Following a major earthquake which struck Pakistan in September 2013, a peculiar thing happened: A new island appeared in the sea off the country’s coast in the northern part of the Arabian Sea.
This strange occurrence soon attracted the attention of the world’s media, and articles about it began appearing – augmented by some stunning before-and-after images from NASA satellites. The rapid conclusion geoscientists drew? The new island was probably a mud volcano.
What Are Mud Volcanoes?
Mud volcanoes are topographical features related to geothermal activity. Unlike their magma-generated counterparts, mud volcanoes are relatively small and transient (the largest known, named Lusi, in Indonesia, has been erupting since 2006, and has been spreading mud over several square kilometres). Submarine mud volcanoes usually erode rapidly.
Although the exact mechanism of their formation isn’t clear (and science has identified different types and settings) there are certain key features which we can identify as common to all mud volcanoes. Essentially, mud volcanoes can form where an impermeable rock layer overlays a permeable shale or sedimentary layer. When, for whatever reason, fluid or gas reaches sufficient pressure to break through at the surface, the mud volcano will erupt.
A.V. Milkov lists a number of key reasons for their formation, under four different headings. Geologic reasons (essentially, those outlined above) are supplemented by tectonic reasons such as an association with folding and faulting but other features include hydrogeological reasons, which include the presence of hydrothermal fluids, or geochemical reasons.
The latter are of particular economic importance because in these cases the mud volcanoes may be associated with the presence of petroleum or gas hydrates, making them significant indicators of the presence of hydrocarbons. An as a footnote, it’s worth noting that some scientists believe that the eruption of Lusi is in fact the result of drilling , although others attribute it to an earthquake which occurred a few days before the eruption began.
Distribution of Mud Volcanoes
Mud volcanoes, though most people don’t know about them, and we don’t talk about them much, are in fact common phenomena. They’re widely distributed both on land and on the sea bed – though there are probably many more on the sea bed.
Mud volcanoes occur “throughout the globe in both passive and predominantly active margins, often situated along faults, fault-related folds, and anticline axes” according to Milkov, and their association with seismic belts illustrates their links to complex Earth processes. Perhaps most famously, the mud volcanoes of Azerbaijan are associated with the convergence of the Indian and Eurasian plate, but such features also occur in California’s Salton Sea (a conservative boundary) and away from plate boundaries as a result of magmatism (for example, in Yellowstone National Park).
Mud Volcanoes and Earthquakes
Although not all mud volcanoes directly associate with tectonic activity, there is considerable evidence to suggest that there is a relationship between the eruptions of these features and earthquakes. As Manga, et al put it, in 2008, “mud volcanoes sometimes erupt within days after nearby earthquakes. The number of such nearly coincident events is larger than would be expected by chance and the eruptions are thus assumed to be triggered by earthquakes.”
In situations where the conditions for mud volcanoes exist, the shaking caused by an earthquake may prove a trigger. Liquefaction, a well-established consequence of earthquakes, occurs when changes in fluid pressure as a result of shaking cause formerly solid ground to become liquid and is strong candidate as a triggering mechanism.
Pakistan’s New Island – A Mud Volcano?
Tectonically speaking, Pakistan is a zone of convergence, and as such we expect it to be an ideal location for mud volcanoes. This is indeed the case: There are a number of them located in the southern part of the country (you can clearly see them on Google Earth) and they also occur offshore, where the subduction of the Arabian plate beneath Eurasia has created a thick layer of loosely-consolidated sediments, which are a typical setting for mud volcanoes.
In view of this, and also of the established, probably statistical association of such features with earthquakes, all the evidence suggests that the new island is, indeed, a mud volcano and was triggered by the M7.7 tremor of 25 September – although how long it will last as the sediments erode remains a matter for debate.
Kinver, M. Indonesian mud volcano flow ‘to last 26 years‘. (2011). BBC News. Accessed October 6, 2013.
Milkov, A.V. Worldwide distribution of submarine mud volcanoes and associated gas hydrates. (2000). Marine Geology Volume 167. Accessed October 6, 2013.
Manga, M., Brumm, M. and Rudolph, M.L. Earthquake triggering of mud volcanoes. (2009). Marine and Petroleum Geology vol 26. Accessed October 6, 2013.
Mazzini, A. Mud volcanism: Processes and implications. (2009). Marine and Petroleum Geology vol 26. Accessed October 6, 2013.