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Y2K, DEEP TIME, AND THEORY CHOICE IN GEOLOGY concept, and only dimly as a reality. It was the Geologists who, through their attempts to understand the formation of earth's landforms, brought the concept of deep time into science. MILLENNIUM MADNESS Madness is rare in individuals, but in groups, parties, nations, and ages, it is the rule. -Friedrich Nietzsche I recall the hoopla that surrounded the transition from 1999 to 2000, the dawn of the 3rd millennium. I considered the fuss important only because some pieces of equipment might have stopped working. Also, we were inconvenienced by the temporary loss of a few services. Actually, the millennium, a thousand year period, is an accident of evolution (we have adopted a base ten number system because we have ten fingers), an accident of cosmology (a year has the particular period or length of time because of the earth’s distance from the sun), an accident of biology (we have a life span that approaches 100 years, so ten times that length is just beyond the understanding of any one person but within the understanding of a culture), and an accident of history (numbering of the current era began at an arbitrary year 1). Thus, the particular year of 2000 (or 2001 if you are a purist) becomes trivial by almost any way of reckoning it. Measured relative to the age of the earth, a 1000-year period is like a drop in the bucket. However, a serious consideration of the earth’s age requires an understanding of the concept of Deep Time. That is, thinking of time in millions and billions of years. These periods of time are so much out of a human’s everyday, lifetime or cultural experience that we can understand them only rationally. That is, we understand "billion" first as a word, secondly as a BY WATER Werner had great antipathy to the mechanical labor of writing, and he could never be persuaded to pen more than a few brief memoirs. -Sir Charles Lyell Before the 18th century there was little speculation about the transformation of the earth's surface. Some troubling things like marine fossils on mountaintops appeared in the discussion but were explained away as failed creations of life or evidence of The Genesis Flood. That a single catastrophe like The Flood or series of flooding catastrophes could explain the earth's landforms gave way to a theory that assumed the earth originally was covered by water. Gradually, the waters receded, and as they did, landforms of the earth appeared above the waves and took shape. Adherents to this theoretical view were called the Neptunists after the classical god of the sea. An important aspect of this view is that it could explain the earth in the 5-10 thousandyear period assumed to have occurred since The Creation. Neptunists explained crystalline rocks like granite and many other minerals as having grown as crystals in the waters of a younger earth. While covered with water, suspended rocks, sands, down to very fine particles 1 differentially settled out and made the layers of sedimentary rocks. German geologist, Abraham Werner (1750-1815), one of the principle Neptunist theorists supposed that the earth's landforms came about by the processes of precipitation and crystallization from a primordial sea. Therefore, older layers lay beneath younger layers. Werner even explained the occurrence of volcanoes as relatively new structures that were formed by subterranean coal fires. eroded the rocks of volcanic origin to produce sedimentary rocks. Hutton assumed that the natural processes at work on the earth now are the same ones that shaped the surface of the earth from the time it was a molten ball. This Uniformitarian Principle required immense amounts of time to wear down mountains, produce sedimentary rocks that then also erode away, etc. This principle became Hutton's major contribution to the sciences, and developed into the cornerstone of Geology. Hutton's vision was quite grand. He saw the earth as a great heat engine with volcanoes producing the raw materials for the renewal of the surface landforms. The goal of this cycle of rocks and erosion was the production of soil. BY FIRE 1 With no vestige of a beginning and no prospect of an end. -James Hutton An alternative theory said that volcanoes were responsible for the origin of the earth's landforms. John Ray (1627-1705), a British scientist, believed that mountains were formed by volcanic action. Also, the layers of sedimentary rock recorded a series of volcanic eruptions. This view required much longer periods of time with the same mechanism repeating again and again. During the 18th century, the followers of Ray (called Vulcanists or Plutonists) explained the volcanic origin of continents, ocean basins, as well as mountains. James Hutton (1726-1797), a contemporary of Abraham Werner, expanded on the Vulcanist theory. Through observations that he made in Edinburgh, Scotland and during his travels, he began to speculate that that the center of the earth is molten. Rocks like granite and basalt came from that molten center via volcanoes. Water then MODERN GEOLOGY IS BORN When we compare the result of observations in the last thirty years with those of the three preceding centuries, we cannot but look forward with the most sanguine expectations to the degree of excellence to which geology may be carried. -Sir Charles Lyell Hutton's vision was set down in a rambling 1000 page book called the Theory of the Earth in 1795. It was poorly written and his explanations were so ambiguous that arguments arose almost immediately as to what his conclusions really were. Sir Charles Lyell, a fellow Scot with an interest in Geology, took parts of the Neptunist theory that explained sedimentation, the formation of sedimentary rocks, and the relative ages of sedimentary rock layers. Also, he retained the view that the earth had an origin and a history. 1 Please consult Geology Through Lyell’s Eyes for an expansion on the contributions of Werner, Hutton, and Lyell. 2 Lyell kept much of the Huttonian Vulcanist theory and expanded on the Uniformitarian Principle. He eliminated Hutton's view of an eternal earth, but retained the requirement of long periods of time. In short, his theory proposed that the earth's surface became more uneven by volcanic or igneous processes and became more even by erosion or aqueous processes. Lyell's Principles of Geology (Volume 1 was first published in 1830) was well written and successfully integrated portions of the Vulcanist and Neptunist theories with the central theme of gradualism, based on the Uniformitarian Principle. The book grew to three volumes and went through 12 editions. It had a major impact on geologists and naturalists of the mid-19th century, including a young naturalist named Charles Darwin on his voyage of discovery. The vestiges of the Neptunist Theory became manifest in a young earth view in which the earth was shaped by a series of catastrophes. Lyell would not budge on his requirement of a gradual, uniformitarian process of landform creation. By the end of the 19th century, Gradualism had won out in the community of Geologists and catastrophes were no longer allowed as explanations. century. Descriptions of minerals and the physical processes involved in the formation of rock layers seemed to be pretty well known. Strata had been classified into broad ages and correlated across the planet 2. All could be explained within a gradualist theory. Geologists theorized that mountains, continents and ocean basins formed on the earth as a consequence of gradual cooling from a molten ball. As it cooled, it shrank slightly, thereby wrinkling the outer crust. It was in consideration of the cooling rate that a crisis arose in the sciences when physicists like William Thompson, Lord Kelvin chose to tackle the problem of the age of the earth using the principle of uniformitarian gradualism, the fundamental principle of geology itself. If the earth cooled from a molten ball to an earth with a molten interior and a crust of only 50 miles thick, then the earth could be no more than 100 million years old. Indeed, some of Kelvin's calculations seemed to say that the earth was as young as 24 million years. This new young earth theory was based on a physical calculation that was confirmed by other physicists. Not until the discovery of radioactivity and the realization that the interior of the earth contains small amounts of radioactive elements did a source of heat appear to counter Kelvin's assertion. At the time, though, his argument had been so compelling that some geologists had begun to accommodate theory to agree with the new dates. They assumed that the Uniformitarian Principle must be modified to say that geological activity was much more intense during the early earth and then it slowed down in more recent times. A PHYSICIST'S CHALLENGE His impulse was to correlate phenomena and arrive at the principle underlying them, and this gave him a certain impatience with branches of science that were still in the observational stage… -A Student of Lord Kelvin Traditional disciplines within geology such as minerology and stratigraphy blossomed during the 19th 2 3 I have expanded on this in Reading the Earth. less dense that basalt, so it appeared that the continents were higher because they "floated" on the denser mantle and ocean basin material. Alfred Wegener (1880-1930) embraced the concept of floating continents at the turn of the 20th century. Although he had received a doctorate in astronomy in 1905, Wegener quickly turned his attention to the weather and made important contributions to the study of meteorology. In the mean time, he became intrigued by the remarkable fit between the continents of South America and Africa. Although he was not the first to do so, Wegener gave the connection his full attention in 1914 as he lay in a hospital recovering from wounds suffered during the First World War. As he convalesced, Wegener began to collect evidence from stratigraphy and paleontology. He discovered that there are places that seem geologically continuous from one continent to another as if they were just torn apart. Also, the two continents shared particular plant and animal fossils that could not have made the transit across the Atlantic Ocean. Wegener collected enough evidence to convince him that all of the earth's continents had once been connected into a supercontinent that he called Pangaea. He published his ideas in 1915 in The Origin of Continents and Oceans (see Figure 5-9). In Wegner's view, the continents then broke apart and drifted to their present positions as they floated or slid over the plastic mantle. He looked in vain for a force sufficient to overcome the enormous forces of friction. He speculated that lunar tidal forces or the centrifugal force exerted by the spinning earth would be enough to move the continents like great ships over the earth's surface. The community of The discovery of radiation and the understanding of the half-lives of the products of radioactive decay led to the opposite problem. This indicated that the earth was immensely old, on the order of billions of years. Also, the cooling rate must have been so slow that ocean basins and mountains could not be explained that way. It was an intellectual Pyrrhic victory. SLIDING CONTINENTS It is probable that the complete solution to the problem of the forces will be a long time coming. The Newton of drift theory has not yet appeared. -Alfred Wegener The view at the turn of the 20th Century was that the earth was very old. Its continents and ocean basins had been altered by fluctuations in water level, mountain formation, and erosion. How were mountains formed? Volcanoes were easier to understand than the large mountain ranges of folded sedimentary rock. What could have generated enough force to cause large expanses of sedimentary rock to buckle and rise? Some postulated that rolling currents developed in the mantle, a hot zone between the molten core and the solid crust. As the mantle moved, parts of crust wrinkled above them. This served as an explanation for why there were mountains of very different ages and how mountains could have been formed in the absence of a cooling, shrinking earth. Also, it was noted that although continents seem to be made of recycled materials, the core of the continents was made of granite, a kind of igneous rock. The ocean basins, on the other hand, were made of basalt, another kind of igneous rock. Curiously, the granite was 4 geologists did not accept Wegener's ideas. His evidence was viewed as circumstantial and his explanations for the movement absurd. What is more, he was an outsider, a meteorologist meddling in geology. Wegener spent his professional life working on meteorology and defending his theory of Continental Drift to no avail. He died during an expedition to Greenland at age 50, and his theory of Continental Drift was relegated to an interesting footnote in the history of geology. to light in sea floor research. First, examination of the sediments of the floor of the Atlantic Ocean showed that they were quite young and thin. Static Earth Theory required that the ocean basins had accumulated sediment continuously for the earth's long history. However, collected sediments were thicker and older nearer the continental margins and younger toward the middle of the Atlantic. The center of the Atlantic Ocean had a ridge that ran the full length (north to south) and was equidistant from continents to the east and west. Hess and others recognized that the ridges were volcanic in origin and were sites of upwelling and spreading of material from the mantle. Hess suggested that the ridge overlay a rising convection current that was fueled by heat from radioactive decay. This explanation came together with evidence that ocean crust returned to the mantle at ocean trenches where the spreading seafloor forced oceanic crust to dive beneath the less dense continental crust. Furthermore, the subducted crustal material must melt when it dives into the mantle and rise again to form volcanoes. That continental volcanoes were associated with these areas of subduction confirmed the theory. In 1962, Harry Hess put all of this together in his theory of Sea Floor Spreading (see Figure 5-10). In this view, the continents did not plow through sea floor, but rode on them as passengers. At last the obstacle to Wegener's Continental drift was overcome. A plausible mechanism for change had been offered. Hess became Wegener's much-needed Newton of drift theory. FIGURE 5-9. Alfred Wegener's illustration of Pangaea from The Origin of Continents and Oceans. NEWTON OF DRIFT THEORY One of the truly remarkable earth scientists of this century. -National Academy of Sciences Memoir Harry Hess (1906-1969) earned a Ph.D. from Princeton in 1932, two years after the death of Wegener. During World War II, Hess served in the U.S. Navy, ultimately rising to the rank of Rear Admiral. He developed a system to track German submarines and made thousands of depth soundings across the Pacific Ocean. Following this experience he continued his interest in ocean basins by forming and directing the Princeton Caribbean Research Project. During this time, Hess considered some lines of evidence that were coming 5 began to crystallize. As the waters receded, more layers formed so that younger layers overlay older layers. This theory did not require a very old earth, but it did require a place for all of that water to go. In the Vulcanist Theory, landforms were created gradually by successive volcanic events. Thus, long periods of time were necessary. Indeed, in the Vulcanist theory of Hutton, time became cyclic, without a beginning or an end, operating on the Uniformitarian Principle. Charles Lyell then took parts of the Neptunist and Vulcanist Theories and wove them into a new way of explaining the earth. He kept the stratigraphy and minerology of the Neptunists and adopted the uniformitarian gradulaism of Hutton with the concept of an earth with a molten center. In this view of deep time, the earth was very old, but it did have a beginning. Lyell's theory did survive challenges by the catastrophists and physicists who said that the earth had to be much younger. Wegener's Continental Drift theory never entered the mainstream of the science of geology. It was interesting, but offered no mechanism to explain how the continents could move. Then following Hess' theory of Sea Floor Spreading, Wegener's Continental Drift transformed into Plate Tectonics, the central paradigm of geology. Still, Plate Tectonics is not completely new. It has vestiges of Neptunism, Vulcanism, Static Earth, and Continental Drift. Above all, it is a fruitful theory allowing our explanations of landforms to go beyond the bounds of this earth and through immense periods of time. FIGURE 5-10. A diagram of sea floor spreading. Sea Floor Spreading is the fundamental concept on which the Theory of Plate Tectonics is built. In this theory, the earth's surface is divided into a number of crustal plates like a cracked egg (see Figure 5-11). The plates move by spreading or slipping past each other. Sometimes they collide (the Indian and Eurasian Plates), and sometimes they slip beneath or subduct beneath another plate (the Nazca and South American Plates). This is a beautiful theory in that all landforms on the earth can be explained by these actions accompanied by erosion. By 1967, the theory was defined and began to be tested. Now, the theory is useful in understanding landforms on Io, Venus, Europa and other planetary bodies. More direct measurements of the movement can now be made by satellite. The Atlantic Ocean spreads at about 5cm per year. Other plates move at different rates. The surface of the earth is anything but static. THEORY CHOICE What better criterion could there be than the decision of the scientific group? Thomas Kuhn Now return to the various theories that I have explored. The Neptunists said that a primary sea had covered the earth. From that sea, rocks and minerals 6 FIGURE 5-11. Map of the major plates and their boundaries. Image from USGS at: http://easternweb.er.usgs.gov/eastern/plates.html Thus, the concept of deep time These things were far more important to me than a celebration of insignificance. became part of Geology, and allowed for -2000; revised 2004 the development of subtle, more general References: theories to explain the nature of the Bowler, Peter. 1992. The Fontana History of the earth. The fruitfulness of deep time as a Environmental Sciences. Fontana Press. principle caused it to weave itself into all London. disciplines of science. Thus, my rational Gould, Stephen. 1987. Time's Arrow, Time's understanding of deep time tempered my Cycle. Harvard University Press. Cambridge, Mass. enthusiasm for celebrating the new Gould, Stephen. 1997. Questioning the millennium. Millenium. Harmony Books. New York. Kuhn, Thomas. 1977. Objectivity, Value Judgement, and Theory Choice. In: Curd & Cover, eds. 1998. The Philosophy of Science, The Central Issues. W.W. Norton & Co., Inc. New York. pp 102-118. Lyell, Charles. 1990 (originally published 183033). Principles of Geology With a new Introduction by Martin J. S. Rudwick. Vols. 1-2. The University of Chicago Press. Chicago. Schuchert, Charles. 1928. The Hypothesis of Continental Displacement. Annual Report of the Board of Regents of the Smithsonian Institution. US Government Printing Office. Washington. pp 249-282. Trefil, James and Robert Hazen. 1998. The Sciences, An Integrated Approach. John Wiley and Sons, Inc. New York. Wegener, Alfred. 1966 (originally 1915). The Origin of the Continents and Oceans. Translated by John Biram. Dover Publications, Inc. New York. Y2K Someday we’ll all look back on this, laugh nervously, and change the subject. -A Bumper Sticker The year 2000 came and went, and much ado was made of that "landmark" time. I was so tired of hearing about it that I welcomed the next week when our attention-deficit culture turned its eyes to something else. After all, during the past 2000 years the Atlantic widened by only 100 meters. The whole Atlantic Ocean began to form only 140 million years ago, a mere 1/30th of the age of the earth. On New Year's 2000, I sang and danced. I celebrated friendships and life. 7 Questions to Think About 1. What were the contributions of Werner, Hutton, and Lyell in the development of modern geology? 2. How did Lord Kelvin and other physicists present a significant challenge to one of the fundamental principles of geology? 3. How was that challenge resolved? 4. What was Alfred Wegener’s theory of the earth? Why was it generally not accepted by geologists? 5. How could Harry Hess be described as The Newton of Drift Theory? 6. What is the theory of seafloor spreading? How does it help to explain the surface features of the earth? 7. How do continental volcanoes fit into the theory? 8. How is Plate Tectonics different from Continental Drift? 9. What do you think of the following comment by Thomas Kuhn: What better criterion could there be than the decision of the scientific group? To what was he referring? 10. Consider the history of geology. Did new theories ever really supplant those that preceded them? How does Plate Tectonics carry the vestiges of the Neptunists, Vulcanists, Hutton-Lyell, and Wegener. 8