Scotland s Time Lords

Transcription

1 Scotland s Time Lords Prof Stuart K Monro, Scientific Director, Our Dynamic Earth and Honorary Professor, University of Edinburgh Prologue This article is an opportunity to reflect on how Earth science was born in the City of Edinburgh and how ideas about Earth processes have changed up to the present day. There have also been some dramatic changes in our attitudes to public engagement with science in the 21 st century. As a geologist, I frequently quote the phrase geologists are good company, especially for other geologists which reflects the unfortunate attitude of some specialists in a variety of professions to engage more with each other than with the wider community. I contrast that with the nature of Society during the Edinburgh Enlightenment when individuals with widely different interests came together in discussion, often over a jug of claret, thus fostering the development of the polymath. It was in this climate that the modern understanding of geology, the science of the Earth, was born, here in Edinburgh over 200 years ago. Here is where our story, represented in the first gallery of Dynamic Earth, begins James Hutton, medic, chemist, farmer and the Father of Modern Geology James Hutton was born in Edinburgh on the 3 rd June 1726, the son of the City Treasurer. At the age of 17, he was apprenticed to a lawyer; but this was not a successful move. According to his biographer John Playfair, happily the force of genius cannot always be controlled by the plans of a narrow and short-sighted prudence. The young man was often found amusing himself and his fellow apprentices with chemical experiments instead of copying papers. With an interest in chemistry, it was medicine that he studied when he attended the University of Edinburgh. In those days, chemistry was an integral part of a medical education though few students actually matriculated for a degree programme, preferring instead to attend individual lectures. Three years later he continued his studies in Paris pursuing with great ardour the studies of chemistry and anatomy and took the degree of doctor of medicine in 1749 at Leyden. His thesis was on the circulation of blood. His departure to the continent to complete his studies was a common enough occurrence then, however it may have been precipitated by having had a son during his early twenties by a Miss Edington! In 1750 he returned to Edinburgh and resumed chemical experiments with his friend James Davie. Their work on the production of sal ammoniac ammonium chloride, a salt used in dyeing and working with brass and tin led to a profitable partnership.

2 Hutton also had a keen interest in agriculture and farmed at Slighhouses and Nether Monynut in Berwickshire, farms he had inherited from his father. Hutton wished to apply his scientific understanding to agricultural practice and travelled extensively to gain new ideas, introducing the Suffolk plough to Scotland. It was undoubtedly these travels that fostered his interest in the processes that shape the natural world. He visited Arran and Jedburgh and discovered the geological relationship, the uncomformity, in both places where horizontal or gently dipping rocks rest on top of more steeply dipping rocks. More importantly, he recognised the story that these exposures were telling. In Berwickshire he had seen the steeply dipping rocks that make up the Southern Uplands and also the more gently dipping rocks around Dunbar. He surmised that somewhere on the Berwickshire coast, these two rock formations would come together. Thus, with two friends, Sir James Hall and John Playfair, he sailed from Dunglass, southwards along the coast to discover Siccar Point and its dramatic unconformity. Here Hutton revealed to Hall and Playfair the true significance of what they were seeing. Hutton recognized that the vertical rocks would have been initially laid down horizontally then subsequently squeezed into a vertical position forming large-scale folds and building mountains. Before the second, gently dipping rocks are laid down; there would need to be a long period of erosion, removing the mountain. Finally the second set of rocks would be laid down. The unconformity between the two sets represents, therefore, a period of time when mountains were raised and then eroded away. These observations should be seen in the context of the establish wisdom of the time, that of Archbishop Ussher of Armagh who pronounced that the world was formed on the evening prior to the 23 rd of October 4004BC. Hutton demonstrated to his friends at Siccar Point that the Earth had to be very much older and thus started a quest for the age of the Earth which continued through to the 1940s when Arthur Holmes produced a revised age akin to that accepted today. Hutton s other great controversial idea was that some rocks had been molten and injected into sediments. He was a Plutonist. In this he was at odds with Robert Jameson, the Neptunist, who believed that these rocks were formed by precipitation from some primeval ocean. The evidence for Hutton s view can be clearly seen at what is now called Hutton s Section at the foot of Salisbury Craigs. The relationship between the Craigs rocks and the underlying sediments indicates that the sediments have been forced up and broken off by forceful emplacement of what must have been molten rock. Charles Lyell, the great communicator Hutton s ideas were the subject of two lectures given to the newly-formed Royal Society of Edinburgh on 7 March 1785 and 4 April However, he did not publish his Theory of the Earth in book form until Unfortunately the book was so badly written that few succeeded in reading it. In 1802, five years after Hutton s death, John Playfair tried to explain Hutton s theories in his Illustrations of the Huttonian Theory of the Earth. However Hutton s concepts were not widely recognised until Sir Charles Lyell included them in

3 his Principles of Geology in Charles Lyell was born in Kinnordy, Forfarshire (now Angus) and was the great communicator of Hutton s geological ideas and concepts. His Principles of Geology brought Hutton s ideas forward. Importantly, Charles Darwin had a copy of Lyell s book with him on his voyage on the Beagle. It was a book which greatly influenced Darwin s thinking, giving him a framework of deep time in which evolution by natural selection could work. Lyell went on to be one of the foremost geologists of his time and held the post of Professor of Geology at King's College London in the 1830s. He was a great advocate of uniformitarianism, a concept that related back to Hutton s thinking that the earth was shaped entirely by slow-moving forces still in operation today, acting over a very long period of time. The other side of the coin was that of catastrophism, where the Earth has been affected in the past by sudden, short-lived, violent events, possibly worldwide in scope. Now of course, it is recognized that both processes are operative; slow-moving change in many areas but violent and catastrophic events like volcanic eruptions, earthquakes and tsunami in others. The scientific understanding of our Earth continues to evolve. The Readers of the Rocks, Ben Peach and John Horne Fundamental to Hutton s ideas was the concept that rocks would be put into a vertical disposition by major compressive forces, building mountains. Though Hutton surmised this process, he was unaware of the mechanism by which it could be achieved. During the 19th century prominent geologists conducted a prolonged and bitter debate, the Highland Controversy, about the geological relationships exposed in the north-west Highlands of Scotland. The protagonists were, on the one hand Roderick Murchison and Archibald Geikie and on the other, James Nicol and Charles Lapworth. This was finally resolved in 1907 by the work of mapping geologists from the Geological Survey, Ben Peach and John Horne. The main issue was that the Moine schists at the top of the crag appeared to be older than the Cambrian and Ordovician rocks lower down. Murchison and Geikie believed the sequence was wrong and that the Moine schists must be the younger rocks. The conundrum was explained by Peach and Horne to be due to the action of a lowangle thrust fault bringing older rocks from the east over the top of the younger rocks. This was evidence of the major compressive forces envisaged by Hutton. Subsequently investigations in areas where there are much younger mountain chains such as the Alps and the Himalayas have confirmed that major compressive forces can build mountains by generating large-scale overfolds called nappes and the stacked, thrust slices which Peach and Horne recognized in the north-west Highlands of Scotland. However, what was yet unknown, was the mechanism by which these major compressive forces could be generated. That was all to change in this evolving story.

4 The Second Time Lord, Arthur Holmes Working in Durham and then in Edinburgh, Arthur Holmes was able to use the emerging science of radioactive decay to date rocks and establish a numerical timescale for the events that shaped the Earth. His interest in geochronology dates back to his undergraduate experience where he dated a Devonian rock from Norway as 370my. For the first time, rocks and the events associated with their formation could be accurately dated and a chronological story of the evolution of the Earth could be started to be written. Holmes was undoubtedly the other historical character who could stand alongside Hutton as a Time Lord. Global impact of these innovators There is an international dimension to this story which starts with a German polar explorer and meteorologist, Alfred Wegener who, in 1912, considered that the geometrical fit between South America and Africa suggested that they had been at one time joined and through time had drifted apart; the theory of continental drift. There was strong scientific evidence for this with identical, terrestrial fossils present on both continents however the scientific world would have none of it there being no mechanisms by which continents could move. Holmes championed Wegener s theory at a time when it was deeply unfashionable with his more conservative peers. Holmes proposed that Earth's mantle contained convection cells that dissipated heat produced by radioactive decay and moved the crust at the surface. The next real advances were stimulated by the Cold War; the Americans wanted to know where Russian submarines were and vice versa. As a result research was funded into the nature of the ocean floor topography. Marie Tharp was an American geologist and oceanographic cartographer working at the Lamont-Doherty Earth Observatory of Columbia University, who, along with her colleague Bruce Heezen, mapped the ocean floor including the Mid-Ocean Ridges, a line of undersea mountains. Their first physiographic map of the North Atlantic was published in 1957 and was part of the evidence that led Harry Hess from Princeton University in the 1960s to the concept of seafloor spreading from a mid-ocean ridge, opening up oceans and moving continents. Further evidence to support this concept came from work on determining the age of the ocean floor and the geomagnetic signature contained within the magnetic minerals. Sampling of the basaltic rocks of the ocean floor revealed a symmetrical pattern either side of the mid-ocean ridge. The ages of the rocks were shown to be progressively older away from the ridge. This pattern was reinforced by evidence from the magnetic minerals which recorded the periodic reversals that have taken place in the Earth s magnetic field through time. This symmetrical pattern revealed by Fred Vine and Drum Matthews working in Cambridge University and Lawrence Morley of the Canadian Geological Survey demonstrated conclusively that magma was rising up along the Mid-Ocean Ridge and moving away from the ridge giving the mechanism that Wegener was looking for to move continents. The corollary of this process is that if the Earth were not to be seen as continually expanding then there must be zones where crust is destroyed, compensating for the

5 generation of new crust in the Mid-Ocean Ridges. A processes known as subduction emerged from studies of location and depth of the epicenters of earthquake activity together with the information from the ocean floor topography. Oceanic trenches were identified with the epicenter of earthquakes becoming progressively deeper away from the trench. From this emerged the concept of a dense oceanic plate moving below a lighter continental plate. As the oceanic plates moves into the hotter areas of the crust and mantle it begins to melt, helped by the addition of water which reduces the melting temperature. In this way the oceanic crust is destroyed producing magma which rises to the surface, producing volcanoes which increase the volume of continental crust. Hutton s ideas revisited This story moves us a long way from the original ideas of Hutton, but taking the time to reflect might well help make meaningful connections between our modern understanding of how the Earth works and the concepts that emerged from Hutton s work. These ideas fall into three categories; deep time, the Earth s heat engine and the concept of cycles. Hutton s idea of Deep Time is fundamental to much of modern thinking on Earth science processes. These generally work slowly through time though the occasional catastrophic event is recorded in the geological time scale. Without this concept our ideas of the nature of the Universe and of evolutionary processes would not have emerged in the way they have. The concept of Cycles that Hutton recognized in his agricultural activities also fed through to his geological understanding. Primarily, he recognized a continuing process of erosion leading on to deposition of sediments which eventually become lithified to form rock. Looking at modern processes of Plate Tectonics, that too operates in cycles with oceans opening, oceans closing building mountains which are then eroded and deposited in the oceans forming new rock. The Heat Engine that Hutton conceived of is the driving force for plate movement with convection in the mantle, though it is now widely recognized that the pull of a subducting cold plate is probably more important than the push from upwelling along the Mid-Ocean Ridge. Epilogue The following letter, purported to come from James Hutton and bearing marks of scorching was, according to A Geological Miscellany, received recently by the editor of Séance de la terre: We all look forward to meeting some of your young colleagues who have made such spectacular advances in plate tectonics, and I am especially glad to hear that they are using my heat to drive their plates The science has moved on since Hutton s day but his legacy remains as the foundation for our modern understanding of how the Earth works.