SUBMARINE MOUNTAINS 6l 



the substratum is topped by basalt) is the trace of such a verti- 

 cal fissure through the crust. To right of it are shown other 

 fissures, along which the molten basalt from the substratum 

 (solid black) rises in successive stages. At the top is a built-up 

 cone, through which the liquid column passes, all the way to 

 the open crater. The two sections respectively picture the con- 

 ditions for basaltic eruption on continent and sea floor, at a 

 geological epoch when the glassy basalt formed a world-circling 

 layer. For the reason stated in the first chapter, the sub-oceanic 

 crust is shown as somewhat thicker than the sub-continental 

 crust. 



As the surface cone grows higher by the piling of frozen 

 lava and ash-beds around the vent, the active liquid column 

 becomes longer and exerts increasing pressure on the sub- 

 stratum. Evidently this back-pressure puts a limit to the height 

 to which the liquid column or the surface crater can reach. 

 That limit is reached when the weight of the crust, plus the 

 weight of air and water on the crust, becomes equal to the 

 weight of the liquid column. 



The highest liquid column of the oceanic volcanoes is that 

 of Mauna Loa, Hawaii, when its crater is occupied by a "lake" 

 of boiling, basaltic lava, about 13,000 feet above sealevel. The 

 10,000-foot Mount Etna in full activity seems to contain the 

 highest column of liquid basalt on the continents. Let us sup- 

 pose that Mauna Loa and Mount Etna have both completed 

 their growth. If we can estimate the average densities of these 

 two liquid columns, and also estimate the respective average 

 densities of the sub-oceanic and continental segments of the 

 crust, we can calculate the approximate thicknesses of the crust 

 in the two segments. With the stated assumptions, the crust 

 under Sicily is found to be not far from 40 miles; the crust 

 under the mid-Pacific, not far from 50 miles. Both estimates 

 are crude but seem to give a reasonable order of magnitude. 



