322 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1933 



From the above accounts it will be seen that a certain amount of 

 direct evidence is supplied only by the craters of Arizona, Henbury, 

 and Wabar, and this has to be supplemented by a considerable amount 

 of speculative deduction. Direct observation of how such craters are 

 formed is, of course, quite out of the question. Meteorites of which 

 the fall has been actually observed have always been of comparatively 

 small size, and their velocity has been reduced by the resistance of 

 the air to that of an ordinary falling body of about 70 meters per 

 second. They make small holes, usually of not more than 1 or 2 

 feet in depth, in the ground. The largest meteorite of which the fall 

 has been observed is a stone of 820 pounds, which fell at Paragould 

 in Arkansas on February 17, 1930. This penetrated clayey soil to a 

 depth of 8 feet, scattering clods to a distance of 50 feet in the pasture. 

 On the other hand, the largest known meteorites, all of which are 

 irons and none observed to fall, have been found by reason of their 

 being partly exposed at the surface of the ground. The 60-ton Hoba 

 meteorite discovered in South-West Africa in 1920 has its upper sur- 

 face level with the surrounding ground, and around it there is no 

 sign of a crater. The large masses of iron near Cape York in the 

 north of Greenland were found loose on the rocky surface. 



It seems therefore that meteorite craters are not merely dents in the 

 ground made by the percussion of a meteorite; but that they are 

 explosion craters due to the sudden vaporization of part of the 

 material, both of the meteorite and of the earth, in the intense heat 

 developed by the impact. When a large mass of iron traveling with 

 planetary velocity is suddenly stopped, the kinetic energy (l^mv ^) 

 is transformed into heat at a localized spot with the development of 

 a very high temperature. Simple calculations give very high figures. 



The materials from the Henbury and Wabar craters give ample 

 evidence of high temperatures. The transformation of kamacite 

 from a-iron to y-iron at 850° C. and the melting points of iron at 

 1,530° C. and silica at 1,700° C. are definite points on such a " geo- 

 logical thermometer." We may further add the boiling point of 

 iron at 3,200° C. and that of nickel at 3,377° C. The boiling point 

 of silica has been estimated at a minimum of 2,590° C, but this is 

 probably too low. These are the boiling points calculated for the 

 pressure of one atmosphere, but under the enormous pressures pro- 

 duced by the explosions at the meteorite craters they must have been 

 considerably higher. 



The upward force of the explosion must be very much greater 

 than the downward force of percussion ; and for this reason the beds 

 exposed on the inside crater walls will dip radially outwards from the 

 center (fig. 3a), instead of inward toward the center as might at 

 first sight be expected. The outward dip could also perhaps be 



