2.2 Evidence for continental drift
2.2.1 Geometric continental reconstructions
Ever since the first global maps were drawn following the great voyages of discovery of the 15th and 16th centuries, it has been realised that the coastline geography of the continents on either side of the Atlantic Ocean form a pattern that can be fitted back together; in particular, the coastlines of western Africa and eastern South America have a jigsaw-like fit (Box 1).
Although some coastline fits are striking, it is important to note that the current coastlines are a result of relative sea level rather than the actual line along which land masses have broken apart. Indeed, coastline-fit is a common misconception - Wegner himself pointed out that it is the edge of the submerged continental shelf, i.e. the boundary between continental and oceanic crust that actually marks the line along which continents have originally been joined.
It was not until 1965 that the first computer-drawn reassembly of the continents around the Atlantic Ocean was produced by the British geophysicist Edward Bullard and his colleagues at Cambridge University. They used spherical geometry to generate a reconstruction of Africa with South America, and Western Europe with North America, which were all fitted together at the 500 fathom (about 1000 m) contour, which corresponded to the edge of the continental shelf. This method revealed the fit to be excellent (Figure 2), with few gaps or overlaps.
Question 1
Figure 2 shows some overlaps in the way in which the continents fit together around the Atlantic. Why might these exist?
Answer
Most of the overlaps are caused by features that have formed since the continents broke up or rifted apart, such as coral banks (Florida), recent river deltas (Niger) and volcanoes (Iceland).
Figure 2 (interactive): A computer-generated spherical fit at the 500 fathom contour (i.e. edge of continental shelf) showing Bullard's fit of the continents surrounding the Atlantic.