In Lecture 21 we defined revolution as "a fundamental departure from any previous historical pattern" and the scientific revolution as a process that replaces qualitative ideas about nature by quantitative experiment. The scientific revolution in ancient Greece went hand in hand with a social revolution, and so did the scientific revolution of the late 17th century, which led to the separation of science from philosophy.
The process from qualitative ideas to quantitative experiment did not occur in all areas of science at the same time. It began with the revolution in physics, continued in the 18th century with the revolution in chemistry, and was completed in the middle of the 19th century with the revolution of biology.
Geology, another core discipline of the natural sciences, took particularly long to sever its links with religion and philosophy. It underwent a first revolution in the late 18th century but had to wait another 150 years before a second revolution finally established it as a quantitative science.
A scientific understanding of the forces that shape the Earth requires the knowledge of all continents. It could therefore only develop after all continents became known to science as a result of the Spanish voyages to the Americas and the expeditions into the Pacific Ocean of the 18th century. Before that time any theory of the evolution of the Earth could only be speculation. The question how the Earth came to acquire its present shape occupied scientific minds on occasions. But the paucity of accurate data and the inability of science to collect them meant that they did not dwell on it at any great length. Among them was Francis Bacon, who commented on the remarkable similarity of shape between the east and west coasts of the Atlantic Ocean already in 1620 in his Novum Organum but did not speculate about its reasons.
Before the scientific revolution of the 18th century geologists had a good understanding of erosion (the process responsible for the breaking up of rocks into sediment) but lacked an understanding of the processes that lead to rock formation. The prevailing view was that of the "catastrophists", who believed that the continents and oceans are the result of cataclysmic events of global proportions. The great flood described in the Bible was taken as the last example of a series of such catastrophes, the momentous event that shaped the continents, mountains and seas into their present form. Accompanying this belief was the concept that the Earth was about 6,000 years old.
This belief, originally based on the biblical description of divine creation, was so strongly established that even scientists and philosophers who did not invoke supernatural forces to explain the face of the Earth accepted it as fact and tried to imagine processes that could shape the Earth in the span of a few thousand years. A natural explanation for the existence of sedimentary rocks, for example, was seen in a process where massive amounts of minerals would precipitate out of the receding flood waters, forming new rock strata more or less instantly.
The Scottish chemist and naturalist James Hutton challenged the rule of catastrophism in 1788 with two papers entitled Theory of the Earth; or an Investigation of the Laws Observable in the Composition, Dissolution, and Restoration of Land upon the Globe. His theory, which from 1832 became known as uniformitarianism, explained the face of the Earth in terms of geological processes that can still be observed in action today. Hutton stated that weathering, erosion, sediment transport and deposition, consolidation into rocks through volcanism and extreme pressure and upthrust into new land constitute a cycle, in which land is very slowly but constantly formed and destroyed. He argued that the Earth must have gone through this cycle many times, which explained that marine fossils are found in high mountain ranges, and that the Earth must therefore be much older than the generally accepted 6,000 years.
Hutton's publications marked the first revolution in geology. They ended the association of geology with religious beliefs and challenged science to clarify the details of the geological cycle.
Hutton's revolutionary ideas did not find immediate acceptance by all. In 1825 the French zoologist George Cuvier gave new credence to catastrophism through his book Discours sur les révolutions de la surface du globe ("Discourse on the Revolutions of the Globe"), in which he used the idea of catastrophic floods to explain the distribution of fossil bones. For several decades catastrophism and uniformitarianism existed side by side, and most geologists were satisfied with the collection and description of observations without giving much thought to physical processes or hypotheses.
The Scottish geologist Charles Lyell advanced the cause of uniformitarianism greatly through publication of his work Principles of Geology in 1830 - 1833. It was the time of the first dinosaur discoveries and of Darwin's voyage on the Beagle, and Lyell's work took the same attitude to science: to collect facts for the purpose of supporting or rejecting a hypothesis. When Darwin's Origin of Species appeared in 1859 Lyell combined the evidence for biological evolution with the uniformitarian approach to geology and published The Geological Evidence of the Antiquity of Man in 1863.
At about the same time William Thomson (Lord Kelvin) raised a new argument in support of a relatively short lifespan for the Earth. He pointed out that the Earth is losing heat to space by conduction and therefore could not be much older than about 100 million years. His argument was laid to rest in the 20th century by the discovery of radioactivity and the recognition that radioactive isotopes in the Earth's interior provide a steady source of heat. While the Earth will eventually cool down, the process takes much longer than envisaged by Thomson.
As the 19th century progressed, uniformitarianism slowly outweighed catastrophism, although defendants of the latter continued to propose new mechanisms. The main problem was that, while the uniformitarianists could plausibly describe the cyclical process of rock formation and destruction, they could not give a physical explanation for the distribution of the rocks at the surface of the Earth or the existence of continents, mountain ranges and deep ocean basins.
The catastrophists had various proposals at hand. Oceans could have been carved out by the rushing waters of the receding flood. Snider-Pellegrini reinterpreted the biblical Creation in his work La Création et ses Mystères dévoilés ("The Creation and its Mysteries Unveiled") of 1858 by claiming that the continents were originally created as one but were blasted apart on the sixth day by volcanic vapours that rose through a fissure between Africa and America. (LeGrand, 1988)
George Darwin, the son of Charles Darwin, became one of the last defenders of catastrophism. In 1879 he wrote that the Earth initially rotated so fast that a large piece of land was torn off and formed the moon, leaving behind the Pacific Ocean. The ocean soon filled with molten lava, on which the continents began to drift. (Golden, 1972) It was one example how continental drift could be incorporated into catastrophism.
The existence of mountain ranges did not cause a problem for catastrophists; mountains were simply assumed to arise as a by-product of continent formation. Uniformitarianists had a greater difficulty with them. A widely accepted proposition was that the Earth is shrinking in size. Its mantle, becoming increasingly too large for it, would respond by folding into mountain ranges. It could also lift in certain places and sag in others, creating new continents and seas in the process, which would explain the various sedimentary layers and fossils in them.
In 1912 Alfred Wegener, a meteorologist at the Geophysical Institute of the University of Hamburg, put forward his view that the continents had once been a single landmass, which had broken apart, and that the individual pieces had then been "displaced" by slow movement. He named the original landmass Pangea (Greek pan = all, gea = Earth).
The idea that continents could have moved was not new. Wegener not only stated that they had moved; he suggested that they were still moving and would continue to move. He approached the issue with scientific rigour by collecting as much geological, biological and fossil evidence as possible to either support or disprove his theory.
Wegener's approach to the history of the Earth could have meant the final transition of geology from speculation to science. It was also of immense practical importance: Knowledge of the position of the continents relative to each a few hundred million years ago would give important clues as to the position of oil deposits in quite different parts of the world.
In 1928 the American Association of Petroleum Geologists organized a special symposium on continental drift. Most of its participants rejected the idea as fanciful and continued to propose formation mechanisms for continents and seas without any regard to the laws of physics. The problem was partly that Wegener himself could not propose a mechanism that drives continental drift. (He occasionally invoked old erroneous concepts such as tidal forces or the tendency of land to move away from the poles as a result of the Earth's rotation but left the question more or less open). But the lack of a mechanism cannot explain the hostility of most geologists, who pointed out Wegener's lack of formal geological training and ridiculed his work.
Soon after the symposium of 1928 the concept of continental drift fell into oblivion, supported only by Arthur Holmes in Scotland and by the South African geologist du Toit, who described Wegener's Pangea as made up of the two supercontinents Laurasia and Gondwanaland with somewhat different history. His work Our wandering continents, published in 1937, contained a much more detailed and accurate reconstruction of Gondwanaland than originally proposed by Wegener. Wegener's untimely death in 1930 contributed to the rapid disappearance of continental drift from the geological literature.
The undeniable verification of continental drift came from studies of the Earth's magnetic field. Magnetism as a phenomenon had been known for centuries; the use of the compass depended on the fact that a freely floating magnetic needle always points in the direction of the Earth's magnetic poles. For more than a millennium this was accepted and used without an understanding of the magnetic field that surrounds the Earth.
Towards the end of the 16th century William Gilbert, physician to Queen Elizabeth I in London, undertook careful laboratory studies of loadstone and came to the conclusion that the Earth acts like a giant bar magnet. His pioneering work De Magnete, Magneticisque Corporibus, et de Magno Magnete Tellure (On the magnet, magnetic bodies, and the great Earth magnet), published in 1600, was translated into English and printed in 1893.
Shortly after 1950 the English geologist Keith Runcorn began to study remnant magnetization in rocks. The molten magma from which rocks are formed often contains magnetized particles. When the rocks solidify these particles align themselves with the Earth's magnetic field. The resulting remnant magnetization thus gives an indication of the position of the rocks ait the time of their formation.
Runcorn began with measurements of the direction of magnetic field lines in English mine shafts but soon expanded his work to other regions. By comparing the direction and inclination of the remnant magnetic field from rocks of different geological age he was able to show that Europe's distance from and orientation relative to the North Pole had varied greatly during the millennia.
Taken in isolation, this finding could be explained either by movement of the pole or by movement of the continent. Movement of the pole over half the Earth was highly unlikely but could not be discarded as a possibility. However, when studies of remnant magnetization in America produced a very similar apparent but clearly distinct path of the pole it became clear that the pole had remained more or less fixed and the continents had not only moved but moved in different ways - in other words, America and Europe had drifted apart.
Like nearly every other geologist of the time Runcorn had not thought much about Wegener's theory. The incoming results from remnant magetization measurements convinced him that Wegener was right. From 1956 onward he dedicated his research to the collection of facts to put the theory of continental drift on solid scientific foundations. His book Continental drift appeared in 1962.
The new evidence for Wegener's theory raised again the question of the driving forces. Geologists still do not all agree on the correct answer, but the assistance of theoretical physics and advances in marine geology have helped to develop a picture of the Earth that will probably only require minor modification in the future.
The starting point is the generation of the Earth's magnetic field. Even in Gilbert's time it had already been difficult to see how the Earth could contain a giant bar magnet. The only physically acceptable method to produce such a magnetic field has to be the dynamo principle, in other words rotational movement of conductive material in the Earth's interior. Arthur Holmes, who had studied the thermal effect of radioactivity, had come to a similar conclusion; in 1929 he had argued that the generated heat could not be dispersed by volcanoes alone but required a layer of convective currents in the outer part of the Earth (Holmes, 1929; Hallam, 1973).
Direct measurements in the Earth's interior below a few tens of kilometres are not possible today, so indirect methods, such as the arrival times at different locations of waves generated by earthquakes, have to be called upon to study its structure. In our present understanding the Earth consists of four major components: A solid inner core is surrounded by a rotating fluid outer core that is responsible for the Earth's magnetic field. Covering the core is the viscous mantle, which is heated from the outer core and therefore undergoes slow convective movement. The ocean floor and continents make up the crust, the fourth and outermost layer.
Because convection in the mantle produces horizontal flow underneath the crust, different parts of the crust move in different ways. The continents assemble above places of downward mantle movement, while new material is brought up into the crust at places of upward movement. This produces a system of mid-ocean ridges, from where the newly formed sea floor spreads sideways until it is subducted under a continent.
This concept of the Earth, in which the continents drift across the mantle as different solid plates that push against each other, has become known as plate tectonics. Since its inception in the 1960s much additional evidence has been collected. One observation that had puzzled geologists for decades and receives an explanation through plate tectonics is the much younger age of the ocean floor compared to continental land masses. In the plate tectonics model the ocean floor is continuously formed at mid-ocean ridges and destroyed through melting in subduction zones, while the continents float at places of converging mantle currents. As a consequence the age of the ocean floor increases from the mid-ocean ridges towards the continents.
Additional confirmation of the concept comes from reversals of the magnetic field. Early in the 20th century studies of basalt that had been extruded in geologically rapid sequence on land had shown that the Earth's magnetic field reverses its polarity every now and then. The effect of such reversals is clearly seen in magnetic anomalies of the sea floor, which align in stripes parallel to the mid-ocean ridges.
While much still remains to be done to clarify the driving mechanisms for the movement of the plates, important elements of plate tectonics are firmly established. Wegener's concept has stood the test of time, and the major framework for continental drift is well understood today. It explains why effects of two ice ages are observed as far afield as in tropical Africa and clarifies many other geological facts that had scientists baffled for 300 years.
The scientific revolution in geology began at the same time when biology discovered evolution. It was not brought to completion before the 20th century and therefore did not attract the same attention from creationists as evolution, which by 1925 had to fight off attempts to ban its teaching in the courts. (Lecture 25)
The development of plate tectonics in the second half of the 20th century created a new challenge for creationism. Faced with overwhelming evidence some creationists now accept continental drift as a fact but try to explain it as the result of the biblical flood. Others reject plate tectonics altogether and insist on traditional versions of catastrophism. Both factions recognize the need to appear scientific in their arguments, and international conferences on creation "science" are held regularly where participants use such advanced investigation methods as computer simulations to verify the biblical flood (Baumgardner, 1994a).
In 2002 the creationist journal TJ published position papers for and against plate tectonics, based on presentations given to the Third International Conference on Creationism of 1994. The case in support of plate tectonics was presented by John Baumgardner, a scientist from the Los Alamos National Laboratory, a state research institution operated by the University of California for the National Nuclear Security Administration of the US Department of Energy (Baumgardner, 1994b). Michael Oard, a meteorologist and lead forecaster of the National Weather Service, presented the case against (Oard, 2001). Baumgardner has a Ph.D. in Geophysics and Space Physics from the University of California Los Angeles, Oard a M.S. in Atmospheric Science from the University of Washington. The editors of TJ called the ensuing debate
The difference between science and religion was discussed in Lecture 1; it does not have to be repeated here. What has to be said about creation "science" in this context has already been said in Lecture 25. The booklet Science and Creationism of the National Academy of Sciences, which barely mentions plate tectonics, nevertheless makes this observation:
It appears that the National Academy of Science of the USA agrees with the central premises of these lectures, that
Baumgardner, J.R. (1994a) Computer Modeling of the Large-Scale Tectonics Associated with the Genesis Flood. In R. E. Walsh (ed.) Proceedings of the Third International Conference on Creationism, 1994. Creation Science Fellowship, Technical Symposium Sessions, 49 - 62.
Baumgardner, J.R. (1994b) Runaway Subduction as the Driving Mechanism for the Genesis Flood. In R. E. Walsh (ed.) Proceedings of the Third International Conference on Creationism, 1994. Creation Science Fellowship, Technical Symposium Sessions, 63 - 75.
Golden, F. (1972) The moving continents. Charles Scribner's Sons, New York.
Hallam, A. (1973) A revolution in the Earth sciences. Clarendon Press, Oxford.
Holmes, A. (1929) Radioactivity and earth movements.Transactions of the Geological Society Glasgow 18, 559 - 606.
LeGrand, H. E. (1988) Drifting continents and shifting theories. Cambridge University Press, Cambridge.
National Academy of Sciences (1999) Science and Creationism, National Academy Press, Washington, DC. Also at http://www.nap.edu/html/creationism/ (accessed 27 August 2004)
Oard, M.J. (2001) Subduction unlikely - Plate tectonics improbable. In J. K. Reed (ed.), Plate tectonics: a different view. Creation Research Society Monograph 10, 93 Ð 145.
van Andel, T. H. (1995) Plate tectonics, Encyclopaedia Britannica 15th ed.
TJ editors (2002) Forum on catastrophic plate tectonics, TJ 16 (1), 57.
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