Landslides in blanket peat on subantarctic islands: Causes, characteristics and global significance

Blanket peat on hillslopes in the British Isles is known to be susceptible to mass movements. A few such peat landslides had also been reported from southern hemisphere islands with climates that favour the formation of blanket bogs. Previous studies of peat landslides on subantarctic islands are re-examined to address two primary hypotheses: (i) that landslides in blanket peat are primarily controlled by natural intrinsic thresholds, and (ii) that the general characteristics and failure mechanisms of the peat landslides on subantarctic islands do not differ significantly from British and Irish examples. Peat slides on Macquarie Island and Marion Island are analysed to establish whether quantitative comparisons can be made with the stability of blanket peat in Ireland. The inventory surveys of the subantarctic islands indicate that there may be several hundred peat landslides, perhaps three times more than in the British Isles. They appear to be common because of the combined effects of several factors including the constituent vegetation and the frequent moderate rainfall that promotes formation of smooth, impermeable iron and organic pans at the base of the peat. The absence of intense convective rainfall, droughts and anthropogenic disturbance leads to the conclusion that the failures result from peat accumulation reaching natural maximum thicknesses with respect to intrinsic stability thresholds. Different vegetation on different slopes seems to influence the shear strength of blanket peat on Macquarie Island, and in general the subantarctic peat landslides are shown to be visually similar but quantitatively different from Irish examples.