the search for a mechanism in the mantle that could account for 

 continental drift and for the youth of the ocean floor has lead to 

 study of the mid-ocean ridge, of magnetic lineations in the deep 

 ocean, of the age and geologic history of rocks from the ocean 

 floor, and of the continental margin; and it will lead very soon 

 to more direct study of the upper mantle at sea. 



THE MID-OCEAN RIDGE 



Reflection profiles across the Mid-Atlantic Ridge reveal it as 

 an uplifted welt of the sea floor some 2 km. high and 2000 km. 

 wide. The edges are buried in sediment, and the amount of sedi- 

 ment decreases, and the relief increases, towards the crest. The 

 crest is marked by a line of earthquake epicentres, is frequently 

 marked by a median " rift " valley, is displaced by transcurrent 

 faults, and near it high heat flow values are frequently measured 

 (though low ones occur as well). Gravity values measured at sea 

 level show no progressive change, so the uplifted welt must be 

 compensated by less dense material beneath. Seismic refraction 

 results indicate that the mantle beneath the crest of the ridge has 

 unduly low velocity (75 km/sec. instead of the usual 8-1) and 

 the corresponding decrease in density must provide the compensa- 

 tion required by the gravity results. Where the ridge comes to land 

 in Iceland it is clearly an active volcanic structure, and the presence 

 of large volcanic seamounts like the Azores, and the dredging of 

 basalts from innumerable places along the ridge confirm its 

 volcanicity. This is all consistent with the idea of convection in 

 the mantle bringing heat up and causing melting in the upper 

 mantle. 



The pattern of magnetic anomalies observed over the crest 

 of the ridge is very striking: three detailed surveys of the crest 

 of the Mid-Atlantic Ridge, all reported during the past year, have 

 shown a pattern of linear anomalies running parallel with the axis 

 of the ridge. An example is shown in Fig. 4. To explain a similar 

 pattern observed during a survey of the Carlsberg Ridge made in 

 1962, Vine and Matthews (1963) proposed the hypothesis that 

 dykes were being injected into the crust near the crest of the ridge. 

 As each dyke cooled it became magnetised in the direction of the 

 earth's magnetic field and was progressively shouldered aside by 

 younger dykes. It was known that the polarity of the earth's field 

 has reversed every million years or so during the past few million 

 years, so that the result of this process would be the formation 

 of strips of the earth's crust alternately normally and reversely 

 magnetised — just the structure needed to explain the linear pattern 

 of anomalies. This "fruitful Vine" hypothesis has recently 

 received additional support when it was found able to deduce a 

 sequence of reversals for the past 10 million years from profiles 

 of the mid-ocean ridge in the East Pacific and to use these correctly 



35 



