A small island that appeared in the Arabian Sea off Pakistan after an earthquake last week most likely formed when the shaking released methane gas and water trapped in undersea sediments. The gas and water forced part of the seabed to the surface, experts said.
"It looked as if a section of shallow seafloor had simply been pushed up," said Game McGinsey, a volcanologist with the U.S. Geological Survey.
Photographs of the island, which measures roughly 100 feet by 250 feet and rises about 60 feet above the water, showed a rough-textured surface suggesting that the seafloor had risen and cracked, he said.
'It's not a mud volcano'
McGinsey said the way the island was created was similar in some ways to that of a so-called mud volcano, in which gas and water force mud up through vents to the surface. In those cases, the flow of mud normally continues for some time, similar to the way that lava flows from a conventional volcano. There are some long-lived mud volcanoes in the region, McGinsey said, but this one appeared to be a one-time event, with no sign of continuous flow.
"It's not a mud volcano in the classic sense," he said.
The magnitude 7.7 quake struck Tuesday, killing more than 500 people and flattening homes in the southwestern province of Baluchistan. It was followed by a 6.8-magnitude aftershock Saturday that killed at least 15 people.
The initial quake was centered about 40 miles north of the city of Awaran and about 250 miles from the port town of Gwadar, where the new island appeared in shallow waters about a half-hour later.
Townspeople and scientists who visited the island told news agencies that it was muddy and rocky and was emitting flammable gas. Methane, the main component of natural gas, is highly flammable.
Makran subduction zone
The quake occurred in the Makran subduction zone, a vast and complex tectonic feature stretching from Pakistan to Iran where three plates, the Indian, Arabian and Eurasian, meet.
As the Arabian plate slides under the Eurasian, sediments containing water and methane are compressed, said Michael Steckler, a geophysicist with the Lamont-Doherty Earth Observatory, a part of Columbia University.
"In subduction zones you get a lot of overpressure," Steckler said.
Even a relatively far-off earthquake can produce enough shaking to fracture the sediments and release the gas and water, he said.
The methane is created through the action of bacteria on organic matter, and would have been trapped in the sediments as free molecules of gas. The Arabian Sea is also home to large quantities of methane hydrates, icy cagelike structures of water molecules with methane molecules inside, and immediately after the island formed there was speculation that hydrates, not free methane, had been released.
But hydrates form only under high pressure and low temperatures, and Carolyn Ruppel, director of the survey's hydrate research program, said water in the area was too shallow, and temperatures in sediments far too high, for hydrates to exist.
Similar islands formed in the Arabian Sea after an 8.1-magnitude earthquake in 1945. (A recent study by scientists in Germany showed that that quake set off the release of free methane from sediments, releases that continue today.) Islands also formed after quakes in 1999 and 2010.
Such islands eventually disappear, eroded by the action of tides and waves. Steckler said that the one that formed in 1999, for instance, was gone in a few months, a victim of monsoon surges.
"This one's coming up after the monsoon," he said, "so we'll see how long it lasts."