the Mid-Atlantic Ridge

dhill757

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Member # 1890

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posted 11-06-2004 03:25 AM
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PROOF OF THE FORMER EXISTANCE OF A LARGE ISLAND ON THE MID ATLANTIC RIDGE

River Systems Extract - Azores as a Large Island

In 1971 we carried out an inspection of parts of the South Coast of San Miguel, the largest of the Azorean Islands. At one point, we found the remnants of a large, boulder filled, river-bed truncated by the shoreline. The rounded boulders were smoothly water-worn and massive (up to two feet across). The river bed, if we remember correctly, was some two hundred and fifty feet from bank to bank; but there was now insufficient width of island to sustain such a river. The boulders were so worn that they had, obviously, travelled a considerable distance, and a strong current of water with a head of thousands of feet would have been required to transport them. There was no room on the narrow island for such a current to be fostered - the rivers source must have lain to the north, on the flanks of a high mountain range. The present mountains on San Miguel are only a little over 3,000 feet high; and we estimate that it would have required a fall of at least 10,000 feet to have reduced boulders of that size, and hardness, to the degree of roundness which they profess.

This discovery so intrigued us that we started an investigation of the mapped sea-bed around all of the islands, with remarkable results. We started with the 1:1 million scale Admiralty Chart - Arquipelago dos Acores - the numerous soundings of which gave a very good general view of the configuration of the sea bed over the whole group of Azorean islands, covering, from east to west - Ilheus das Formigas, Santa Maria, Sao Miguel, Terciera, Soa Jrge, Graciosa, Pico (with its 7,613 feet high, conical, volcanic peak). These were supplemented by larger scale charts where they were available.

We started by contouring the sea bed at intervals of 100 fathoms (600 feet), and it immediately became clear that the river systems that now modestly drain the southern flanks of Sao Miguel were merely head-water tributaries whose channels continued far out to sea, joining into one great, winding, submarine valley some 40 miles further to the south. Other islands contributed similar results and, outstanding, were the triple group of Fial, Sao Jorge and Terciera whose combined results spawned two long river-like valleys which joined into one large valley to give a system that extended for 180 miles.