Can magma-injection and groundwater forces cause massive landslides on Hawaiian volcanoes?

Landslides with volumes exceeding 1000 km³ have occurred on the flanks of Hawaiian volcanoes. Because the flanks typically slope seaward no more than 12 °, the mechanics of slope failure are problematic. Limit-equilibrium analyses of wedge-shaped slices of the volcano flanks show that magma injection at prospective headscarps might trigger the landslides, but only under very restrictive conditions. If static magma weight is the sole source of magma pressure, hypothetical flank failures can have any size, but can occur only if slip-surface friction angles are less than about 16 °. If slip surfaces have friction angles more typical of fragmented or intact rocks (30–40 °), flank failures can occur only if mean magma pressures exceed static equilibrium pressures. Landslide length then scales with the excess magma pressure divided by the buoyant unit weight of the volcano flank. For typical excess magma pressures, buoyant unit weights and rock friction angles, the largest landslides that might be triggered in this manner have lengths of only several kilometers. This is at least an order of magnitude too small to explain the occurrence of giant Hawaiian landslides. The growing mass of active Hawaiian volcanoes can compress the edifice and substrate rocks and consequently produce groundwater head gradients that might destabilize larger sectors of the volcano flanks. However, calculations show that volcano growth at an estimated long-term vertical rate of 0.02 m/yr can generate significant head gradients only if an areally extensive, buried clay layer exists that has a great thickness (~ 200 m) or very low hydraulic diffusivity (~ 10⁻¹¹ m²/s). Additional calculations show that groundwater head gradients associated with topographically induced flow and sea-level change are less likely to be important. Thus a simple, quantitative explanation for failure of Hawaiian volcano flanks remains elusive, and more complex scenarios may merit investigation.