at all depths, from very shallow surface 

 movements to movements at depths of 

 over 400 miles. 



Although news reports of earthquakes 

 are quite infrequent, quakes are, in real- 

 ity, almost daily occurrences. Ninety 

 per cent of them are of such small mag- 

 nitude as to be unnoticed. Earthquake 

 shock waves are, at least in part, like 

 sound waves. Experience has shown us 

 that we can often see an event before we 

 hear it. We see a man in the distance 

 fire a gun but do not hear the report un- 

 til after a lapse of time. Similarly, when 

 an earthquake generates shock waves a 

 period of time elapses before they reach 

 the surface and are recorded by the seis- 

 mograph. The time it takes to cover a 

 given distance depends directly on the 

 density and uniformity, or lack of uni- 

 formity, of the rocks through which the 

 waves must pass. 



Experiment has shown that the ve- 

 locity at which shock waves travel is 

 higher in denser rock and lower in rela- 

 tively less dense rock. A liquid, on the 

 other hand, will almost entirely absorb 

 these waves so that none of them gets 

 through. Furthermore, from experience 

 we know that sound waves can be re- 

 flected, or echoed, off walls, canyon 

 sides, hills, etc. Similarly, shock waves 

 in the earth will reflect partly or wholly 

 when they encounter a radical change in 

 density, as would happen if definite lay- 

 ers of different density existed within the 

 earth. 



Seismologists have been gathering da- 

 ta on the velocities and reflections of 

 earthquake shock waves for over seventy 

 years. The result of their interpretations 

 is the familiar picture of our earth as a 

 layered planet (Fig. 1). Our chief inter- 

 est here is in the layer called the mantle, 

 which makes up about 84 per cent 



of the volume of the whole earth. 



In 1909, the Yugoslavian seismologist, 

 A. Mohorovicic (pronounced Moe- 

 hor-o-veetch'-ic) observed that shock 

 waves from a local quake near Zagreb 

 were partially reflected and changed ve- 

 locity from about four miles per second 

 to five miles per second at a depth of 

 about twenty miles. He interpreted this 

 as an abrupt change in the density of the 

 layers through which the waves were 

 traveling. Over the past fifty years nu- 

 merous observations by many seismol- 

 ogists have verified this break in uni- 

 formity within the earth's interior and 

 have named it, in honor of its discoverer, 

 The Mohorovicic Discontinuity (Fig. 2). 

 It marks the bottom of the earth's crust 

 and the top of the earth's mantle. 

 Above this discontinuity (usually short- 

 ened to "Moho") the average density is 

 2.8 grams per cubic centimeter. Below 

 it, the density jumps to 3.3 grams per 

 cubic centimeter. Further refinements 

 over the last fifty years have indicated a 

 slight rise in density just above the base 

 of the crust, before the sharp rise at the 

 Moho itself. 



The question is, then, just exactly 

 what change occurs at the Moho? The 

 top of the crust can be observed directly 

 and is known to consist mostly of a light- 

 colored rock called granite, usually com- 

 posed of three common minerals: feld- 

 spar, quartz, and biotite (black mica). 

 The base of the crust, at which the slight 

 rise in density (and wave velocity) 

 occurs, cannot be observed directly. 

 Hence we must make an educated 

 guess. Since many volcanic lava cham- 

 bers lie in the lower parts of the crust 

 we may assume that the lava which 

 comes out is largely representative of 

 this level. The lavas are dominantly ba- 



salt, which is composed of the minerals 

 plagioclase, pyroxene, and olivine. Thus 

 it appears that the crust gradually 

 changes downward from granite to 

 basalt, which has a density of about 

 3.0 grams per cubic centimeter. How- 

 ever, below the Moho, which lies be- 

 tween the base of the crust and the 

 mantle, there are several possibilities and 

 no observable evidence of which may be 

 correct. The known density of 3.3 fits a 

 rock called peridotite (olivine and py- 

 roxene), one called dunite (mostly oli- 

 vine alone), and a rare rock called eclo- 

 gite (pyroxene, amphibole, and garnet). 

 In addition, it has been conjectured that 

 under very high pressures, such as those 

 at the top of the mantle, the mineral oli- 

 vine would be squeezed to a more com- 

 pact form with a higher density. Thus 

 an olivine-bearing basalt would only re- 

 quire that its olivine content be conver- 

 ted to the higher density type to achieve 

 the appropriate jump in the average 

 density of the rock as a whole. Lab- 

 oratory attempts to make the higher 

 density form of olivine have not suc- 

 ceeded, however, because of the diffi- 

 culty of maintaining a high enough pres- 

 sure for a long enough time. 



There is no way to choose among 

 these various hypotheses as to the com- 

 position of the earth's mantle, although 

 it is generally believed that the eclogite 

 theory is the least likely. Thus it comes 

 down to three choices: peridotite, dun- 

 ite, or simply a continuation of basalt in 

 which olivine has been converted to a 

 high density form. 



The existence of The Mohorovicic Dis- 

 continuity is well established, with all 

 seismologists in agreement on this point. 

 Furthermore, since its recognition in 

 1909 refinements in measurement have 

 been made and it is now known that the 



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