This article was originally published on The Conversation.
Rice farmers living in Regency Sidoarjo, Indonesia, awoke to a strange sight on May 29, 2006. The ground had cracked overnight and was emitting steam.
In the weeks that followed, water, boiling mud, and natural gas were added to the mix. As the eruption intensified, mud began to spread across the fields. Alerted residents were evacuated in hopes of safely awaiting the outbreak.
Only it didn’t stop. Weeks passed and the spreading mud engulfed whole villages. In a frantic race against time, the Indonesian government began building dams to contain the mud and stop it from spreading. As the mud topped these dams, they built new ones behind the first set. The government finally managed to stem the advance of the mud, but not before the torrents wiped out a dozen villages and forced 60,000 people to relocate.
Why would the earth suddenly start spewing out huge amounts of mud like this?
Introduction to mud volcanoes
The Lusi Structure – a contraction of Lumpur Sidoarjo, meaning “Sidoarjo Mud” – is an example of a geological feature known as a mud volcano. They form when a combination of mud, liquids and gases erupt at the Earth’s surface. The term “volcano” is borrowed from the much more familiar world of magmatic volcanoes, where molten rock comes to the surface. I’ve been studying these fascinating structures using underground seismic data for the past five years, but nothing quite compares to seeing one actively erupting.
In mud volcanoes, in many cases, the mud bubbles up to the surface quite quietly. But sometimes the eruptions are quite violent. Additionally, most of the gas that emanates from a mud volcano is methane, which is highly flammable. This gas can ignite and create spectacular fiery eruptions.
Mud volcanoes are little known in North America but much more common in other parts of the world including not only Indonesia but also Azerbaijan, Trinidad, Italy and Japan.
They form when liquids and gases accumulated under pressure inside the earth find an escape route to the surface through a network of fractures. The fluids move up these cracks, carrying mud with them and creating the mud volcano as they escape.
The idea is similar to a car tire containing compressed air. As long as the tire is intact, the air stays inside. However, once the air has found a way out, it begins to escape. Sometimes the air escapes as a slow leak – other times a blowout occurs.
Overpressure in the earth builds up when underground fluids cannot escape under the weight of the overlying sediments. Some of this liquid was trapped in the sediment as it was deposited. Other fluids can migrate from deeper sediments, while still others can be generated in situ by chemical reactions in the sediments. An important type of chemical reaction produces oil and natural gas. Finally, fluids can become overpressurized when compressed by tectonic forces during mountain construction.
Overpressures are common when drilling for oil and gas and are usually planned for. A primary way to deal with overpressures is to fill the wellbore with dense drilling mud of sufficient weight to contain the overpressures.
If the well is drilled with insufficient mud weight, pressurized fluids can rush up the well and explode at the surface, causing a spectacular blowout. Famous examples of blowouts include the 1901 Spindletop Gusher in Texas and the more recent 2010 Deepwater Horizon disaster in the Gulf of Mexico. In these cases it was oil, not mud, that gushed out of the wells.
In addition to being fascinating in their own right, mud volcanoes are useful for scientists as windows into conditions deep inside the Earth. Mud volcanoes can contain materials from as deep as 10 kilometers below the Earth’s surface, so their chemistry and temperature can provide useful insights into processes deep in the Earth that cannot be obtained any other way.
For example, analysis of the mud erupting from Lusi has revealed that the water was heated by an underground magma chamber associated with the nearby Arjuno-Welirang volcanic complex. Each mud volcano reveals details about what’s happening underground, allowing scientists to create a more comprehensive 3D view of what’s going on inside the planet.
Lusi’s mud is still erupting
Today, more than 16 years after the eruption began, the Lusi structure in Indonesia continues to erupt, but at a much slower pace. Its mud covers a total area of about 7 square kilometers, more than 1,300 football pitches, and is behind a series of dikes built to a height of 30 meters.
Almost as interesting as the efforts to stop the mud were the legal battles aimed at assigning blame for the disaster. The first rupture occurred about 650 feet (200 meters) from an actively drilled gas exploration well, prompting widespread allegations that the oil company responsible for the well was at fault. Well operator Lapindo Brantas countered that the eruption was natural, triggered by an earthquake that had occurred a few days earlier.
Those who believe the gas well triggered the eruption argue that the well erupted due to insufficient mud weight, but that the eruption did not make it all the way to the surface of the well. Instead, the fluids only partially made their way up the well before entering lateral fractures and erupting at the surface several hundred meters away. As evidence, these proponents point to measurements taken downhole during drilling. In addition, they suggest that the earthquake was too far from the well to have any impact.
In contrast, earthquake trigger proponents believe the Lusi eruption was caused by an active underground hydrothermal system similar to that of Old Faithful in Yellowstone National Park. They argue that such systems have long been affected by very distant earthquakes, so the argument that Lusi was too distant from the earthquake is invalid.
Additionally, they suggest that a downhole pressure test conducted after the eruption began showed the wellbore was intact, unruptured by fractures and leaking fluid. Consistent with this interpretation, there is no evidence that drilling muds ever came from the Lusi eruptions.
In 2009, the Indonesian Supreme Court dismissed a lawsuit accusing the company of negligence. That same year, police dropped criminal investigations into Lapindo Brantas and several of his associates due to lack of evidence. Although the lawsuits have been settled, the debate continues, with international research groups lining up on both sides of the dispute.
Michael R. Hudec is a senior research scientist at the Bureau of Economic Geology at the University of Texas at Austin. He receives financial support from the Applied Geodynamics Laboratory, an oil industry-funded research consortium supported by more than 20 companies.