Nexus - 1703 - New Times Magazine-pages

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he ridge crest always have present-day (normal) polarity. Stripes of rock parallel to the ridge crest were shown to have alternated in magnetic polarity normal-reversed-normal, etc.), suggesting that the Earth's magnetic field has reversed many times hroughout the planet's history. By explaining both the zebra-like magnetic striping and the construction of the mid-ocean-ridge system, the sea floor spreading hypothesis quickly gained converts. Furthermore, the oceanic crust now came to be universally appreciated as a natural "tape recording" of he history of the reversals in the Earth's magnetic field. Subsequent work by the Commission for the Geological Map of the World and UNESCO during the 980s led to the publication in 1990 of the bedrock Geological Map of the World (figure 2). In this global map, the magnetic striping discussed above was taken a step further. By dating the ages of the ocean floor bedrock at regular intervals throughout each of the oceans and comparing these ages with the magnetic striping, scientists were able to display ocean floor crust according to the ages of the rocks. What this means is that the yellow stripes in figure 2, for instance, located between the younger red stripes and the older orange stripes, represent volcanic rocks that erupted along the ancient mid-ocean ridges during symmetrical about the mid-ocean ridges. Alternating stripes of rock were shown to be laid out in parallel rows on either side of the mid-ocean ridge, one stripe with normal polarity and the adjoining stripe with reversed polarity. The overall pattern, defined by these alternating bands of normally and reversely polarised tock, became known as "magnetic striping". The discovery of this symmetrical magnetic striping pattern suggested a close relationship between the mid- ocean ridges and the stripes. In 1961, scientists (mos notably the American geologist Harry Hess) began to theorise that the mid-ocean ridges mark structurally weak zones where the ocean floor is being ripped apar lengthwise along the mid-ocean-ridge crest. It was suggested that new volcanic magma from deep within the Earth must rise through these weak zones and eventually erupt along the crest of the ridges to create new oceanic crust. This process, later called "sea floor spreading”, operates over many millions of years and continues to form new ocean floor all along the 60,000- kilometre-long system of mid-ocean ridges now known to be present in all of the oceans. This hypothesis is supported by several lines o evidence. At or near the crest of the mid-ocean ridges, the rocks are very young and they become progressively older away from the ridge crest. The youngest rocks a 36 * NEXUS APRIL - MAY 2010 Figure 2: Geological Map of the World (Commission for the Geological Map of the World and UNESCO, 1990) www.nexusmagazine.com