Mud volcanism: Processes and implications

Mud volcanoes can be large and long lived geological structures that morphologically resemble magmatic volcanoes. Because of their capricious behaviour and their spectacular morphology and landscapes, mud volcanoes have attracted attention since antiquity. More recently, mud volcanoes have become the focus of extensive studies for natural science research, including geologists and biologists. Mud volcanoes can be essentially divided in two groups: those associated with magmatic complexes and those related to petroleum provinces. Their occurrence is broadly distributed throughout the globe in both passive and predominantly active margins, often situated along faults, fault-related folds, and anticline axes. These structures act as preferential pathways for deep fluids to gather and ultimately reach the surface. Mud volcanoes episodically experience violent eruptions of large amounts of gas mixed with water, oil, mud and rock fragments forming the so called ‘‘mud breccia’’. The periodical eruptions can produce volcano-shaped mountains that can reach kilometres in size. Detailed studies of mud volcanoes have been conducted for decades (e.g. Jakubov et al., 1971; Higgins and Saunders, 1974; Barber et al., 1986; Brown, 1990; Camerlenghi et al., 1992; Kholodov, 2002; Kopf, 2002). Below I summarize the main findings so far, combined with my own suggested mechanisms (Fig. 1). The main driving engine of the eruptions is overpressured methane rising from source rocks and hydrocarbon reservoirs at greater depths. Other known overpressure buildup mechanisms that contribute to the brecciation of the deep sedimentary units include for example the dewatering of thick clay-rich sedimentary units, and geochemical reactions in sedimentary units with high temperature gradients. These fluids overpressured fluids gather along morphological discontinuities and favorable geological structures (e.g. fault planes, anticline axes, preexisting deformations). During this overpressure buildup a dome or diapir-shaped feature of brecciated sedimentary units forms in the subsurface. The rise of the fluids and the growth if this diapir is partly self-sustained by buoyancy and by the constantly increasing volume of fluids at shallower depth. A suggested scenario summarizing the birth of a mud volcano and the eruption mechanisms envisages that when the subsurface overpressure reaches a threshold depth where the overburden weight is exceeded, fracturing and breaching of the uppermost units occur, sometimes facilitated by external factors (e.g. earthquakes). Brecciated sediments throughout the feeder channel have a reduced cohesion. As breaching of the overburden occurs,