Oceans | VOL. 11, ISSUE 69, November-December 2011

Beneath the Ocean Floor

From the late 1930s, new techniques have opened up in submarine geology. Gravity measurements and geotectonic imagery has allowed accurate mapping of the sea surface and the bottom structure. The ocean floor is marked by huge mountain ranges - the mid-oceanic ridges that form part of a global network, extending for more than 80,000 kilometres (Fig. 1). In places such as Iceland, Ascension and the Galapagos Islands - the ridges rise above sea level. The ocean floor is also cut by deep trenches which marks subduction zones and are punctuated by isolated seamounts. One of the key pieces of information came from paleomagnetic studies along the Mid Atlantic Ridge. It was found that only half the rocks on each side of the ridge-axis near Iceland showed normal magnetic polarity; the remainder had a reversed polarity (a magnetic needle would point south). The pattern of normal and reversed polarity was manifested in a magnetic striping of the oceanic crust, mirrored on each side of the ridge crest. The alternating pattern of normal and reversed polarity rocks is produced as successive belts of lava are extruded at the site of a divergent plate margin. At the mid oceanic ridges and associated rift zones, new sea floor is generated which is then carried away from the ridge axis by lateral mantle motions. When individual stripes were dated, it...

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