notes-6 - F1) Our understanding of global geology during...

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F1) Our understanding of global geology during the 20 th century There was a fundamental revolution in geology during the twentieth century - beginning in about 1910, and culminating in the 1970s with the widespread acceptance of the Theory of Plate Tectonics 1 . The significant points of plate tectonic theory are that: the earth's crust is made up of a number of semi-rigid plates, these plates move around with respect to each other, largely in response to convection taking place within the earth's interior, this process has been going on for much of the earth's history, and most of the major geological features of the earth can be explained by processes that take place at plate boundaries. Geologists are continually learning more about plate tectonics, how the process has affected the earth's crust and mantle, and how it has implications for virtually every aspect of geology. As part of our investigation into plate tectonics we will look first at the history of this scientific revolution, which is an interesting and important lesson in science itself and an insight into how scientific thinking evolves, why this evolution can be so slow and dysfunctional in some cases. Firstly - it is important to know what was generally believed about global geology before plate tectonics. At the beginning of the 1900s geologists had a reasonable understanding of how most rocks were formed and their relative ages, but there was considerable controversy regarding the origin of mountains, and in particular of mountain belts - such as the Alps. One of the prevailing views on the origin of mountains at the end of the 19th century was known as contractionism - the idea that the earth is slowly cooling and shrinking - so that mountain ranges form like the wrinkles on a dried up apple, and in which the oceans were merely submerged parts of former continents. Another widely held view was described as permanentism - in which it was believed that the continents and oceans have always been generally as they are today. This view incorporated a mechanism for creation of mountain chains known as the geosyncline theory. A geosyncline is a belt of sedimentary deposits that accumulate within the ocean along the margin of a continent - on the continental shelf and slope. The sediments are derived from erosion of the continent (see figure below). Geosynclinal sediments - which eventually turn into sedimentary rocks - can be many thousands of metres thick. As they accumulate they push down the pre- existing crustal rocks. Extenisve geosynclinal deposits exist around much of the coastline of most of the continents. Even into the early 1970's it was widely believed that mountain belts were formed when geosynclines were compressed by forces pushing from either side, although no one was exactly sure what force was to do the pushing.
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notes-6 - F1) Our understanding of global geology during...

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