Moho dips to greater depths under con- 

 tinents and rises to shallower depths un- 

 der the oceans. This is a direct conse- 

 quence of the fact that continents, which 

 are granitic, literally float on deeper 

 rock, depressing it beneath them (Fig. 2). 

 Thus, beneath any continent the Moho 

 lies at least twenty miles below sea level, 

 while in mountainous regions, the added 

 load depresses the Moho to depths of 

 thirty to forty miles. Under an ocean, 

 however, the Moho layer lies only about 

 six miles below sea level, or some three 

 miles below the ocean bottom. Further- 

 more, the distance approaches two and 

 a half miles below the bottoms of certain 

 deep sea trenches. 



In addition to all this, it is known that 

 even under fairly level ocean bottoms 

 the Moho possesses slight rises and dips 

 which cannot be due to different weights 

 of overlying rocks since these rocks are 

 fairly uniform in thickness. It has been 

 conjectured that the Moho may repre- 

 sent the original, primordial land surface 

 of the earth before the granitic conti- 

 nents were formed. 



This, then, is the background to the 

 present Moho story. Speculations, hy- 

 potheses, and educated guesses concern- 

 ing the true nature of the Moho have 

 been heaped upon each other without 

 any means of judging which may be 

 correct — if any is at all ! 



In 1957, the American Miscellaneous 

 Society (AMSOC) was formed by a 

 group of scientists from the Earth Science 

 Committee of the National Science 

 Foundation to investigate the feasibility 

 of drilling to the Moho. They reasoned 

 that billions of dollars (and rubles) will 

 be spent in the next twenty years to 

 reach the moon, Mars, and other plan- 

 ets, while we will still be ignorant of 

 what lies three miles below the ocean 

 bottom, although only a few hundred 

 pounds of samples taken from this area 

 would give us a major clue to the nature 

 and composition of 84 per cent of the 

 volume of the earth on which we live. 

 The deepest hole that had been drilled 

 was an oil exploration hole that went to 

 23,000 feet. Since it was on the con- 

 tinent it did not, of course, hit the Moho. 

 Current drilling technology indicated 

 that a 25,000 to 30,000 foot hole (five to 

 six miles) would be possible. If drilling 



Page 6 



THIN CRUST 

 (3-40 MILES) 



MANTLE 

 (RERlDOTlTE?) 



OUTER CORE 

 (LIQUID) 



INNER CORE 

 (IRON-NICKEL) 



Fig. 1 



took place on the open sea, above a deep 

 sea trench, the drill could simply be low- 

 ered the first three miles and would have 

 to drill through rock for only about an- 

 other 16,000 feet to tap the Moho. Fur- 

 thermore, no drilling through upper 

 crustal granite would be required. If 

 this could be achieved, it would mark a 

 major breakthrough for the sciences of 

 geology, geophysics, and seismology. 



Enormous technical difficulties still 

 would have to be overcome, the chief 

 one of which would be to keep the 

 drilling ship in one position over the 

 hole. It is not our purpose here to dis- 

 cuss these difficulties, which have been 

 amply covered in the popular press. Suf- 

 fice it to say that the AMSOC committee 

 reported that the project could be accom- 

 plished. 



Two sites were chosen which fitted all 

 the technical requirements of depth to 

 the Moho, sea conditions, weather con- 

 ditions, and the like. One is near 

 Guadalupe Island, south of Los An- 

 geles. The other is in the Puerto Rican 

 Trench, about 120 miles north of San 

 Juan, Puerto Rico. It was decided that 

 preliminary test runs would be made at 

 the former site, and the final drill hole 

 probably placed at the latter. For these 

 purposes the drilling ship, Cuss I, was 



modified for deep sea drilling. 



Although the primary objective of the 

 project is, as we have said, to settle the 

 question of the composition of the earth's 

 mantle rock, this is by no means the only 

 aim. On the way down to the Moho, 

 sedimentary materials on the ocean bot- 

 tom will also be sampled. Deep oceanic 

 sedimentation is extremely slow, and a 

 few feet of sediment can represent many 

 millions of years of time. Scientists 

 have long asked the question: just when 

 did the first seas form? Examination of 

 sedimentation samples may finally make 

 it possible to determine the age of our 

 oceans. Farther down, it should also be 

 possible to verify the existence of the 

 basaltic layer at the base of the earth's 

 crust. 



Further, very accurate density meas- 

 urements of mantle rock will enable us to 

 judge the density of the earth's core with 

 greater accuracy than in the past. At 

 the same time, an age determination of 

 mantle rock samples should tell us if the 

 Moho does indeed mark the primordial 

 earth's surface. 



Gases seeping into the hole can also be 

 collected and will yield information 

 about the composition of deeper levels 

 yet. Heat flow, temperature, and heat 

 conductivity can be measured with great 



