As the name implies, mud volcanoes are eruptions of mud –
not molten rock as in igneous volcanoes.  They’re found all around the world,
amounting to about a thousand in total number known. The one thing they have in
common is hot or at least warm water, so they occur in geothermal areas
especially, but they also are found in the Arctic.

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They range in size from tiny, just a few meters across and
high, to big things that can cover several square miles. In Azerbaijan some mud
volcanoes reach 200 meters, 650 feet, in height, and around the world many of
them do have conical, volcano-like shapes. But there are others that are just
low mounds, more like a shield volcano. <o:p></o:p>

A little (15-cm) mud volcano in New Zealand. Photo by Richard Gibson.

The mud is often enough just a slurry of suspended fine-grained
sediment that mixes with the hot water. And by hot water, we don’t necessarily
mean incredibly hot – mud volcano temperatures as cold as a couple degrees Centigrade
are known, but most are associated with temperatures approaching the boiling
point of water.  In some places, like Yellowstone,
the water is acidic which helps it dissolve rocks down to the tiny fragments in
mud, and in other places it may just be the weathered soil and debris picked up
by the water that makes the mud. <o:p></o:p>

Mud volcanoes can erupt violently, or seep slowly, and
emissions can last from minutes to years. I think it’s fair to think of some of
them as geysers in which the water contains a lot of sediment, while others are
more like thick, viscous muddy warm springs. <o:p></o:p>

Besides water and fine sediment, mud volcanoes often contain
natural gas – most commonly methane, but sometimes carbon dioxide, nitrogen, or
other gases. The pressure of these gases is often the driving force behind
eruptions, and with a hydrocarbon gas like methane present you might think mud
volcanoes would be associated with oil and gas fields, and you’d be right. The
hundreds of mud volcanoes in Azerbaijan and in the adjacent Caspian Sea are in
the midst of the first great oil province to be exploited, and some of the petroleum
deposits there are related to structures in the rocks and sediments caused by
the upward force of the mud, which can bend its confining rocks as it rises,
just as a salt dome can do. And since methane is flammable, often enough there
are flames associated with mud volcanoes. In 2001, near Baku, Azerbaijan,
flames shot 15 meters, near 50 feet, into the air. Gobustan in Azerbaijan is a
World Heritage Site for its abundant rock carvings dating to 5000 to 20,000
years ago or more. The flaming methane eruptions of mud volcanoes in Azerbaijan
have been linked to the development of the Zoroastrian religion, and in fact
the name Azerbaijan derives from words meaning Land of the Eternal or Sacred
Fire.<o:p></o:p>

The most destructive mud volcano eruption in recent years
was on the island of Java, in Indonesia, in May 2006. It erupted in the middle
of a rice paddy, and ultimately killed 20 people, caused nearly 3 billion
dollars in damage, and displaced 60,000 people. The mud it erupted covers about
seven square kilometers, nearly three square miles, and in 2018 it continues to
erupt something like 80,000 cubic meters of mud every day – that’s almost 3
million cubic feet, 32 Olympic swimming pools each day. <o:p></o:p>

What caused the violent and extensive eruption of the Lusi
Mud Volcano, also called the Sidoarjo mud flow, on Java is not clear. It may be
simply part of the ongoing natural tectonic and magmatic processes in the
region, which is dotted with many real volcanoes, the kind that carry molten
rock to the surface as lava, and there’s a fault system that may provide a conduit
for hot water from a volcano about 50 kilometers away. Lusi may be an entirely
natural phenomenon. But there are also interesting possible trigger mechanisms.
One suggests that a large earthquake two days before the mud volcano erupted
changed the plumbing system enough to spur the eruption. That’s reasonable,
since we know that earthquakes can have significant effects on geyser systems.
Old Faithful in Yellowstone changed its eruption period following the strong Hebgen
Lake earthquake in 1959. The other possible trigger is nearby drilling by a gas
exploration company, which may have encountered an open pocket of gas or some
other feature that ultimately may have allowed enough pressure to build up to
make the mud volcano erupt. Good science on all sides of this issue have not
resolved its origin with certainty, but on the whole I think the consensus is
that the mud eruption was indeed triggered by the drilling. Studies continue,
and there are legal cases in progress too, of course. <o:p></o:p>

Sidoarjo Mud Flow, Indonesia, 2008 NASA image created by Jesse Allen, using data from NASA/GSFC/METI/ERSDAC/JAROS, and the U.S./Japan ASTER Science Team. Caption by Michon Scott, based on interpretation by Geoffrey S. Plumlee, U.S. Geological Survey Crustal Imaging and Characterization Team. Source

Another mud volcano that was recently in the news is in
Taiwan. Taiwan has at least 17 mud volcanoes which have been known for
centuries, and the flammable natural gas associated with them was used in
brick-making in southern Taiwan. The gas is probably methane, and it sometimes
ignites naturally. The Wandan mud volcano in this area has a sporadic history,
dormant for 9 years in the 1980s but erupting with damage in 2011 and 2016.
Taiwan is on the subduction zone between the Philippine plate and Eurasia,
complicated by a change in orientation of the subduction zone where Taiwan
sits. This complex tectonic setting, together with the heat liberated by
subduction, is probably the ultimate cause of the earthquakes, geologically
recent volcanism, and the mud volcanoes on Taiwan.

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Mud volcano eruptions are probably no more predictable than
real volcanoes or earthquakes, but their similarity to geysers might give at
least an element of predictability to them. A mud volcano that erupted in
Trinidad in February 2018 seems to have a period of about 25 to 30 years, but
that’s obviously a pretty wide range. The most recent event at Trinidad’s
Devils Woodyard mud volcano covered an area about 100 meters across and tossed
mud six meters into the air. Like the features in Azerbaijan, the mud volcanoes
in Trinidad are closely associated with hydrocarbon deposits, including
Trinidad’s famous pitch lake – thick tarry oil at the surface of the land. <o:p></o:p>

Most of the hot mud activity in Yellowstone isn’t really
what you’d call mud volcanoes. It’s more boiling mud-rich hot springs like the
Fountain Paint Pots, but every now and then they can make small cones, less
than a meter high, and in the past there have been mud-rich geyser eruptions at
Yellowstone. <o:p></o:p>

By some estimates there are many more mud volcanoes on the
sea floor than there are on land. The known offshore mud volcanoes are often
associated with methane hydrates – methane gas frozen into ice in the sediment
beneath the sea floor. So it would be no s