Page:The Scientific Monthly vol. 3.djvu/423

 THE SOIElSr-TIFIC

��NOVEMBER, 1916 \j$ N0V"71916

��EXPLOSION CEATEBS Bt n. h. dabton

U. S. GEOLOGICAL' SURYBT^

ALTHOUGH most great craters are on top of volcanic peaks^ a few big holes of volcanic origin are on plains and not connected with lava outflows. These are rimmed by a ridge of fragmental material evidently blown out of the hole, so that they are clearly the results of an explosion. One crater of this character, however. Crater Moimd (formerly Coon Butte) in Arizona, is believed by some observers to have been caused by impact of a meteor. As there is no direct evidence as to the origin of this great hole and its rim of fragmental ejecta, it may be interesting to present for comparison facts regarding some craters which are closely similar in all essential respects.

Among long-known holes ascribed to volcanic explosion and not connected with lava outbursts are the "maars'* in the Ehine valley and the craters of Eiffel, Auveme, Montecchio, Albani, Nemi, Astromi, Faif a, etc., of Nassibe, in Madagascar and at Lonar, India. The latter, as described by Dr. Blandford, is a hole about a mile in diameter, 300 to 400 feet deep, in a great lava plain. Except on the north and north- east sides, there is a rim 40 to 100 feet high composed of loose blocks of basalt similar to the rocks on the sides of the hole. The latter are bent up very slightly. As to the competency of volcanic explosion to cause a crater, many illustrations have been observed, notably in the great Bandai-San eruption^ in Japan in 1888, which made a vast crater in a mountain where there had been no activity for many cen- turies. There was no lava effusion connected with this outburst. The eruption of Krakatoa in 1883 is another impressive instance.

In the course of the present European war many a huge *' crater *' or *' entonnoir '^ has resulted from the explosion of '* mines ^* intended to destroy fortifications or dislodge troops, and it is interesting to have an example of this kind to compare with some natural craters. One

1 Published by penniBsion of the Director.

a "The Eruption of Bandai-San," Seis. Soc. Japan, Vol. 12, pp. 139-222, 1890.

VOL. in. — ^29.

�� �