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independent thinkers who instead considered that the __ in the level of the sea floor in the central Atlantic Ocean; opening of the oceans could be attributed to an _ this rise is now known as the "mid-ocean ridge". They increase in the Earth's radius. Roberto Mantovani, in also found that the floor of the seabed beneath the layer 1889 and again in 1909, published a theory of "Earth of sediments consisted of basalt—not granite, as expansion and continental drift". In this theory, he previously assumed, which is one of the main considered that a closed continent covered the entire | constituents of continents. In addition, they found surface on a smaller Earth. He suggested that thermal oceanic crust to be much thinner than continental crust. expansion led to volcanic activity, which broke the land All of these new findings raised important and mass into smaller continents. These continents then _ intriguing questions about the way we perceive oceanic drifted away from each other because of further crust. The most important of these is that the ocean is expansion at the "rip-zones", where oceans currently lie. not simply "oceanised" continental crust covered by sea This theory was followed by the pioneering work and __ water, as previously thought. publications of B. Lindemann in 1927, Ott Christophe Beginning in the 1950s, scientists using magnetic Hilgenberg in the 1930s, Professor Sam Carey during the 1950s to late idhooean ri 1990s, Jan Kozier during the 1980s, Mi ridge and Klaus Vogel in the 1980s and | | 1990s. it These researchers all showed that i —_— each of the continents were physically fitted together, they would neatly |] envelop the Earth with continenta . crust on a small Earth globe some 55 Reversed magnetic b to 60 per cent of its present size. This polarity coincidence led Hilgenberg, Carey and Vogel in particular to conclude tha terrestrial expansion brought abou the splitting and gradual dispersal o continents as they moved radially outwards during geological time. The perceived failings and shortfalls of each of these theories, owever, eventually led to the c acceptance of Plate Tectonics theory in the 1960s. This theory is now credited to have arisen out of the Figure |: Symmetrical magnetic striping across part of the Atlantic Ocean ypothesis of continental drift, as mid-ocean ridge (Source: http://en.wikipedia.org/wiki/ irst proposed by Alfred Wegener. Plate_tectonics#Explanation_of_magnetic_striping) Most of us are now reasonably amiliar with the concept of Plate Tectonics, whereby the instruments (magneto-meters), adapted from airborne Earth's outer crust is said to be made up of a series of | devices developed during World War II to detect arge, rigid, plate-like crusts that randomly move over submarines, also began recognising odd magnetic he Earth's surface under the influence of mantle patterns across the ocean floor. This finding, though convection currents. In the process of random unexpected, was not entirely surprising because it was migration, the crustal plates are said to rift, slide past known that basalt, the iron-rich volcanic rock making up one another and/or periodically collide to form _ the ocean floor, contains a strongly magnetic mineral mountains and subduct beneath continental crusts. called magnetite, which can cause local distortions in The primary assumption and absolute basis of Plate compass readings. More importantly, because the Tectonics is that the radius of the Earth has remained presence of magnetite gives the basalt measurable constant, or near constant, throughout its 4,500-million- | magnetic properties, these newly discovered magnetic year life span. sea floor patterns provided an important means to ated tha dinteihutian Af eralannian eanka theawe hawt anch in the level of the sea floor in the central Atlantic Ocean; this rise is now known as the "mid-ocean ridge". They also found that the floor of the seabed beneath the layer of sediments consisted of basalt—not granite, as previously assumed, which is one of the main constituents of continents. In addition, they found oceanic crust to be much thinner than continental crust. All of these new findings raised important and intriguing questions about the way we perceive oceanic crust. The most important of these is that the ocean is not simply “oceanised" continental crust covered by sea water, as previously thought. Beginning in the 1950s, scientists using magnetic and Figure |: Symmetrical magnetic striping across part of the Atlantic Ocean mid-ocean ridge (Source: http://en.wikipedia.org/wiki/ Plate_tectonics#Explanation_of_magnetic_striping) instruments (magneto-meters), adapted from airborne devices developed during World War II to detect submarines, also began recognising odd magnetic patterns across the ocean floor. This finding, though unexpected, was not entirely surprising because it was known that basalt, the iron-rich volcanic rock making up the ocean floor, contains a strongly magnetic mineral called magnetite, which can cause local distortions in compass readings. More importantly, because the presence of magnetite gives the basalt measurable magnetic properties, these newly discovered magnetic sea floor patterns provided an important means to study the distribution of volcanic rocks throughout each of the ocean floors. As more and more of the sea floor was mapped during the 1950s, it was discovered that the magnetic patterns were not random or isolated occurrences; instead, they revealed recognisable zebra-like stripes which were CONTRIBUTIONS TO MODERN TECTONICS THEORY utilising the Woods Hole Oceanographic Institution's research vessel Atlantis, confirmed the existence of a rise APRIL - MAY 2010 NEXUS ¢ 35 www.nexusmagazine.com