Abstract The development of overpressure in continental margins is typically evaluated with hydrogeological models. Such approaches are used to both identify fluid flow patterns and to evaluate the development of high pore pressures within layers with particular physical properties that may promote slope instability. In some instances, these models are defined with sediment properties based on facies characterization and proxy values of porosity; permeability or compressibility are derived from the existing literature as direct measurements are rarely available. This study uses finite-element models to quantify the differences in computed overpressure generated by fine-grained hemipelagic sediments from the Gulf of Cadiz, offshore Martinique and the Gulf of Mexico, and their consequences in terms of submarine slope stability. By comparing our simulation results with in situ pore pressure data measured in the Gulf of Mexico, we demonstrate that physical properties measured on volcanic-influenced hemipelagic sediments underestimate the computed stability of a submarine slope. Physical properties measured on sediments from the study area are key to improving the reliability and accuracy of overpressure models, and when that information is unavailable, literature data from samples with similar lithologies, composition and depositional settings enable better assessment of the overpressure role as a pre-conditioning factor in submarine landslide initiation.

Most phases of silicic lava dome growth have some associated explosive activity. Tephra produced during this activity have depositional characteristics, grain sizes, and grain shapes that reflect different mechanisms of dome growth and destruction. It is therefore possible to interpret the explosive history of a dome through study of adjacent tephra deposits even though the dome may no longer be present. Five stages of dome growth and their associated tephra deposits are considered here. (1) Crater formation before extrusion of a dome, including phreatic, phreatomagmatic (ph-m), and Plinian pumice eruptions, produces a tephra sequence at the base of a dome consisting of deposits rich in accidental lithic clasts from crater walls, overlain by beds of fine-grained tephra and coarse-grained pumice. (2) Magma pulses during dome growth (ph-m, in part) produce tephra consisting of mixtures of juvenile pumice and clasts derived from the partly solidified dome. (3) Ph-m interaction between new magma and a water-saturated dome produces uniform tephra consisting of angular clasts of dome lava. (4) Explosive eruptions that follow collapse of a gravitationally unstable dome produce tephra that consists of angular, partly pumiceous clasts of dome lava which fragment due to expansion of metastable water after release of confining pressure. (5) Posteruptive destruction of the dome by phreatic eruptions results in pyroclasts consisting of fine-grained, hydrothermally altered clasts derived from dome lavas. Major kinetic processes before explosive dome eruptions are the relatively slow diffusion of magmatic volatiles from magma to fracture planes and foliations within the dome, and the relatively fast diffusion of meteoric water into magma by mechanical mixing. These basic processes control most explosive activity at domes in cases of either expulsion of new magma or collapse of an unstable dome.