The planet we live on: The beginnings of the Earth Sciences

Figure 3.7: <strong>The</strong> structure of the Earth,showing the lithosphere and asthenosphere(not to scale).Figure 3.8:mantle.Convection currents in thetherefore recorded the magnetism of the Earth at the time and held this record as theocean floor was moved sideways, producing the series of stripes they had detected. Thiswas excellent evidence to support Hess’s ‘Sea Floor Spreading’ theory.It was later in the 1960s that the Canadian geologist, John Tuzo Wilson, put together hiswork on volcanic hotspots with that of Wegener, Hess, Vine and Matthews and others tosuggest that the whole Earth was covered by a series of slabs of rock that moved sideways.His ideas <strong>we</strong>re later called plate tectonics, the slabs of rock are plates and their movementis tectonics. He realised that plate margins are found not only at ridges and trenches,but also where there are long faults in the Earth, like the San Andreas Fault in the USA.<strong>The</strong>se large faults at plate boundaries he called transform faults.<strong>The</strong> new ‘<strong>The</strong>ory of Plate Tectonics’ included Hess’s ‘Sea Floor Spreading’ theory as<strong>we</strong>ll as much of Wegener’s ‘Continental Drift’ theory. It sho<strong>we</strong>d that continents didn’tplough through oceans, as Wegener had thought, but <strong>we</strong>re carried on plates over theEarth’s surface. It also provided the mechanism for sideways movement that Wegenercouldn’t find, since the plates <strong>we</strong>re carried by currents in the mantle beneath them.It took a few years for geologists across the world to change their ideas, but by the mid-1970s most geologists believed the new theory. Soon, the theory that the whole surfaceof the Earth had been in sideways motion for millions of years, was being used to explainmany puzzling geological features. Like Hutton’s ‘Principle of Uniformitarianism’ androck cycle ideas before it, plate tectonics revolutionised the thinking of geologists, andprovided great insights into Earth processes that are still being used today.Since the 1970s geologists have continued to investigate the details of plate tectonics and<strong>we</strong> now have a much better idea of how the plate tectonics machine works, creating newplate material in some parts of the Earth, and destroying it elsewhere, whilst plates slidepast each other in other areas. <strong>The</strong> plates are about 100km thick and are composed of thecrust and the upper part of the mantle, together called the lithosphere (‘litho’ is Greekfor ‘stone’ and it is a sphere because it forms the outer part of the Earth). <strong>The</strong> lithosphere78