As described in the text , several scientists noted the similarity of the continental outlines of South America and Africa (in particular) and speculated that these two continents had been joined together in the past. With time, other evidence was gathered (much of it located in the southern hemisphere) which supported the concept although the mechanism by which rigid continental blocks could move with respect to one another remained highly speculative. Several terms were introduced which are in use today:
divergent plate boundaries
passive continental margins
continent/continent collision zones
The second and third parts of the animation shows how the apparently globe-wide wandering of the magnetic poles can be explained as a product of continental drift. Choose Inclination in this animation.The second view, Inclination Change, shows, in cross section, the gradual change in inclination of the magnetic field over India during the past 180 million years. The change can be explained by allowing the India subcontinent to drift from left to right (south to north) relative to a steady magnetic field that dips to either side of the equator.
Choose Apparent Polar Wandering in this animation.
Paleomagnetic poles recorded on different continents were frequently inconsistent, as if North America and Europe had separate magnetic north poles, for example. This conflict was resolved by the concept of apparent polar wandering. To under stand how it works, select "Apparent Polar Wandering" from the menu. Red and blue spots are paleomagnetic poles recorded in rocks of the red and blue continents, respectively. The continents collide, move together for a while and then split up again, much as North America and Eurasia did in the Paleozoic. The declination and inclination of the paleomagnetic field is recorded by magnetite crystals in cooling igneous rocks at several locations. Averaged over a short period (tens of thousands of years), the magnetic pole coincides with the geographic pole, as indicated by the red and blue spots. These records move with the continent where the source rocks are located and so some time later, the apparent position of the magnetic pole differs from the current geographic pole.
With the advent of Plate Tectonics, it became apparent that the continents move but the poles stay constant (keeping in mind the results from the first part of this simulation).
!Good Idea! Between now and the end of the semester you should prepare a chart on which you record information about the geologic characteristics of each plate boundary type. Included in your information should be rock type (igneous, metamorphic and sedimentary), type of stress (tensile, compressive, shearing), distribution of Earthquake foci, type of major structural features, etc. This is a good way to review as well as portray some of the patterns we have discussed.
The velocities of the lithospheric plates vary considerably. Two major types of spreading centers are recognized:
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Copyright by John C. Butler, July 29, 1995