230 BULLARD [chap. 11 



late in the process of solidification. On the continents it is probable that a large 

 part of the radioactivit}' is now concentrated in the granites and gneisses of the 

 crust. The oceanic basalts are less radioactive than continental rocks and 

 produce about three times less heat per unit volume ; the bulk of the observed 

 oceanic heat flow must, therefore, come from beneath the "Moho". If the rocks 

 beneath the "Moho" had as low a radioactivity as ultra-basic rocks or meteo- 

 rites, the heat would have to come from great depths in the mantle and the 

 temperature gradients needed to convey the heat to the surface by conduction 

 would lead to melting. There are two ways of resolving this difficulty : either the 

 radioactivity must be concentrated towards the top of the mantle so that 

 most of the heat is generated in the upper 200 km. or there must exist some 

 more efficient process than conduction to bring heat to the surface from great 

 depths. Convection currents in the mantle might provide such a mechanism. 



The high heat flow^s over the ocean ridges can hardly be accounted for by 

 local concentrations of radioactive material and it seems most probable that 

 their immediate cause is the intrusion of igneous rocks beneath the ocean floor. 

 Vulcanism occurs today at several jDoints on the Mid-Atlantic ridge and there 

 seems nothing improbable in suggesting that dykes may be injected along the 

 line of the ridge. Since a dyke 1 km thick cools in a time of the order of 10^ years, 

 a series of intrusions would be required. The temperature beneath the ridges 

 must rise very rapidly with depth. The observed temperature gradient at the 

 station on the Mid-Atlantic Ridge in Fig. 8 is 315°C/km. Beneath the sediments 

 this will fall to about 130°C/km owing to the conductivity of basalts being 

 greater than that of the sediments. The temperature would, therefore, reach 

 the melting point of basalt at a depth of 5 to 10 km below the ocean floor. 

 Intrusion and vulcanism seems likely to occur under such circumstances. 



It is perhaps significant that the high heat flow on the Mid- Atlantic Ridge 

 was measured in the central valley. This valley is a frequently observed feature 

 of the ridge and is believed to run more or less continuously through the North 

 and South Atlantic, round South Africa and up the ridges of the Indian Ocean, 

 one of which runs into the Red Sea. In the Red Sea there is a trough similar to 

 that on the ridge. From gravity and magnetic surveys, Girdler (1958) has con- 

 cluded that this valley marks the site of a dyke, or series of dykes, 50 km in 

 width. It would be of great interest to know if the heat flow is high in this 

 valley also. 



The relation of the Red Sea to the ridges of the Indian Ocean and the Atlantic 

 appears similar to that of the Gulf of California to the East Pacific Rise. Here 

 again, where the ridge runs into the narrow sea there are high heat flows. There 

 is, however, a less well-marked trough and gravity high. 



If high heat flows prove to be characteristic of mid-ocean ridges and to be 

 due to intrusion, it will be necessary to explain why intrusion should occur on 

 such a scale over such great distances. It has been suggested that the pattern 

 of heat flow might indicate the locus of an ascending convection ciu-rent in the 

 mantle, though there is no compelling reason to suppose that this is so ; if it 

 were, the low heat flows on each side of the East Pacific Rise could be interpreted 



