Natural Explosive Processes in the Permafrost Zone

Abstract

Two groups of natural explosive processes in the permafrost zone are considered. The first group was described long ago and is associated with freezing of water under constrained environmental conditions (explosions of hydrolaccoliths and icing mounds). The second group has been identified in the last three years. It is associated with the release of underground gases formed during dissociation of gas hydrates contained in permafrost. In both cases, explosion is caused by overpressure in the soil mass containing free water or gas. Release occurs when pressure exceeds the strength of the top of the permafrost. A number of common features related to preparation of explosive processes in permafrost can be identified. The first is a local zone where an explosive substance is concentrated: a frozen streambed of a groundwater flow, a water concentration zone in the frozen soil mass, or gas hydrates in frozen soil. The second feature is pressure compressing the substance. The third feature is deformations in overlying rocks. If pressure increases slowly and the roof has time to deform, then plastic deformation takes place and frost mounds expressed in the topography are formed. If pressure increases quickly, plastic deformation may not occur. The fourth feature is the explosion itself. As many authors have described, explosion impacts on objects of various origin have common characteristics: ejection of gas-saturated water, gas, and ground and ice debris to a distance of up to tens and sometimes hundreds of meters. Dissociation of gas hydrates in frozen ground first produces microcracks, then, ascending at quite high pressure, gas hydrates form subvertical channels and elongated pores. Ascent of gas hydrates to the surface and gas evaporation are impeded by a durable monolithic overlying ice-soil “cover.” As a result of this, a crack–pore structure of the frozen ground forms under the cover. The width of the crack opening and pore size increase as pressure grows due to gas filtration from the source of gas hydrate dissociation. Cracks and pores merge to form a cavity, into which gas leaks. Once the ultimate strength limit is exceeded, the cover may not bear stresses and the accumulated gas potential energy in the cavity is released (i.e., it is transformed to kinetic energy) through an explosion. During development of the Arctic, the hazard of explosive processes for engineering structures will increase. Nevertheless, this group of hazards is not only not taken into account in designs and forecasts, they are not even treated as dangerous geological processes.