Mud volcano floods - Sidoarjo, Java

Disaster-plagued Indonesian island faces new threat.

What has happened?

For 3 months a sea of hot mud has been gushing from the ground in Sidoarjo, East Java, 35 kilometres south of Indonesia's second largest city, Surabaya. The steaming mud pool is growing at an estimated 50,000 cubic metres a day, accompanied by hydrogen sulphide gas, and now reportedly covers more than 25 square kilometres. The flow has not yet been stopped; thousands of people have lost their homes.
How bizarre... has this sort of disaster happened before?

The Sidoarjo disaster is an example of a 'mud volcano'. Mud and gas accumulates when sea sediments are trapped in subduction zones, where one tectonic plate slides under another, and can erupt out of volcanic cones or simply from a crack in the ground. Mud volcanoes have burst on every continent, but are abundant in the South Caspian region (offshore and onshore Azerbaijan) and offshore Indonesia in the East Java Basin.

But the Sidoarjo mud volcano is rather unusual. It's huge. And, says Sam Rice, a geologist at the University of Cambridge, UK, reports of the mud eruption suggest that it is a hybrid between typical mud volcanoes and hydrothermal vents. The mud is of an unusually high temperature (60 °C) and contains enormously high concentrations of hydrogen sulphide gas. This suggests that some kind of volcanic, hydrothermal activity is going on at the same time.
What creates the conditions for a mud volcano?

Achim Kopf, a geologist from the University of Bremen, Germany, who has studied mud volcanoes extensively, explains that marine sediment can be scraped off an oceanic tectonic plate as it slides underneath a continental plate. If the sediment accumulates rapidly and water is trapped in its pores, this can stop the sediment being cemented by pressure. The resulting reservoir of mud can be trapped underground. In the case of the East Java mud flow, the mud is thought to have come from a reservoir some 2.7 kilometres below the Earth's surface.
And what triggers an eruption?

A number of things can create a crack that allows trapped mud to bubble to the surface; particularly earthquakes and drilling.
And in Java specifically?

In Java both of these things have happened recently. The oil and gas exploration company PT Lapindo Brantas is drilling in the area, and the gas and hot mud first spewed from the company's drilling rig on 28 May.

Geologist Georg Delisle of the Federal Institute for Geosciences and Natural Resources (BGR), Hannover, Germany, explains that the drilling apparently penetrated into the liquid sediment and created a connection back to the surface. The pressure then squeezed up the mud, like toothpaste from a tube. But it is likely that other connections were made to the surface, he adds not just through the drilling pipe because attempts to pump concrete into the pipe to block the flow of mud have failed.

On 27 May an earthquake struck and devastated Yogyakarta on Java, and this too could have cracked the ground, potentially helping to release the mud. But the quake's epicentre was some 300 kilometres away from the mud volcano (making it only 2 on the Richter scale in that area).

The issue of what, exactly, caused this disaster is highly politically charged. It is still under investigation by police, the government and international experts.
Just how big is the eruption?

According to many geological experts, the scale of this mud volcano is unprecedented at least on land.

In 1945, the Makran earthquake in Pakistan triggered the sudden emergence of three offshore mud volcanoes, and in March 1999 a mud volcano rose out of the water overnight to form Malan Island, 3 kilometres from Pakistan's coast. It is hard to estimate the volume of mud created by such underwater eruptions. And, notes Rice: "Because the extrusion of mud and toxic gas occurs on the seabed it does not threaten human life and does not make the headlines."

'Well-kick' the sudden surface eruption of gas and mud during offshore oil drilling is common, but usually stops after a few days. Delisle recalls a smaller-scale incident in the 1960s where a geothermal well in the Wairakei geothermal field, New Zealand, ran wild: it took 3 months to stop the geothermal steam that found its way to the surface alongside the original borehole.
Can the disaster be stopped?

Nobody knows. So far, nothing has worked. PT Lapindo Brantas's senior vice-president Imam Agustino has been quoted saying: "The best-case scenario [for stopping the mudflow] is now mid-November, but I have to admit it might never be stopped."