Have you heard about mud volcanoes? If not much, let's go to the east part of Java, the densest populated island in the world with ca. 1000 people/km2. On 29 May 2006 a new mud volcano, Lusi, was born there. The birth was violent because >120,000 m3 of mud oozed out of the ground during that day. Geysers of water and huge amounts of gas streamed out of a hole in the ground. 13 people were killed and 60,000 made homeless. The eruption of hot mud still goes on and will probably continue for another 20 years or so. 40 villages have been covered with up to 40 m thick, hardening mud. The world's greatest building, Boeing's Everett Factory — assembling site for the company's largest aircraft — has a volume of 13.3 million m3. It would take Lusi 112 days to fill it with mud up to its roof. At the time of writing, Lusi has delivered mud during 4410 days. There are more mud volcanoes than active volcanoes on the continents, and the sea-beds have many times more of both kinds.

Mud volcano at Bledok Kuwu in Central Java, Indonesia. Photo: Sven Laufeld April 1990, 1/1000 sec.

Long before the Lusi mud volcano was born I spent a day to get acquainted with the Bledok Kuwu mud volcano in Java. It has not built a cone, and is located in a flat area as a rounded basin of mud where it from below inflates 1-2 m wide bubbles of mud that burst without much sound and collapse with a thud. The locals collected the liquid, poured it in halves of bamboo tubes which they stored in the sun until the water had evaporated and the salt crystallized. They used it locally because they had no money for buying salt from the ocean.

Mud volcanism has little to do with typical volcanism. The ejected material from mud volcanoes is less than 60 °C, has neither been molten, nor belonged to a magma. When a tectonic plate is formed by extruding lava and spread outwards from a Mid-ocean ridge, it will collide with something after about 250 million years. An example: the East Pacific plate has collided with the South American continental plate and is forced to plunge downwards under the continental plate. Geologists call such a submerging zone a Benioff zone. The oceanic rocks at that collision zone become hot when they dive deep under the continental plate and melt lower parts of the continental rocks. So the magma mix formed has less density than the overlying chiefly sedimentary rocks.

Like a cork under water this magma moves upwards like huge "drops" through the overburden. If and when a magma drop reaches the surface of the earth a volcano is born. When the magma sees light we call it lava. Lava is hot, >800 °C. This is why there is a long chain of volcanoes parallel with South America's west coast. In the zone of collision the rocks are compressed and folded and lifted upwards as a mountain chain — the Andes. Parallel with the mountain chain and the long row of volcanoes there is a zone of compression but not strong enough to have folded the rocks into mountains or hills. That zone is the birth place of mud volcanoes.

The mud is formed by the microscopic particles that were embedded in the ocean sediments that turned into sedimentary rocks. In tropical areas limestones are commonly formed near the coasts. The mud in mud volcanoes oozes out from porous and permeable marine limestones, young limestones of Tertiary age. In compresssion areas the underground mud reservoirs will become mud volcanoes sooner or later. Erosion will take away the seal or cap rock. A fault through a subterranean mud reservoir can generate an earthquake or a landslide that can start a mud volcano. When we drill for hydrocarbons, we can also trigger mud volcanoes.

When Lusi was born it began degassing greenhouse gases, a mix of 98% water vapor, 1.5% carbon dioxide (CO2) and 0.5% methane (CH4). The volumes are 800,000 tons, 30,000 and 2300 tons, respectively, each year, so far during 13 years. But the recorded values are few and the ejected volumes irregular over time. The volume of ejected methane varies by the minute. Methane is the most interesting because it is >20 times more powerful than the CO2 as a greenhouse gas and because Lusi is producing twice as much CH4 as measured at most other mud volcanoes. Climate modellers have to pay attention to mud volcanism.

Another interesting thing with mud volcanoes is that when an eruption starts, some catch fire when pieces of rocks collide and the sparks ignite the methane, ethane and other flammable hydrocarbons. From Burma and Azerbaijan we know that the initial flame can become >1 km high and scorch all vegetation 100 m from the crater. Another risk involved at the beginning of a mud volcano eruption is the degassing of another flammable but toxic gas, H2S, easily distinguished by its smell of rotten eggs. Hydrogen sulfide is as poisonous as carbon monoxide and occurs in crude oil and natural gas. Hydrocarbon plant workers have personal safety gas detectors which go into high alert at 15 ppm of H2S. At very low concentration it is an eye irritant, but at 50 ppm can cause eye damage. When a mud volcano starts with an eruption the concentration of this stinking gas can be 500 ppm, which can be lethal. Like CO2 hydrogen sulfide is heavier than air. Both gases are common at ordinary volcanoes and can kill plants, animals and people in the craters, canyons and at valley bottoms.

Mud volcanism is of great strategic interest because of its environmental importance not only locally. In many parts of the world mud volcanoes are abundant in hydrocarbon producing areas where their eruptions — natural as well as manmade — can cause great disasters. Hydrocarbon production is ongoing around and in the Black and Caspian seas where intensive exploration drilling also is taking place among one of the earth's largest concentrations of mud volcanoes.

According to Arrianos, Alexander the Great's army saw mud volcanoes close to the Indian Ocean before they reached River Indus in the 320s BCE. In his Naturalis Historia, Pliny in 77 CE described the huge eruption — with a flame visible far away — of the mud volcano at Sassuolo not far from Modena in Italy. Mud volcanoes like volcanoes are an interesting natural resource too. In Indonesia salt is extracted from them, in Italy the mud is used in equestrian medicine. In areas where volcanoes are absent, some mud volcanoes can provide sulfur rich mud that cures skin problems. They can also provide geothermal energy. Mud volcanism exists in compression zones all over the world.