THE EARTH'S INTERIOR— ADAMS 263 



only about 2.8, it is obvious that the central density must be much 

 higher than 2.8 — perhaps 8 or 10 or 12 — in order for the average to 

 come out right. This high density in the central region might be due 

 to either of two causes: (1) The squeezing of ordinary rock into a 

 much smaller volume under the enormous pressure due to the weight 

 of superincumbent material, or (2) the presence of some other, intrinsi- 

 cally heavier, substance such as a metal. The first alternative was 

 ehminated by using seismologic data to tell us the compressibihty of 

 rocks at great depth and then computing the amount by which the 

 volume of silicate rocks could be reduced at depths well toward the 

 center. The maximum by which the density of the material could be 

 increased turns out to be surprisingly large, but entirely inadequate for 

 giving the required average density of the earth. We must conclude, 

 therefore, that it is impossible to account for the high density of the 

 earth by compression alone, and that at and around the center there 

 is a considerable amount of an intrinsically heavy substance. The 

 only reasonable choice is metalhc iron. This element is the fourth in 

 order of abundance in ordinary rocks, it is also abundant in the sun 

 as shown by the spectroscope, and in both the metalhc and combined 

 form it is the dominant constituent of meteorites. By analogy with 

 meteorites we should expect that the core would not be pure iron but 

 rather an alloy of iron with several percent of nickel. The notion of an 

 iron core is not a new one; it was suggested by Dana in 1873, and 

 developed by Wiechert and others in later years. Still earlier the earth 

 was considered to be a great ball of granite, chemically homogeneous 

 throughout, but we have now passed beyond what may be called the 

 granitic era in geophysics, and our present convictions are based on 

 quantitative evidence of the presence of some heavy material at 

 the center. 



We may therefore speak with confidence of an ii-on or nickel-iron 

 core the diameter of which is fixed by seismologic data at 6,400 kilo- 

 meters, or a little more than one-half the diameter of the earth, and 

 confirmed by the moment of inertia determined by astronomical 

 observations. The core is plastic rather than rigid, since it does not 

 transmit transverse earthquake waves; it is nonmagnetic and there- 

 fore has no appreciable influence on the earth's magnetism; and the 

 pressure at its center, as is easily calculated, reaches the enormous 

 value, 3,200,000 atmospheres. We know the core to be very hot, 

 but it has not yet been possible to arrive at an entirely satisfactory 

 estimate of the central temperature. From considerations connected 

 with the origin of the earth, the conclusion has been reached ^° that 

 the temperature m the far interior is of the order of 3,000° C. 



Although the existence of an iron core is generally accepted by those 

 who have interested themselves in the subject, a few investigators 



"> Personal communication from Dr. Ross Qunn. 



