From time to time submerged volcanic peal<s 

 erupt and become new islands. This 

 photograph shows the 1946 eruption of 

 Myojin, part of the 1500-mile-long chain 

 of islands extending from Japan to Guam. 



mountain range extends about a thousand miles across its base and 

 stretches down the middle of the Atlantic for more than ten 

 thousand miles. Recent surveys suggest that similar ridges in other 

 oceans link with the mid- Atlantic Ridge and form a chain of world- 

 encircUng mountains some forty thousand miles in total length. 

 A feature on such a vast scale must be explained along with the 

 formation of the oceans and continents themselves. 



We know very little about the composition of the mid- Atlantic 

 Ridge. Photographs and dredge hauls show that the peaks are 

 submarine volcanoes that have erupted many times over the years, 

 spilUng their lava flows out over the surrounding sea bed. Under 

 the influence of the cold water, the flows have cooled rapidly and 

 have subsequently been broken up by internal stresses. We know 

 from a study of the distribution of earthquakes over the world that 

 one belt of shallow earthquakes runs right down the ridge itself 

 and that it is a region of quite intense crustal movement today. 

 Deep seismic refraction work has shown that under the ridge the 

 oceanic crust has been thickened, possibly by low density material 

 rising up out of the mantle, giving "roots" to the range. 



In the center of the ridge in many places is a deep valley running 

 along its length. In the North Atlantic we have followed this valley 

 for about a hundred and fifty miles, and noticed in some places that 

 it has been blocked by volcanic outpourings from neighboring 

 volcanoes. Studies of other mid-ocean ridges reveal similar median 

 valleys, and it now appears that such a valley is common to all 

 ridges. How do we explain such a feature? By comparing the mid- 

 Atlantic Ridge valley with the African Rift valley and the trench in 

 the Red Sea, some geophysicists conclude that these are tears in the 

 Earth's crust — weak seams that are gradually widening as our 

 planet slowly expands. 



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