found, in accordance with the results of seismic research, the value of 7 × 1011 for the zone of silicate rocks of the earth, originally estimated as 1500 km. in thickness,[1] and for the core, probably made of iron, according to Wiechert’s observations on earthquakes, a value of about 20 to 24 × 1011. The difference between these figures is of little importance. It is quite sufficient for our purpose to know that the earth as a whole is more rigid than steel.
Schweydar has also examined the question suggested by the earthquake observations as to whether a fluid magmatic layer exists beneath the earth’s crust: “It appears that a magmatic layer the fluidity of which is only to be compared with that of sealing-wax at room-temperature, and the thickness of which is only 100 km., cannot be present. By calculation, it is seen that the assumption of a liquid layer approximately 600 km. thick, the viscosity coefficient of which is of the order 1013 to 1014, beneath a crust of the earth of thickness 120 km., is in closest agreement with the observed facts.” The viscosity coefficient of sealing-wax at ordinary temperature is of the order of 109, or, in other words, Schweydar finds that the sima beneath the continental blocks is about 10,000 times as rigid as sealing-wax at room temperature.
It is certainly not to be wondered at if these quite reliable results are felt to contradict the ideas developed above on the viscosity of the earth.
The solution of this apparent contradiction lies in
- ↑ The school of Wiechert (B. Gutenberg, “Über Erdbebenwellen,” Nachr. d. Ges. Wiss. zu Göttingen, 1914) finally found from the propagation of earthquake-waves four surfaces of discontinuity, namely, at the depths of 1200 km., 1700 km., 2450 km., and 2900 km., of which the first and last appear most marked, so that the best assumptions at present are an approximately 1200 km. thick silicate covering, a median layer about 1700 km. thick, and a core of the earth measuring 3500 km. in radius.
