Pyroclastic Andesite Tile Melting during the First Two Seconds after the Explosion of the A-bomb at 8:15 a.m. on August 6, 1945 in Hiroshima

The extreme effect of the heat rays of the A-bomb explosion on August 6, 1945 at 8:15 a.m. in Hiroshima was studied on two tile fragments that had been excavated during the period from 1977 to 1982 from the west bank of Motoyasu River, about 100 m down the river from the Motoyasu bridge. A number of very hot and melted fragments, which the shock wave brought from buildings that were smashed at the hypocenter 1.318 s after the explosion, were deposited on the west bank of the river. The pieces of tile possibly came from the destroyed stone wall of the Sei Hospital, the Saikoji Temple, and/or the Sairenji Temple, and were quickly cooled by the river water. The tile fragments were composed of andesitic pyroclastic rock and their surfaces were melted to a depth of 3.18 mm. The glass crust had a variable andesite and basalt-andesite composition, which are the melt products of cristobalite and/or tridymite, pigeonite (XFe = Fe/(Fe + Mg) = 0.37-0.44), hornblende (XFe = 0.33-0.42), labradorite (Ab48.2-40.6An51.8-55.5Or0-3.9), and K-feldspar (Ab8.2Or91.8). The temperature of 6287°C was calculated on the surface of an object at the hypocenter after the explosion, according to the depth of 3.18 mm of the melt and different depths and melting points of above mentioned minerals. This surface temperature was deduced by the extrapolating of the depth-temperature relationship obtained by the mineral-relicts between 2.68 and 3.18 mm of depth. According to the regression line T = -1715.1d + 6287 (d is the depth) with R2 = 0.989, the temperature gradient in the andesite tile was 1715°C/mm, reaching a depth from 2.86 to 3.18 mm, where the volume of glass and volume of primary minerals (rock) are equal. For a depth of more than 3.64 mm, the structure and mineral assemblage of pyroclastic andesite rock has an initial composition.