cphil_evolve3

a.  In 1908 F. B. Taylor in the U.S. and in 1910 the meteorologist Alfred L. Wegener of Germany independently suggested mechanisms that could account for the large lateral displacements of the earth’s crust and thus show how the continents could be driven apart. Wegener advanced a number of correlations, drawn from geology and paleontology, indicating a common historical record on both sides of the Atlantic Ocean. He proposed that all the continents were once joined into a single vast land mass before the start of the Mesozoic Era (about 200 million years ago). Wegener called this supercontinent Pangea or Pangaea. Wegener in 1912 argued that if the earth could flow vertically in responce to vertical forces, it could also flow laterally. Under the action of forces associated with the rotation of the earth, the continents had broken apart, opening up the Atlantic and Indian oceans. Between 1920 and 1930 Wegener’s hypothesis excited great controversy. Physcists found the mechanism that he proposed inadequate and expressed doubts that the continents could move laterally in any case. Geologists showed that some of Wegener’s suggestions for reassembling the continents into a single supercontinent were certainly wrong and that the drift was not necessary to explain the similarities of geology in many areas. They could not dispute the validity of most of the transatlantic connections. Indeed, more such connections were being steadily added.

b.  Many geologist of the Southern Hemisphere, led by Alex. L. Du Toit of South Africa, welcomed Wegener’s views. They sought to explain the mounting evidence that an ice age of 200 million years ago had spread a glacier across over the now scattered continents of the Southern Hemisphere. At the same time, according to the geological record, the great coal deposits of the Northern Hemisphere was being formed in tropical forest as far north as Spitzbergen. To resolve the climate paradox Du Toit proposed a different reconstruction of the continents. He brought the southern continents together at the South Pole and the northern coal forests toward the Equator. Later, he thought, the southern continent broke up and its component subcontinents had drifted northward.

c.  Many suggestions have been made to explain how to create and destroy land bridges needed to explain the biological evidence without moving the continents. Some involved isthmuses and some involved whole continents that had subsided below the surface of the ocean. But the chemistry and density of the continents and ocean floors are now known to be so different that it seems more difficult today to raise and lower ocean floors than it is to cause continents to migrate.

d.  One of the best leads to a mechanism that would move continents came about in the 1930 when the sensitive techniques of gravimetry that had established the rule of hydrostatic equilibrium, or isostasy, ashore, was extended to the sea floor. The Dutch geophysicist, Felix A. Vening-Meinesz, demonstrated that a submerged submarine would provide a sufficiently stable platform to allow the use of gravimeter at sea. Over the abyssal trenches in the sea floor that are associated with the island arcs of Indonesa and the western side of the Pacific, Vening-Meinesz found some of the largest deficiencies in gravity ever recorded. It was clear that isostasy does not hold in the trenches. Some force at work there pulls the crust down into the depths of the trenches more strongly than the pull of gravity.

Arthus Holmes of the University of Edinburg and D. T. Griggs at the University of California at Los Angeles, were stimulated by these observations to re-examine and restate in modern terms an old idea of geophysics: that the interior state of the earth is that of extremely sluggish thermal convection, turning over the magma like boiling water does in a pan. They showed that convection currents were necessary to account in full for the transfer of heat flowing from the center of the earth through the poorly conductive material of the mantle; the region lies between the core and the crust of the earth. The trenches, they said, mark the place where currents in the mantle descend again into the interior of the earth, pulling down the ocean floor.

Convection currents in the mantle now play the leading role in every discussion of large-scale and long-term processes that go on in the earth. It is true that the evidence for their existence is indirect; they flow too deep in the earth and too slowly, a few centimeters a year, to be directly observed. Never the less, their presence is supported by an ever increasing body of independently established evidence. Perhaps the strongest evidence has come from the discovery of the regions where the currents ascend towards the earth’s surface. This was the major discovery on the floor of the oceans of a continuous system of ridges. Across the floor of all oceans, for a distance of 40,000 miles, there are these ridges. In the mid-Atlantic for long streches these ridges are faulted and rifted by under the tension of forces acting at right angles to the axis of the ridge. Measurements undertaken by Sir Edward Bullard of the University of Cambridge showed that the flow of heat is usually great along these ridges, exceeding by two to eight times the average flow of a millionth of a calorie per square centimeter per second observed on the continents and elsewhere on the ocean floor. Most of oceanographers are now agreed that the ridges form where convections currents rise in the earth’s mantle and that the trenches are pulled down by the descent of these currents into the mantle. Here then is a mechanism, in harmony with physical theory and much geological observations, that provide a means for disrupting and moving continents. This theory, in contrast to earlier theories of continental drift where the continents drive through the crust like ships through a frozen sea, this mechanism conveys contiments passively, carrying them along by the lateral movement of the crust from the source of the convection current to its sink. The continents having been built up by the accumulation of lighter and more siliceous materials brought up from below, are not dragged down at the trenches where the currents descend but pile up there as mountains. The ocean floor, being essentially altered mantle, can be carried downward; such sediments as have accumulated in the trenches descend also and, by a complicated processes, may add new mountains to the continents. Since the materials near the surface are chilled and brittle, it fractures, causing earthquakes, until it is heated by its descent. [2]