Natural explosion structures in rocks

Abstract

We have observed, in a few hydrothermal breccia bodies near Hong Kong, many peculiar natural explosion structures that bear a strong resemblance to the fracturing patterns produced by quarry blasting. The structures, occurring in hydrothermally altered late Mesozoic volcanic and sedimentary rocks, range in size from a few cm to about 80 cm in diameter, and exhibit a central cavity, a crushed zone around the central cavity, and a radial-crack zone like those formed by explosive blasting. A damage boundary marking the outer extent of the radial fractures is sometimes present. Some of the structures are affected by pre-existing cracks, with the damage boundary stretched in the direction of the crack. Results from detailed field measurements of the morphology, geometric characteristics, and orientations of over a hundred such explosion structures show a spherical geometry of the structure. These natural explosion structures probably formed when rapidly ascending superheated fluids reached the boiling curve, which led to depressurization of the fluids and expanding vapor explosions. The formation mechanism and thermodynamic implications are discussed in this paper, which is the first report of such naturally formed vapor explosion structures in rocks.