338 ANNUAL. EEPOET SMITHSONIAN INSTITUTION, 19 31 



DIASTROPHIC FORCES 



Tliere are no known forces which have their origin outside of the 

 earth's material which can exert horizontal stresses on the crustal 

 material of the earth of such strength as to form mountains and 

 plateaus and cause earthquakes. It is true that the attractive forces 

 of the sun and moon are exerted on the earth, and, since the portion 

 of the earth that is nearest to the sun or the moon is attracted more 

 than the material that is farther away, a stress is set up. This stress 

 is not of sufficient magnitude, however, to rupture the material or to 

 make it move out of its normal place, except to the extent of a slight 

 elastic deformation called the earth tide. These tide-producing 

 forces of the sun and the moon change phase every few hours as the 

 earth turns on its axis. 



I think we can eliminate the attractive effect of the sun and the 

 moon as being the cause of any geological phenomena involved in 

 mountain forming, earthquakes, etc. Of course, the time of an 

 earthquake on an island or near the continental coast may be decided 

 by an exceptionally high or low water tide in the vicinity, but it is 

 reasonably certain that the crustal material is brought nearly to the 

 breaking point by some other cause and that the high or low tide 

 supplies merely the small increment required to increase the stress 

 beyond the breaking strength of the rock. The real causes of the 

 major features of diastrophism must lie within the earth itself. 



Much has been written in recent years about the effect of the heat 

 resulting from radioactivity of certain minerals in the outer portion 

 of the earth. This, it seems to me, may be a factor in earth move- 

 ments, but I am inclined to think it is one of minor importance. In 

 the first place, the radioactivity is largely confined to the granitic 

 material which is supposed to be only from 15 to 20 miles in thick- 

 ness under the continents. There is no granite under the oceans, but 

 some of the strongest earthquakes occur there and much of the 

 ocean bottom is quite active from a geological standpoint. Broken 

 ground with very steep slopes is found under the oceans, and many 

 oceanic islands are due to volcanic activity. All of this implies that 

 movements are going on in the crust under the oceans, and these 

 surely can not be due alone to the radioactivity of minerals. The 

 basalts which are supposed to underlie the granites of the continental 

 areas and to form the bottoms of the oceans have present in them 

 some radioactive minerals but not in such large proportions as are 

 present in the granites. 



Again, we have the problem of accounting for physical or chemical 

 activity that probably occurs even to the depth of 60 miles below 

 sea level. Earth students, who have been writing on radioactive 



