The mechanical genesis of “fairy circle” depressions

Abstract

Fairy circles are sub-circular structures characterized by distinctive vegetative growth linked to natural hydrogen seepage. These features typically appear as subtle depressions with depth-to-diameter ratios as low as 1:100. Numerical models incorporating two-phase flow and volumetric deformation due to changes in effective stress successfully simulate depressions formed by mechanical compaction, closely resembling their natural counterparts. Mechanical compaction requires a decrease in pore fluid pressure and a consequent increase in effective mean stress, a condition that arises when gas inflow from a localized subsurface source ceases. Model results demonstrate that the diameter and depth of fairy circle depressions generally scale with both the gas pressure and the depth of the source. For the first time, this study explains the mechanical formation of fairy circle depressions, paving the way for future models to support the exploration and production of natural hydrogen as a sustainable energy resource.