Exercise 11 continental drift and plate tectonics depression, or rift zone, is superimposed along the crest of the mid-atlantic ridge along most of its length. not only was the rift recognized in the atlan- tic, but the linear topographic low appears to be part of a major series of rifts and ridges associated with mid-oceanic areas of basaltic volcanic activity and seismic activity. the distribution of this worldwide system is shown in figure 11.2. the topographic and structural features were documented at about the same time that parallel belts of alternating reversed and normal polarity of the magnetic intensity, or magnetic field strength, were recognized in rocks along the ridge trend these alternating bands of normal and reversed polarity in the basalts occur symmetrically along the rift zone in the atlantic ocean in the fashion illus trated in the generalized diagram in figure 11.3. the location of the segment of the seafloor represented in figure 11.3 corresponds with line a-a on figure 11.2. the sequence and ages of polarity reversals for the past 5.5 million years are shown in figure 11.4. procedure part a evidence of continental drift outline maps of south america, africa, europe, and north america are given in figure 11.1. the stippled area around each of the continents repre- sents the area between the present shoreline and the 500-fathom (900-m) line (continental shelf). the latter is approximately the edge of the granitic con- tinental crustal margin a fitting of the continents is best done on a small globe, but various map projectionns have been used by others. because of the limitations of representing a curved surface on a plane, most map projections distort either the shape or size of the continents and alter their margins. because of these difficulties, a modified mercator projection has been used in the present exercise. this projection minimizes distor- tion of the coastline to be fitted. various sections of central america and europe have been shifted. slightly out of their present-day positions, but these are areas of relatively recent orogenic activity and may have had different positions in the past 1. why are the belts of polarity contrast situated symmetrically on either side of the rift zone on the mid-atlantic ridge? 1. trace the continental outlines following their true margin at the outer edge of the continental shelf from figure 11.1. adjust their relative posi- tions until the best fit is achieved, not only of continental outlines but also of geologic data, which are summarized on some of the maps 2. using figures 11.3 and 11.4, calculate as closely as possible the rate, in centimeters per year, at which seafloor spreading is occurring in the atlantic ocean, based upon the paleomagnetic data. your calculation is the rate at which a sin- gle plate is moving part b seafloor spreading and the magnetic timescale available evidence suggests that until the late paleozoic, the continents of africa and south amer ica were essentially side-by-side, and that drifting and opening of the atlantic ocean began early in the mesozoic era. if the continents drifted and the atlantic ocean is still enlarging, there must be some evidence on the present ocean floor. various work ers studied the topography, sediments, and igneous rocks of the mid-atlantic ridge to see if there were clues or evidence to document movement. it soon became apparent that a major linear topographic 3. enter the rate of plate motion determined in question 2 above in the blue box labeled "b." on figure 11.2