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Unit II.Plate tectonics and Man
Science is cumulative, and advances are made in the light of knowledge gained painstakingly by many researchers. A survey of the development of plate tectonics illustrates the progressive and cooperative nature of science and the way in which research in diverse fields produces unifying concepts of practical value to society.
The basic understanding of plate motions was considered as a conceptual revolution as profound for the earth sciences as were earlier developments of the concept of evolution in biology and of the concept of atomic and molecular structure in physics and chemistry. We know the new fields as plate tectonics: the “plate” is the basic unit of the system, and “tectonics” (from the Greek word “tekton”, meaning builder) refers to the processes and products of motions within the Earth.
According to the theory of plate tectonics the Earth’s crust is broken into moving plates of “lithosphere”. The plates tend to be internally rigid, and their interact mostly at their edges. All plates are moving relative to all others. Although velocities of relative motion between adjacent plates are low by human standards, they are extremely rapid by geologic ones. Plates are pulling apart primarily along the system of great submarine ridges in the world’s oceans. Where plates converge, one tips down and slides beneath the other. Generally, an oceanic plate slides (“subducts”) beneath a continental plate or another oceanic plate. A trench is formed where the undersliding plate tips down, and the ocean-floor sediments it carries is scrapped off against the front of the overriding plate.
We now know much about the mechanics of these motions from geophysical studies and particularly from seismic-reflection profiles made with instruments developed for oil-field exploration.
New oceanic-plate material is generated by the upwelling processes at spreading ridges. Old lithosphere is consumed, and recycled deep into the mantle. The balance is global only: the formation of lithosphere at the Mid-Atlantic Ridge is compensated by subduction primarily in the western Pacific.
Although the integrated concepts of plate tectonics were proved primarily by geophysical studies of the ocean basins, they have revolutionized our understanding of continental geology. Earthquakes are the most dramatic way in which these plate motions affect man. Most volcanic eruptions are also produced by the plate motions. The distribution of the mineral deposits and fossil fuels upon which our civilization depends has to a large extent been controlled by plate motions and interactions.
The course of evolution of life on Earth has been much influenced by plate motions too. The Earth has had contrasted areas of land and sea throughout its geologic history, although how much of the present water was early at the surface and how much has since been differentiated out by volcanic processes is debatable. The initial continents of an internally stable Earth, without the rejuvenating processes of uplift, mountain building, formation of new continental material, and magnetism, would long since have disappeared beneath the sea levelled by land and sea erosion, limiting potential life forms.
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