Mud Volcanoes Are Not Volcanoes

A mud volcano looks like a volcano that has been left in the rain. Small grey cones, sometimes a few metres high and sometimes a few hundred, with a crater at the top full of something the consistency of thick soup, which bubbles, occasionally belches, and periodically overflows.

The word volcano is doing something misleading here. There is no magma involved, no lava, and no heat in the volcanic sense: the mud arriving at the surface is usually somewhere between cool and mildly warm. What they share with actual volcanoes is a mechanism for getting something pressurised from underground to the surface, and a habit of building a cone out of whatever comes up.

The confusion is common enough that we have given the subject a page, and there is a second reason: two of the destinations on this site are mud volcanoes, and travellers arrive at both expecting something else.

Start with a thick pile of sediment, usually in a basin where a river has been dumping mud for millions of years. Bury it, and the layers at the bottom get compressed and heated. Organic material in them breaks down and produces gas, mostly methane.

Now the problem: the fine clay above is close to impermeable, so the gas cannot escape. Pressure builds until it exceeds the weight of the rock above, and the system fails. Gas forces its way up through any weakness it can find, carrying water and clay with it, and arrives at the surface as a mud slurry.

That is the whole mechanism. It is a petroleum-geology phenomenon rather than a volcanic one, which is why mud volcanoes and oil fields are found in the same places. Azerbaijan is not coincidentally both the mud volcano capital of the world and one of the oldest oil regions on earth.

Estimates of the global total run to around seven hundred, and something like three hundred of those are in Azerbaijan and the adjacent Caspian, which makes it far and away the densest concentration on earth.

The reason is geological luck. The South Caspian basin is one of the thickest sedimentary accumulations anywhere, over twenty kilometres in places, laid down fast and buried deep, generating enormous quantities of gas beneath layers that will not let it out. The mud volcanoes are the pressure relief.

They are also large. The biggest Azerbaijani examples are hundreds of metres high and kilometres across, which puts them in a different category from the metre-high cones people usually picture, and the field at Gobustan sits alongside a UNESCO-listed collection of thousands of prehistoric petroglyphs, so the visit is archaeological as well as geological.

The other Azerbaijani speciality is fire. Because the gas is methane, a mud volcano can ignite, and when a large one does it produces flames reported at hundreds of metres. Nearby Yanardag, the burning hillside, is a related phenomenon: natural gas seeping from the ground and alight, apparently continuously, for a very long time.

Colombia's contribution is a cone about fifteen metres high near Cartagena, with a crater at the top that holds ten to fifteen people at a time, and the tradition is that you get in.

The mud is dense enough that you float regardless of body composition, which people find genuinely disconcerting: you cannot sink, and you cannot easily stand up either, because there is nothing solid to push against. The usual description is somewhere between warm porridge and wet cement. There are attendants who will move you around the crater and offer a massage, generally for a tip, and then you climb down and rinse off in the lagoon at the bottom.

Two practical notes that nobody mentions until you are standing there in your swimming costume. The mud does not come off easily, particularly out of hair and swimwear, and the rinsing is done in the lagoon by local women who also expect a tip: bring small notes and something you do not mind staining. And leave the camera, or hand it to someone dry, because the combination of fine clay and a phone is permanent.

It is one of the odder things in this catalogue, and it has essentially no dependence on season, weather, altitude or volcanic activity. The only real seasonal factor is that the access track and the rinsing are more pleasant when it is not raining, and Cartagena's dry season runs from December to April.

What it is not is a volcano in any sense that would interest a volcanologist. It is a hole full of cold clay, and it is enormously good fun.

Two things are worth knowing, because they explain why the sites are managed the way they are.

They can erupt violently. A large mud volcano releasing pressure can throw material and, if the gas ignites, produce a flame column visible for tens of kilometres. This is rare and it happens in the large Azerbaijani and Caspian systems rather than in the small ones tourists visit.

They can be triggered by drilling, and the consequences can be permanent. The case that defines this is in East Java, where a mud eruption began in May 2006 beside a gas exploration well and never really stopped. It has buried villages under metres of mud across several square kilometres, displaced tens of thousands of people, and required embankments to be built and raised continuously for years to keep the flow from spreading further. The vent has been reported discharging on a scale that makes it one of the largest mud eruptions ever documented.

Whether it was triggered by the drilling or by an earthquake two days earlier has been argued in the scientific literature ever since, with reputable groups on both sides and a good deal of money resting on the answer. Either way it demonstrates the thing worth carrying away: these systems are pressurised, the pressure is deep, and disturbing them has consequences that outlast everyone involved.

For a visitor at a managed site, neither of these is a practical concern. They are the reason the managed sites are managed.