270 



EXPLORATION GEOPHYSICS 



^ — /C/fi/e e</ye su/>f3orf 



that the discovery of large negative anomalies in the vicinity of island arcs 

 is probably the most important contribution of the century in regard to the 

 nature of mountain building. Such negative anomalies are in the form of 

 a relatively narrow strip, or belt, about 90 miles wide, and up to 5,000 

 miles long in the case of the East Indies. The gravity anomaly in such 

 a strip may be over —150 milligals with a maximum of —300 milligals, 

 which is greater than anomalies known on the continents. 



The strong negative anomaly belt lies, in most cases, on the outer (or 

 convex) side of the main island arc, either approximately over the axis of a 

 long narrow ocean deep or a little to one side thereof. This belt is bordered 

 on each side by belts of positive gravity anomalies which generally coincide 

 with topographic swells (geanticlines) in the ocean floor. The inner swell 

 commonly emerges to form the main island arc. 



It is concluded that such belts of nega- 

 tive anomalies represent abnormal mass 

 distribution in the earth's crust below the 

 strip. They may indicate a downward 

 buckling of the earth's crust into a vertical 

 isoclinical fold with upsqueezed zones 

 flanking them, one of which may form an 

 island arc or chain. Mountain building 

 experiments tend to confirm these ideas. 



mass 171 Simple Pendulum. — A simple pendu- 



lum consists of a small, heavy mass sus- 

 pended by a theoretically massless, perfectly 

 flexible, string of unvarying length. Ob- 

 viously, such an object does not exist but 

 it ofifers a simple theoretical basis for the 

 discussion of more complicated pendulums. 

 As shown in Figure 132, the downward force on the bob is mg, where 

 g is the acceleration due to gravity and m is the mass of the bob. The 

 component of this force acting perpendicular to the string is mg sin i, 

 where i is the angle between the string and the vertical. The acceleration 



dH 

 of the mass is / -rj where / is the length of the string. Hence, from New- 

 ton's second law of motion, 



my 



Fig. 132. — Simple pendulum, (mg is 

 the force of gravity acting vertically 

 downward. The force tending to move 

 the mass m along tha arc A A' is 

 ntg sin t.) 



ml 



dH 

 dt^ 



— — mg sm t 



d i , g . . „ 

 or -7-r 4- 7 sm I = 

 dt'^ I 



The solution of this equation is a periodic function with period T, 

 where T is the time for the pendulum to swing from A to A' and back to 

 A; where points A and A^ represent the highest points of the swing.* T is 



* In some of the literature the half period (time to swing from A to A') is referred 

 to rather than the time for a comolete oscillation. 



