Part 3. the Legacy of Lyell

Downloaded from http://sp.lyellcollection.org/ by guest on September 26, 2021 Part 3. The legacy of Lyell Lyell would have approved of plate tectonics. Indeed, by implication, he almost anticipated it. Recognizing the connection between climate and the global distribution of land and sea, as Fleming points out (pp. 164-165 of this volume), Lyell explained climatic change in the geological past in terms of changes in the geographical distribution of land and sea - continental drift in disguise. Furthermore, since the paradigm of plate tectonics is founded upon an understanding of processes active at present, whose rates can be measured, then applying them to the past, it is entirely in accord with Lyell's principle that geological 'phenomena should be explained only in terms of causal agencies that are observably effective, both in kind and degree' (Rudwick p. 4 of this volume). In a new synthesis of the Lower Palaeozoic tectonic evolution of the northern Appalachians and British Caledonides, Cees van Staal and his colleagues draw upon the modem analogue of the southeast Asia region. Intriguingly, they are able to turn the comparisons around to predict that the modem tectonic activity of southeast Asia will eventually end up in structures looking much like those of the Appalachian-Caledonian orogenic belt. This carefully argued work, paying close attention to field observations, is in the true tradition of Lyell and is firmly set within his principles. The legacy of Lyell is equally evident in Andrew Scott's paper in which he has literally followed in Lyell's footsteps across to North America. Scott discusses on-going research into the occurrence of reptiles preserved inside the upright trunks of Upper Carboniferous trees at Joggins, Nova Scotia, first discovered by Lyell and Dawson in 1852. Scott examines Lyell's ideas on the formation of coal and brings them up to date using modem analogues from southeast Asia. However, he sounds a note of caution in the use of modem analogues since it has now been established that Carboniferous coal-forming plants had life habits and growth mechanisms that were radically different from those of modem peats. The recognition by Lyell of charcoal in coal deposits resulted only many years later in an understanding of its origin from ancient forest fires. Scott explains how current research on ancient charcoal is providing new insights into climatic change, atmospheric composition, and processes of erosion and sedimentation. Lyell visited Nova Scotia twice during his visits to North America. The influence of Lyell on Nova Scotian geology and the use that Lyell made of his fieldwork in his publications is examined by John Calder. Calder provides us with a succinct account of the Carboniferous evolution of Nova Scotia and places Lyell's observations and conclusions in context. Clearly Nova Scotia provides an important link between Europe and North America. Lyell was impressed by the geology of Nova Scotia and his enthusiasm was reciprocated by the local geologists. Particularly important was the influence that Lyell had on Sir William Dawson who was to become one of Nova Scotia's most important and influential geologists. Lyell used many of his discoveries in Nova Scotia to illustrate subsequent editions of his volumes including his discovery of tetrapods within sandstone-filled lycophyte trunks at Joggins. Unravelling the stratigraphic record was always a fundamental objective of Lyell. His desire to understand rates of sedimentation, rises and fall of land and sea level changes as seen both in ancient rocks and as active processes in the modem world is evident from his field observations as recorded in his diaries, travel, and geological books and papers. Our current understanding of the stratigraphic record has been revolutionized in the past twenty years with the advent of sequence stratigraphy as shown by Chris Wilson. The paper by Wilson looks at how the sequence stratigraphic model evolved and its utility. Wilson argues that the application of sequence stratigraphy has provided us with a revolutionary new way of interpreting the stratigraphical record. However, Wilson adds caution in that a new global stratigraphy, allowing global correlations based upon eustatic signals, is not yet a reality as their recognition can not yet be detected unequivocally in the record. Clearly again Lyell takes us back to looking at the rock record but as he also maintained, we must also consider the processes involved that were responsible for what we observe. This point is amplified by Chris Talbot in a review of salt tectonics in the Zagros mountains of Iran. Talbot notes the parallels between the controversy in Lyell's day about whether ice could flow uphill and the more recent controversy about whether salt can flow across the surface of the land or, indeed, the sea bed. Talbot takes the argument a stage further in demonstrating how field measurements of the rates of salt flow and observations of the domal shapes of extrusions are used to quantify the dynamics of the process. He hints at the possible analogy between salt extrusion and the extrusion and gravity spreading of metamorphic core complexes. This richly illustrated, authoritative account of salt tectonics in a classic area is yet further evidence of Lyell's influence on the approach to modem research. Lyell was much concemed with the most tangible expressions of current geological activity in the form of volcanic eruptions and earthquakes. He spent a major part of his life in the field studying volcanoes in Italy and elsewhere, as Wilson describes on pp. 24-31 of this volume, and he was well aware of their Downloaded from http://sp.lyellcollection.org/ by guest on September 26, 2021 198 PART 3 destructive power. Mount Etna was of particular significance, so it is an essential part of Lyell's legacy that volcanic activity on Etna should be closely monitored, both to give greater quantitative understanding of the process of eruption and to give predictive power to mitigate the risks from future eruptions. Hazel Rymer is a member of an international, multidisciplinary team of scientists who have set up a huge array of instrumentation to monitor every waking moment of this volcanic giant. Their paper gives a historical account of volcano monitoring and describes the modern techniques being utilized on Etna, which are advancing all the time. This is especially so with satellite measurements, including observations of ground deformation using GPS and SAR, which they confidently expect to revolutionize monitoring methods within the next decade. Bruce Bolt brings a wealth of experience to review the advancement of seismology since Lyell's day. Arguably the most significant advance was the construction of the Milne-Shaw seismograph and its use in a global network of observatories set up at the end of the nineteenth century. This duly advanced to the Worldwide Standardized Seismograph Network in the 1950s which has now evolved into the Global Digital Network of seismograph stations. Bolt traces the history of seismology from the valuable descriptive accounts of earthquakes in Lyell's time to the present day analysis of seismic waveforms which gives detailed information about both the nature of the earthquake itself and the nature of the Earth's interior along the pathways of the seismic waves. Seismology has become one of the most powerfol tools for exploring and imaging the Earth's interior, well beyond anything that Lyell could have dreamed of, and continues to be the main means of monitoring earthquakes, in efforts to mitigate their risks. Earthquake prediction, as Bolt explains, remains elusive despite huge research efforts in recent years. The current view is that it is likely to remain so because of the frictional nature of the earthquake mechanism and the heterogeneity of the Earth's crust. A better strategy for risk mitigation is to concentrate on reducing the vulnerability of the population at risk by means of planning, education and the construction or upgrading of buildings and other structures to appropriate levels of earthquake resistance. Lyell was intensely interested in the antiquity of Man, as Cohen (pp. 83-93) and other authors in Part 1 of this book have made clear. Lyell was also intensely interested in the natural environment since this is the laboratory in which to record and measure geological phenomena active at present which are the basis for understanding the geological past. Lyell was well aware of the power of nature over Man and of human dependence on the Earth for its resources, but there was little regard in Lyell's day for the need to protect the Earth from human activities. Nowadays it is a major issue and Sir John Knill completes this book with a telling essay on Man and the modern environment to make the point that, more than ever, we need to apply Lyell's principles. It is now imperative to understand and quantify the mechanisms of global climate change in the geological past, especially over the last 500 000 years, in order to predict future climate change including the anthropogenic contribution. Equally, geological processes on all scales must be better understood in order to build strategies for the environmental sustainability of the Earth. There is thus a critical role for geologists in modern society. The old textbook shorthand for Lyell's principles that 'the present is the key to the past' has to be turned around into the title of this book: the past is the key to the present - and, indeed, to the future. Derek J. Blundell Andrew C. Scott Downloaded from http://sp.lyellcollection.org/ by guest on September 26, 2021 The Cambrian-Silurian tectonic evolution of the northern Appalachians and British Caledonides: history of a complex, west and southwest Pacific-type segment of Iapetus C.

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