How Does Mud Volcano Occur?

The biggest mud volcano from Vulcanii Noroioși de la Pâclele Mici.

Upon encountering the term “volcano,” our minds typically conjure images of flowing lava and molten magma. However, an alternate form of volcanic activity exists, capable of wreaking comparable havoc. These phenomena are referred to as mud volcanoes. In this scenario, molten rock and ash clouds are absent; instead, the focal point is on the discharge of frigid, muddy fluids.

Compared to conventional volcanoes driven by magmatic activities, mud volcanoes remain comparatively enigmatic. Nevertheless, the Lusi outburst in Sidoarjo, an emerging mud volcano in Indonesia, in 2006 served as a reminder of their potential to cause significant destruction. The resulting mudflows from this volcanic event uprooted over 40,000 individuals and inflicted widespread harm on properties. Yet, the mechanics underlying this form of volcanic activity beckon elucidation.

Volcanoes in Very Specific Geological Conditions

The formations resulting from this kind of eruption bear a resemblance to traditional volcanoes, although they generally possess smaller dimensions. They showcase a cone-like structure and a crater from which a mixture of water- and gas-laden sediments is expelled. The more sizable ones, however, have the potential to span several kilometers in width and reach heights of up to 600 meters (1970 feet). On the other hand, some may manifest as uncomplicated depressions in the earth’s surface.

These formations come about due to the forceful expulsion of various gases, primarily methane, carbon dioxide, nitrogen, and other lightweight hydrocarbons. These gases get intermingled with water vapor and a blend of clay and water known as mud. On occasion, the expelled gases can ignite, giving rise to a towering column of flames.

Mud volcanoes can be witnessed on both land and in oceanic settings. Among the most renowned instances are Italy’s Macalube, Indonesia’s Bledug Kuwu and Sidoarjo, Romania’s Berca Volcano, and Pakistan’s Zalzala Jazeera.

This form of volcanic activity can emerge within two distinct geological settings: subduction zones or within the deltas of significant rivers, whether current or historical. These environments share the common characteristics of loose sediments and water, which are both essential components for the initiation of a mud volcano. Yet, there’s more to it. The generation of this kind of volcanic phenomenon also necessitates the pressurization of fluids.

Water- And Gas-Logged Sediments Put Under Pressure

 Sidoarjo region of East Java, Indonesia mud volcano
On May 29, 2006, hot mud began to pour from the ground near a gas exploration well, covering fields and villages in the Sidoarjo region of East Java, Indonesia. Image: NASA, Public Domain.

Within the expansive marine domain, mud volcanoes exhibit a distinct prevalence within sediment accretion prisms situated at the entrances of subduction trenches. These geographical zones manifest as a consequence of the overriding plate’s sweeping action, dislodging sediments from the oceanic plate undergoing subduction. This process leads to the gradual buildup of sediment in a prism-like configuration. Within these substantial sedimentary formations, there is a vigorous circulation of water, subjected to compressional tectonic stresses that expel it outwards, akin to the act of regularly squeezing a sponge. The expelled fluids, possessing lower temperatures and laden with sedimentary particles, subsequently emerge either as widespread diffuse seepages or as localized springs, thereby culminating in the creation of mud volcanoes.

In terrestrial landscapes, the emergence of mud volcanoes is closely linked with the liberation of water and gas, ensnared due to relatively swift sediment deposition. Generally, the sediment accumulation transpires at a gradual pace, permitting the incremental release of fluids. Nevertheless, certain environmental settings give rise to accelerated sedimentation, causing water to become confined at greater depths, occasionally alongside organic material. The decomposition of this organic matter generates gases over time. As the sediment load progressively accumulates, the deep layers saturated with water become pressurized.

Subsequent liberation of the trapped fluid results in the formation of mud volcanoes. Instances may arise where an impermeable layer obstructs the upward migration of fluids. Consequently, these fluids accumulate, progressively heightening the internal pressure. Any perturbation to this system (such as an earthquake or drilling activity) can instigate its rupture, leading to an abrupt discharge of pressure. This scenario triggers an enduring and catastrophic outpouring of muddy fluids.

The Nirano mud volcanoes in the Po Plain are an example of how mud volcanoes frequently appear against the backdrop of historical sedimentary basins as well as within particular oil-bearing basins. While a considerable number of mud volcanoes exist below sea level or within sparsely populated terrains, instances of their emergence within urbanized zones also occur. An illustration of this can be observed in the Sidoarjo volcano on the Java island. The genesis of this particular volcano can be attributed to extensive sedimentary accumulation spanning millions of years within an extensional basin, which subsequently became integrated into a compressional tectonic framework.