Glass produced by underground nuclear explosions

Abstract

Detonation of an underground nuclear explosive deposits a large amount of energy in what can be considered to be a point source in space and time. The strong shock wave produced propagates spherically outward, initially vaporizing the explosive and nearby rock and subsequently melting, then heating the surrounding rock as the energy density decreases. The vaporized material expands adiabatically forming a cavity. As the energy in the vaporized material is dissipated during the cavity formation process, the explosive and rock debris condense and mix with the melted rock. The melt flows to the bottom of the cavity where it is cooled and quenched by fractured rock fragments falling from above as the cavity collapses. Measurements on a large number of underground nuclear explosions indicate that approximately 740 tons of rock and/or soil are melted for every kiloton (1012 cal) of explosive energy, or approximately 25% of the explosive energy goes to melting rock. The resulting glass composition reflects the composition of the unaltered rock with debris from the explosive added. A wide range of physical appearances has been observed, ranging from white pumice to dense, dark lava. In general, bulk composition and color vary with the amount of explosive iron incorporated into the glass. By inference, all the refractory explosion products are mixed with the solidified melt, although the degree of mixing inferred is highly variable. Recent electron microprobe studies of glasses produced by the low yield Rainier event in welded tuff have produced the following results: (1) glasses are dehydrated relative to the host media, (2) glasses are extremely heterogeneous with respect to major element chemistry on a 20 μm scale, (3) a ubiquitous feature is the presence of dark “marble-cake” regions in the glass; these are locally enriched in iron and may be related to the nuclear device debris, (4) optically amorphous regions with chemistries of the crystalline phases are found in the glasses and provide evidence of shock melting, (5) only limited major element redistribution and homogenization occur within the cavity. The short duration of the thermal pulse and the high viscosity of the melts produced by the Rainier event precluded re-homogenization by diffusion or convection, respectively.