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Geology (Greek, ‘Earth-study’) is the scientific study of the Earth. There is a very long history of practical knowledge of the rocks, minerals and structures found at the surface and used for building, pigment or metallurgy, but, in spite of some tentative steps by the Greeks and Romans, scientific geology hardly began before the 19th century. Geology\'s problem, which is also the reason for its fascination, is that it asks fundamental questions about the origin, development, structure and behaviour of the solid Earth. For most of human history such questions have been beyond the reach of science and were seen as too important to be left to rational investigation. The situation has been transformed in the last two centuries, but only in the last few decades have convincing answers emerged.
A key figure in the emergence of modern geology was a Scot, James Hutton (1726 - 1797). His contribution was to put forward a theory of the Earth which was rooted in his own observations. His explanations of what he observed were not all correct, but they embodied an approach that became dominant in the 19th century, which was uniformitarianism.
Hutton lived most of his life in Edinburgh, then one of the great intellectual centres of Europe. He developed an early interest in chemistry which he maintained while studying law, medicine and agriculture. He travelled in Scotland and Europe and preferred field and laboratory investigation, often in collaboration with other investigators, to reading existing texts. As a result, his Theory of the Earth, first read in 1785, published as a paper in 1788 and only extended into a book in 1895, was highly original. He discussed rock formation, uplift, erosion and deposition (first proposing what we now call the rock cycle), and even anticipated later views of the role of glaciation in transporting boulders and of the relations between river systems and their basins. Some of his conjectures later proved wrong, but he had already shown himself willing to test his ideas against observation and change them if they did not fit the facts. It is ironic that the generation which followed him were more impressed by the Neptunist arguments of Werner and the catastrophist beliefs of Cuvier. Those arguments were gradually abandoned, though Werner\'s work on mineralogy and crystallography was an important forerunner of the later work of Berzelius (1779 - 1848) on the chemical principles of mineralogy.
A crucial practical contribution to the development of geology was made by William Smith, who worked as a surveyor, first in the Somerset coalfield and later on canals, turnpike roads and estates throughout England and Wales. His work both stimulated detailed local studies of geological structure and clarified the sequence of rocks in the stratigraphic column. He recognized that the same sequence of rock strata could be found in different places and that a particular stratum would have a distinct set of fossils. As a result he was able to add together the local sequences of rocks to build up the whole stratigraphic column, showing all the sedimentary rocks of the UK from the oldest to the youngest. His observations, plus the principle of faunal sequence could then be used by other geologists to refine the overall picture. With this knowledge, field geologists could both map the outcrops of rocks on the surface and begin to identify the three-dimensional geological structure which underlay them. They were able to show that layers of rock had been tilted and folded, that faults marked where layers of rock had been torn apart and displaced vertically, and that in places there were unconformities between older rocks and much younger ones, marking periods when deposition had either not occurred in that location or had been removed by erosion. Smith and his followers were showing that the Earth has a long and dynamic past rather than a short static one. He published the first geological map of England and Wales in 1815 and went on to produce more detailed county maps. This work was developed by the Board of Ordnance from the 1820s and later by the Geological Survey of Great Britain.
Much of the later work in the 19th century was an extension and elaboration of the work of these pioneers, whether in more painstaking survey and analysis of geological structure (often in association with mining activity), exhaustive investigation of mineralogy (aided by new instruments like the polarizing microscope) or in the analysis of surface processes. A Swiss, Louis Agassiz, showed that glacial features could be found in much of northern Europe and America and so identified the extent of the recent Ice Age. American geomorphologists began the investigation of fluvial processes in both shorter and longer terms. But for most of the century the knowledge of geologists was restricted to the outer layers of the planet.
The key advances in the 20th century stemmed from the development of geophysics, which offered a number of new ways of investigating the internal structure of the Earth. Seismology had a long history as the observation and recording of earthquakes but from the 1880s it progressed rapidly and in 1906 R.D. Oldham was able to deduce the internal structure and composition of the Earth, especially the existence of the Earth\'s dense core and less dense mantle and crust. Measurement of gravity anomalies had begun to show variation in the strength of gravity at different places, and that mountain ranges did not exhibit the positive gravity anomalies which would be expected if their extra mass was simply piled on top of the underlying rock. As a result it was proposed that the extra mass must be counterbalanced by ‘roots’ of less dense rock, suggesting that the continental rocks are ‘floating’ on the underlying rock. This idea, formalized as the concept of isostacy, was proposed in 1899 and supported by studies of gravity anomalies and rates of uplift in areas recently covered by thick ice sheets.
The notion of substantial vertical movement of parts of the Earth\'s crust was supported by studies in experimental petrology: Bridgeman showed that at the sorts of temperatures and pressures which would occur 30 to 40 km below the surface rock would be more plastic and possibly able to flow. Bowen\'s studies of silicate melts and rates of cooling did much to clarify the origins of igneous rocks.
The discovery of radioactivity and the existence of a constant half-life for the transition of a radioactive isotope to its daughter product was soon recognized as both a method of heating the Earth from within and a possible method of giving absolute dates to the rocks of the stratigraphic column. Boltwood used ratios of uranium to lead to date samples in the range 410-2,200 million years. Later, this was shown to be about 20% too high, but the method finally refuted Kelvin\'s thermodynamic argument for ages as little as 20 million years (see dating the Earth).
In 1912, Alfred Wegener published a theory which was as startling as Hutton\'s had been a century earlier. He proposed that the continents had moved over time, and indeed that they had once formed a single super continent. He was able to produce a lot of evidence, including the fit in shape (most noticeable for the two sides of the Atlantic), the evidence of climate change provided by geological deposits (for example the red sandstones of the UK show it experienced a desert climate and was thus well to the south of its current latitude), many geological structures and fossil assemblages appear as if torn apart and separated by thousands of miles. In spite of the evidence, most geologists regarded continental drift as a new form of catastrophism because no current mechanism seemed to exist.
From the 1960s new observations of the mid-ocean ridges showed that ocean spreading was occurring at rates of 2-10 cm per year, and geologists began to accept a new view of the Earth. The theory of plate tectonics brings together many previously disparate observations and explains how they result from the movements of about 20 crustal plates. In its way it is a change similar to the Copernican revolution. No longer do we stand on a static Earth whose surface is a complex riddle. Now we stand on a mobile Earth whose past movements have created the pattern of rocks and structures that past geologists described and present-day geologists can explain. PS |
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