Lesson 2: Earth Planet of Life 2.5 Drifting Continents Figure 2.5.1 The Atlantic Ocean "Rift" When contemplating the distribution of land and water on the home planet, we noted that there is a land-dominated hemisphere and one that has only ocean, with the respective centers somewhere in the Middle East for one and around Christmas Island for the other. We also noted that the Atlantic Ocean looks like a giant rift, its western and eastern shores running more or less parallel, and that it is increasing in size every year. The scientist who first suggested a reason for the unequal distribution of land and sea was Alfred Lothar Wegener (1880-1930), a German astronomer and meteorologist. Wegener proposed that there had been a single super-continent (which he called "Pangea" or "All-Earth") that broke up beginning sometime in the early Mesozoic. Its pieces drifted apart since, to form the continents we know today. In the process, the Atlantic formed. Thus, in Wegener's view, published in 1915, the continents as they exist today are puzzle pieces that fit together when they are re-arranged as they were two hundred million years ago. Wegener's hypothesis met with general disbelief among the geologists of the day, and remained in disfavor (at least in the U.S.A.) until well into the 1950s. Figure 2.5.2 The evolution of Wegener's supercontinent Pangea. Fifty years later, Edward C. Bullard (1907-1980) and collaborators produced a striking confirmation of the puzzle-piece idea. Using a theorem of the Swiss mathematician Leonhard Euler (1707-1783), which says that uniform motion on a sphere is equivalent to rotation about a pole, Bullard and associates showed that the submerged edges of the continents on either side of the Atlantic could be fitted to each other with great precision. Earlier, in the 1920s and 1930s, the South African geologist Alexander L. Du Toit (1878-1949) had argued for such a former fit in the South Atlantic, from comparing the rock formations in South Africa with those in South America, formations that he found to be practically identical. Most geologists rejected Wegener's ideas for the decades following publication. One notable exception was the British geologist Arthur Holmes (1890-1965), who worked on assigning absolute age values to the geologic time scale, based on evidence from radioactivity. Holmes, in 1929, proposed that continents moved because of convection in the mantle. The mantle, he deduced, is being heated by the radioactive decay of elements within it. Mantle material, he thought, comes up in the Atlantic and pushes the continents apart. Wegener's concepts (or lack thereof) about how continents were supposed to move were a major stumbling block in accepting his theory. Holmes's answer (while correct, in essence) in turn had to wait for acceptance another 35 years or so, when the geophysical properties of the ocean floor became known from surveys done after WWII, with instruments developed during the war. Surprisingly, then, while we understand the motions of our celestial neighbors quite well for the last 300 years or so, the major features of Earth's land and sea distributions are only understood since the late 1960's. Much of what was written before that time on the subject has been assigned to the dustbins of history. Earlier notions, in particular, favored such things as "land bridges" to explain how similar terrestrial fossils could be found in the Paleozoic rocks of continents on either side of the Atlantic. These "land bridges" supposedly subsided and disappeared from view at the end of the Paleozoic. Wegener pointed out that the ocean crust is fundamentally different from continental crust (as established by gravity measurements, showing that continental crust is made of lighter material). So it is impossible to make land go up and down like an elevator. The reaction to Wegener's objections was to make land bridges smaller, so that not finding their remnants would be less surprising. Continental Drift is now well established and no longer a point of contention. We can actually measure the movements of continents, using GPS technology, that is, through the use of precise positioning by communication with satellites. The proud title of Wegener's book, "The Origin of Continents and Oceans", is somewhat misleading in that the existence of continents and oceans is postulated to begin with, and only the present distribution patterns of continents and ocean basins is at issue. Wegener introduced the concept of large-scale horizontal movement of continental landmasses without deformation of the landmasses themselves, except at their edges, where mountains can be made (as in the western Americas). This concept revolutionized geological thinking, eventually. He did not come up with a mechanism to move the continents (nor did he claim that he did, stating that this was a problem that still waited for a solution). Wegener's insistence that continents and ocean basins are made of fundamentally different materials so that one cannot be readily turned into the other (an error perpetuated by Plato in his legend of Atlantis) has proved its worth. This basic duality is the reason that Earth has two preferred zones of elevation, one just above sea level and the other corresponding to the average depth of the ocean. It is interesting to note that the two elevations, which are ultimately a result of differences in the density of crust of continents and oceans, are tied to the amount of water on Earth. This indicates that there is a dynamic adjustment of the height of continents, depending on sea level, and that the thickness of the continents is thereby controlled by the amount of water on the planet. Where continents stick out too far above sea level, they are vigorously eroded. Where they are below sea level, deposition takes place. From this simple fact, we can deduce that where there are mountains, the land is rising, and where there are deep depressions on continents, it is sinking. Just how these vertical motions are tied to continental drift is a topic that belongs to "plate tectonics".