DYNAMICS OF MUD VOLCANOES: AN OSMOTIC PUMP MECHANISM?

Introduction: Mud volcanoes occur both on land (e.g., near Bologna) and in the ocean depths, and in the latter case they are associated with the formation of methane hydrates, for which reason there is considerable interest in them at present. Submarine mud volcanoes are geological formations raised from the sea-bed by the extrusion of mud where fluid-rich and fine-grained sediments ascend owing to their buoyancy. Their origin has puzzled earth scientists during the last two-hundred years and is not yet well understood. Objectives: Our particular motivation in this study is to understand whether they are a geologic example of an osmotic pump: a self-organized mechanism that is generally studied in so-called chemical gardens; structures formed in laboratory experiments with the dissolution of metal salts that involve the formation of a semipermeable membrane. Development and conclusions: Buoyancy forces are the immediate driving force of the upward migration of sediments; however, this phenomenon on its own does not explain the existence of mud volcanoes as long-lived geological structures. The formation of a membrane as part of an osmotic pump is necessary to understand the dynamics. Clay minerals in the presence of gas hydrates formed by methane gas can form a seal in the sediments of the sea floor. This seal can act as a semipermeable membrane and water molecules will be able to enter by osmosis to the lower part of the membrane. This entrance of water will increase the fluidity of the lower sediments. This phenomenon along with the presence of the methane gas will increase the upward pressure until this fluid will break through the membrane and will escape to the surface by buoyancy, forming the cone of a mud volcano. This osmotic pump mechanism can produce both continuous-flow and oscillatory regimes that are observed in mud volcanoes in analogy to the regimes observed in the dynamics of chemical-garden growth in the laboratory.