such as the Hawaiian Island chain, explained by the mantle plume theory?
The mantle plume theory also conjectures that plumes rise, but rather than breaking up the plate, they act as eruptive centres beneath a moving plate. The result is a line of extinct volcanoes that increase in age away from an active volcano directly above the plume.
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