Mud volcanoes are associated with the Alpine-Himalaya fold belt from northern Italy on the west to New Guinea on the east. The most significvant of these are on the south of the Greater Caucasus in Azerbayjan and in the South Caspian depression. More than half the mud volcanoes of the world are concentrated in the South Caspian depression, and this does not even take into account those in the deep-water part of the Caspian Sea.
The sedimentary section of the South Caspian has been studied by continuous geoacoustical sounding and common depth point seismic surveys. The Pliocene-Quaternary section has been intensively faulted and deformed into long, high-closure linear folds and is host to numerous mud volcanoes. Most of these folds are diapiric and are characterized by chaotic seismic records.
The most common type of mud volcanoe in the study area is associated with faults on anticlinal and diapiric folds, where the eruptive apparatus penetrates the fold and at the crest forms a debris cone. When an eruption takes place offshore in water depths up to 50-70 m, an island may form - only to be rapidly eroded. A dredge sample from a mud volcano in the south of the Shatskiy offshore zone consisted of blue-gray gas-saturated sediment, which contained a large amount of yellowish-white gas hydrate. These crystals were quite large (5-7 cm), and in the air they melted and disappeared. When ignited they burned with a blue flame. In association with the gas hydrate was a large quantity of clay-carbonate concretions.
The regional distribution of the mud volcanoes coincides with that of gas hydrates. Consequently, the South Caspian is a hydrate province. This is in contrast to land areas where the gas of mud volcanoes escapes into the atmosphere. Under marine conditions the gas is converted into hydrate and is retained. With passage of time and deposition of younger sediments on top of this hydrate, the temperature increases, the hydrates melt, and the gas migrates upward and may again go into hydrate.
Association of mud volcanism with deep faults is unquestioned. The latter serve as conduits for flow of super-heated water and hydrocarbon gases from great depth. This in conjunction with plastic rocks in the section leads to formation of mud volcanoes. The most important factor for mud volcanism is geostructural, and in particular the presence of a large depression zone with consider- able sedimentary fill. This depression zone must have experienced active subsidence in recent time. Thickness of Quaternary sediments in the most subsided sectors of the South Caspian is 2000 m. This subsidence led to creation of overpressure, which contributed to the mud volcano phenomenon. Another important factor has been the high thermal state of the asthenosphere in the South Caspian. (Taken from Kulakova and Lebendev, 1990; digested in Petroleum Geology, vol. 30, no. 4, 1996) Copyright 2001 James Clarke. You are encouraged to print out this News Letter and to forward it to others. Earlier News Letters are available at our web page: http://geocities.com/internetgeology/ This News Letter is distributed without charge in the interest of our science of petroleum geology. To be added to the mailing list please send your e-mail address to: jamesclarke@erols.com